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SH(1)                       General Commands Manual                      SH(1)

NAME
     sh - command interpreter (shell)

SYNOPSIS
     sh [-abCEeFfhIiLmnpquVvXx] [+abCEeFfhIiLmnpquVvXx] [-o option_name]
        [+o option_name] [command_file [argument ...]]
     sh -c [-s] [-abCEeFfhIiLmnpquVvXx] [+abCEeFfhIiLmnpquVvXx]
        [-o option_name] [+o option_name] command_string
        [command_name [argument ...]]
     sh -s [-abCEeFfhIiLmnpquVvXx] [+abCEeFfhIiLmnpquVvXx] [-o option_name]
        [+o option_name] [argument ...]

DESCRIPTION
     sh is the standard command interpreter for the system.  The current
     version of sh is in the process of being changed to conform more closely
     to the POSIX 1003.2 and 1003.2a specifications for the shell.  This
     version has many features which make it appear similar in some respects
     to the Korn shell, but it is not a Korn shell clone (see ksh(1)).  This
     man page is not intended to be a tutorial or a complete specification of
     the shell.

   Overview
     The shell is a command that reads lines from either a file or the
     terminal, interprets them, and generally executes other commands.  A
     shell is the program that is running when a user logs into the system.
     (Users can select which shell is executed for them at login with the
     chsh(1) command).  The shell implements a language that has flow control
     constructs, a macro facility that provides a variety of features in
     addition to data storage, along with built in history and line editing
     capabilities.  It incorporates many features to aid interactive use and
     has the advantage that the interpretative language is common to both
     interactive and non-interactive use (shell scripts).  That is, commands
     can be typed directly to the running shell or can be put into a file and
     the file can be executed directly by the shell.

   Invocation
     If no arguments are present and if the standard input, and standard error
     output, of the shell are connected to a terminal (or terminals, or if the
     -i flag is set), and the -c option is not present, the shell is
     considered an interactive shell.  An interactive shell generally prompts
     before each command and handles programming and command errors
     differently (as described below).  When first starting, the shell
     inspects argument 0, and if it begins with a dash `-', the shell is also
     considered a login shell.  This is normally done automatically by the
     system when the user first logs in.  A login shell first reads commands
     (as if by using the "." command) from the files /etc/profile and .profile
     in the user's home directory ($HOME), if they exist.  If the environment
     variable ENV is set on entry to a shell, or is set in the .profile of a
     login shell, and either the shell is interactive, or the posix option is
     not set, the shell then performs parameter and arithmetic expansion on
     the value of ENV, (these are described later) and if no errors occurred,
     then reads commands from the file name that results.  Note that no error
     messages result from these expansions, to verify that ENV is correct, as
     desired, use:
           eval printf '%s\\n' "${ENV}"
     Otherwise if ENV appears to contain a command substitution, which is not
     performed here, or if there were no expansions to expand, the value of
     ENV is used as the file name.

     Therefore, a user should place commands that are to be executed only at
     login time in the .profile file, and commands that are executed for every
     shell inside the ENV file.  To set the ENV variable to some file, place
     the following line in your .profile of your home directory

           ENV=$HOME/.shinit; export ENV

     substituting for ".shinit" any filename you wish.  Since the ENV file can
     be read for every invocation of the shell, including shell scripts and
     non-interactive shells, the following paradigm is useful for restricting
     commands in the ENV file to interactive invocations.  Place commands
     within the "case" and "esac" below (these commands are described later):

           case $- in *i*)
                   # commands for interactive use only
                   ...
           esac

     If command line arguments besides the options have been specified, and
     neither -c nor -s was given, then the shell treats the first argument as
     the name of a file from which to read commands (a shell script).  This
     also becomes $0 and the remaining arguments are set as the positional
     parameters of the shell ($1, $2, etc).  Otherwise, if -c was given, then
     the first argument, which must exist, is taken to be a string of sh
     commands to execute.  Then if any additional arguments follow the command
     string, those arguments become $0, $1, ...  Otherwise, if additional
     arguments were given (which implies that -s was set) those arguments
     become $1, $2, ...  If $0 has not been set by the preceding processing,
     it will be set to argv[0] as passed to the shell, which will usually be
     the name of the shell itself.  If -s was given, or if neither -c nor any
     additional (non-option) arguments were present, the shell reads commands
     from its standard input.

   Argument List Processing
     Currently, all of the single letter options that can meaningfully be set
     using the set built-in, have a corresponding name that can be used as an
     argument to the -o option.  The set -o name is provided next to the
     single letter option in the description below.  Some options have only a
     long name, they are described after the flag options, they are used with
     -o or +o only, either on the command line, or with the set built-in
     command.  Other options described are for the command line only.
     Specifying a dash "-" turns the option on, while using a plus "+"
     disables the option.  The following options can be set from the command
     line and, unless otherwise stated, with the set built-in (described
     later).

           -a allexport           Automatically export any variable to which a
                                  value is assigned while this flag is set,
                                  unless the variable has been marked as not
                                  for export.

           -b notify              Enable asynchronous notification of
                                  background job completion.  (Not
                                  implemented.)

           -C noclobber           Don't overwrite existing files with ">".

           -c                     Read commands from the command_string
                                  operand instead of, or in addition to, from
                                  the standard input.  Special parameter 0
                                  will be set from the command_name operand if
                                  given, and the positional parameters (1, 2,
                                  etc.)  set from the remaining argument
                                  operands, if any.  -c is only available at
                                  invocation, it cannot be set, and there is
                                  no form using "+".

           -E emacs               Enable the built-in emacs style command line
                                  editor (disables -V if it has been set).
                                  (See the Command Line Editing section
                                  below.)

           -e errexit             If not interactive, exit immediately if any
                                  untested command fails.  If interactive, and
                                  an untested command fails, cease all
                                  processing of the current command and return
                                  to prompt for a new command.  The exit
                                  status of a command is considered to be
                                  explicitly tested if the command is used to
                                  control an if, elif, while, or until, or if
                                  the command is the left hand operand of an
                                  "&&" or "||" operator, or if it is a
                                  pipeline (or simple command) preceded by the
                                  "!" operator.  With pipelines, only the
                                  status of the entire pipeline (indicated by
                                  the last command it contains) is tested when
                                  -e is set to determine if the shell should
                                  exit.

           -F fork                Cause the shell to always use fork(2)
                                  instead of attempting vfork(2) when it needs
                                  to create a new process.  This should
                                  normally have no visible effect, but can
                                  slow execution.  The sh can be compiled to
                                  always use fork(2) in which case altering
                                  the -F flag has no effect.

           -f noglob              Disable pathname expansion.

           -h trackall            Functions defined while this option is set
                                  will have paths bound to commands to be
                                  executed by the function at the time of the
                                  definition.  When off when a function is
                                  defined, the file system is searched for
                                  commands each time the function is invoked.
                                  (Not implemented.)

           -I ignoreeof           Ignore EOFs from input when interactive.
                                  (After a large number of consecutive EOFs
                                  the shell will exit anyway.)

           -i interactive         Force the shell to behave interactively.

           -L local_lineno        When set, before a function is defined,
                                  causes the variable LINENO when used within
                                  the function, to refer to the line number
                                  defined such that first line of the function
                                  is line 1.  When reset, LINENO in a function
                                  refers to the line number within the file
                                  within which the definition of the function
                                  occurs.  This option defaults to "on" in
                                  this shell.  For more details see the
                                  section LINENO below.

           -m monitor             Turn on job control (set automatically when
                                  interactive).

           -n noexec              Read and parse commands, but do not execute
                                  them.  This is useful for checking the
                                  syntax of shell scripts.  If -n becomes set
                                  in an interactive shell, it will
                                  automatically be cleared just before the
                                  next time the command line prompt (PS1) is
                                  written.

           -p nopriv              Do not attempt to reset effective UID if it
                                  does not match UID.  This is not set by
                                  default to help avoid incorrect usage by
                                  setuid root programs via system(3) or
                                  popen(3).

           -q quietprofile        If the -v or -x options have been set, do
                                  not apply them when reading initialization
                                  files, these being /etc/profile, .profile,
                                  and the file specified by the ENV
                                  environment variable.

           -s stdin               Read commands from standard input (set
                                  automatically if neither -c nor file
                                  arguments are present).  If after processing
                                  a command_string with the -c option, the
                                  shell has not exited, and the -s option is
                                  set, it will continue reading more commands
                                  from standard input.  This option has no
                                  effect when set or reset after the shell has
                                  already started reading from the
                                  command_file, or from standard input.  Note
                                  that the -s flag being set does not cause
                                  the shell to be interactive.

           -u nounset             Write a message to standard error when
                                  attempting to obtain a value from a variable
                                  that is not set, and if the shell is not
                                  interactive, exit immediately.  For
                                  interactive shells, instead return
                                  immediately to the command prompt and read
                                  the next command.  Note that expansions
                                  (described later, see Word Expansions below)
                                  using the `+', `-', `=', or `?' operators
                                  test if the variable is set, before
                                  attempting to obtain its value, and hence
                                  are unaffected by -u.

           -V vi                  Enable the built-in vi(1) command line
                                  editor (disables -E if it has been set).
                                  (See the Command Line Editing section
                                  below.)

           -v verbose             The shell writes its input to standard error
                                  as it is read.  Useful for debugging.

           -X xlock               Cause output from the xtrace (-x) option to
                                  be sent to standard error as it exists when
                                  the -X option is enabled (regardless of its
                                  previous state.)  For example:
                                          set -X 2>/tmp/trace-file
                                  will arrange for tracing output to be sent
                                  to the file named, instead of wherever it
                                  was previously being sent, until the X
                                  option is set again, or cleared.

                                  Each change (set or clear) to -X is also
                                  performed upon -x, but not the converse.

           -x xtrace              Write each command to standard error
                                  (preceded by the expanded value of $PS4)
                                  before it is executed.  Unless -X is set,
                                  "standard error" means that which existed
                                  immediately before any redirections to be
                                  applied to the command are performed.
                                  Useful for debugging.

              cdprint             Make an interactive shell always print the
                                  new directory name when changed by the cd
                                  command.  In a non-interactive shell this
                                  option has no effect.

              nolog               Prevent the entry of function definitions
                                  into the command history (see fc in the
                                  Built-ins section.)  (Not implemented.)

              pipefail            If set when a pipeline is created, the way
                                  the exit status of the pipeline is
                                  determined is altered.  See Pipelines below
                                  for the details.

              posix               Enables closer adherence to the POSIX shell
                                  standard.  This option will default set at
                                  shell startup if the environment variable
                                  POSIXLY_CORRECT is present.  That can be
                                  overridden (set or reset) by the -o option
                                  on the command line.  Currently this option
                                  controls whether (!posix) or not (posix) the
                                  file given by the ENV variable is read at
                                  startup by a non-interactive shell.  It also
                                  controls whether file descriptors greater
                                  than 2 opened using the exec built-in
                                  command are passed on to utilities executed
                                  ("yes" in posix mode), whether a colon (:)
                                  terminates the user name in tilde (~)
                                  expansions other than in assignment
                                  statements ("no" in posix mode), the format
                                  of the output of the kill -l command, where
                                  posix mode causes the names of the signals
                                  be separated by either a single space or
                                  newline, and where otherwise sufficient
                                  spaces are inserted to generate nice looking
                                  columns, and whether the shell treats an
                                  empty brace-list compound statement as a
                                  syntax error (expected by POSIX) or permits
                                  it.  Such statements "{ }" can be useful
                                  when defining dummy functions.  Lastly, in
                                  posix mode, only one "!" is permitted before
                                  a pipeline.

              promptcmds          Allows command substitutions (as well as
                                  parameter and arithmetic expansions, which
                                  are always performed) upon the prompt
                                  strings PS1, PS2, and PS4 each time, before
                                  they are output.  This option should not be
                                  set until after the prompts have been set
                                  (or verified) to avoid accidentally
                                  importing unwanted command substitutions
                                  from the environment.

              tabcomplete         Enables filename completion in the command
                                  line editor.  Typing a tab character will
                                  extend the current input word to match a
                                  filename.  If more than one filename matches
                                  it is only extended to be the common prefix.
                                  Typing a second tab character will list all
                                  the matching names.  One of the editing
                                  modes, either -E or -V, must be enabled for
                                  this to work.

   Lexical Structure
     The shell reads input in terms of lines from a file and breaks it up into
     words at whitespace (blanks and tabs), and at certain sequences of
     characters that are special to the shell called "operators".  There are
     two types of operators: control operators and redirection operators
     (their meaning is discussed later).  The following is a list of
     operators:

           Control operators:
                 & && ( ) ; ;; ;& | || <newline>

           Redirection operators:
                 < > >| << >> <& >& <<- <>

   Quoting
     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, or keywords.  There
     are four types of quoting: matched single quotes, matched double quotes,
     backslash, and dollar preceding matched single quotes (enhanced C style
     strings.)

   Backslash
     An unquoted backslash preserves the literal meaning of the following
     character, with the exception of <newline>.  An unquoted backslash
     preceding a <newline> is treated as a line continuation, the two
     characters are simply removed.

   Single Quotes
     Enclosing characters in single quotes preserves the literal meaning of
     all the characters (except single quotes, making it impossible to put
     single quotes in a single-quoted string).

   Double Quotes
     Enclosing characters within double quotes preserves the literal meaning
     of all characters except dollar sign ($), backquote (`), and backslash
     (\).  The backslash inside double quotes is historically weird, and
     serves to quote only the following characters (and these not in all
     contexts):
           $ ` " \ <newline>,
     where a backslash newline is a line continuation as above.  Otherwise it
     remains literal.

   Dollar Single Quotes ($'...')
           Note: this form of quoting is still somewhat experimental, and yet
           to be included in the POSIX standard.  This implementation is based
           upon the current proposals for standardization, and is subject to
           change should the eventual adopted text differ.

     Enclosing characters in a matched pair of single quotes, with the first
     immediately preceded by an unquoted dollar sign ($) provides a quoting
     mechanism similar to single quotes, except that within the sequence of
     characters, any backslash (\), is an escape character, which causes the
     following character to be treated specially.  Only a subset of the
     characters that can occur in the string are defined after a backslash,
     others are reserved for future definition, and currently generate a
     syntax error if used.  The escape sequences are modeled after the similar
     sequences in strings in the C programming language, with some extensions.

     The following characters are treated literally when following the escape
     character (backslash):
           \ ' "
     The sequence "\\" allows the escape character (backslash) to appear in
     the string literally.  "\'" allows a single quote character into the
     string, such an escaped single quote does not terminate the quoted
     string.  "\"" is for compatibility with C strings, the double quote has
     no special meaning in a shell C-style string, and does not need to be
     escaped, but may be.

     A newline following the escape character is treated as a line
     continuation, like the same sequence in a double quoted string, or when
     not quoted - the two characters, the backslash escape and the newline,
     are removed from the input string.

     The following characters, when escaped, are converted in a manner similar
     to the way they would be in a string in the C language:
           a b e f n r t v
     An escaped `a' generates an alert (or `BEL') character, that is, control-
     G, or 0x07.  In a similar way, `b' is backspace (0x08), `e' (an extension
     to C) is escape (0x1B), `f' is form feed (0x0C), `n' is newline (or line
     feed, 0x0A), `r' is return (0x0D), `t' is horizontal tab (0x09), and `v'
     is vertical tab (0x13).

     In addition to those there are 5 forms that need additional data, which
     is obtained from the subsequent characters.  An escape (\) followed by
     one, two or three, octal digits (`0'..`7') is processed to form an 8 bit
     character value.  If only one or two digits are present, the following
     character must be something other than an octal digit.  It is safest to
     always use all 3 digits, with leading zeros if needed.  If all three
     digits are present, the first must be one of `0'..`3'.

     An escape followed by `x' (lower case only) can be followed by one or two
     hexadecimal digits (`0'..`9', `A'..`F', or `a'..`f'.) As with octal, if
     only one hex digit is present, the following character must be something
     other than a hex digit, so always giving 2 hex digits is best.  However,
     unlike octal, it is unspecified in the standard how many hex digits can
     be consumed.  This sh takes at most two, but other shells will continue
     consuming characters as long as they remain valid hex digits.
     Consequently, users should ensure that the character following the hex
     escape sequence is something other than a hex digit.  One way to achieve
     this is to end the $'...' string immediately after the final hex digit,
     and then, immediately start another, so
           $'\x33'$'4...'
     always gives the character with value 0x33 (`3'), followed by the
     character `4', whereas
           $'\x334'
     in some other shells would be the hex value 0x334 (10, or more, bits).

     There are two escape sequences beginning with `\u' or `\U'.  The former
     is followed by from 1 to 4 hex digits, the latter by from 1 to 8 hex
     digits.  Leading zeros can be used to pad the sequences to the maximum
     permitted length, to avoid any possible ambiguity problem with the
     following character, and because there are some shells that insist on
     exactly 4 (or 8) hex digits.  These sequences are evaluated to form the
     value of a Unicode code point, which is then encoded into UTF-8 form, and
     entered into the string.  (The code point should be converted to the
     appropriate code point value for the corresponding character in the
     character set given by the current locale, or perhaps the locale in use
     when the shell was started, but is not... currently.)  Not all values
     that are possible to write are valid, values that specify (known) invalid
     Unicode code points will be rejected, or simply produce `?'.

     Lastly, as another addition to what is available in C, the escape
     character (backslash), followed by `c' (lower case only) followed by one
     additional character, which must be an alphabetic character (a letter),
     or one of the following:
           @ [ \ ] ^ _ ?
     Other than `\c?' the value obtained is the least significant 5 bits of
     the ASCII value of the character following the `\c' escape sequence.
     That is what is commonly known as the "control" character obtained from
     the given character.  The escape sequence `\c?' yields the ASCII DEL
     character (0x7F).  Note that to obtain the ASCII FS character (0x1C) this
     way, (that is control-\) the trailing `\' must be escaped itself, and so
     for this one case, the full escape sequence is "\c\\".  The sequence
     "\c\X" where `X' is some character other than `\' is reserved for future
     use, its meaning is unspecified.  In this sh an error is generated.

     If any of the preceding escape sequences generate the value `\0' (a NUL
     character) that character, and all that follow in the same $'...' string,
     are omitted from the resulting word.

     After the $'...' string has had any included escape sequences converted,
     it is treated as if it had been a single quoted string.

   Reserved Words
     Reserved words are words that have special meaning to the shell and are
     recognized at the beginning of a line and after a control operator.  The
     following are reserved words:

           !        {        }        case
           do       done     elif     else
           esac     fi       for      if
           in       then     until    while

     Their meanings are discussed later.

   Aliases
     An alias is a name and corresponding value set using the alias built-in
     command.  Whenever a reserved word (see above) may occur, and after
     checking for reserved words, the shell checks the word to see if it
     matches an alias.  If it does, it replaces it in the input stream with
     its value.  For example, if there is an alias called "lf" with the value
     "ls -F", then the input:

           lf foobar <return>

     would become

           ls -F foobar <return>

     Aliases provide a convenient way for naive users to create shorthands for
     commands without having to learn how to create functions with arguments.
     They can also be used to create lexically obscure code.  This use is
     strongly discouraged.

   Commands
     The shell interprets the words it reads according to a language, the
     specification of which is outside the scope of this man page (refer to
     the BNF in the POSIX 1003.2 document).  Essentially though, a line is
     read and if the first word of the line (or after a control operator) is
     not a reserved word, then the shell has recognized a simple command.
     Otherwise, a complex command or some other special construct may have
     been recognized.

   Simple Commands
     If a simple command has been recognized, the shell performs the following
     actions:

           1.   Leading words of the form "name=value" are stripped off, the
                value is expanded, as described below, and the results are
                assigned to the environment of the simple command.
                Redirection operators and their arguments (as described below)
                are stripped off and saved for processing in step 3 below.

           2.   The remaining words are expanded as described in the Word
                Expansions section below.  The first remaining word is
                considered the command name and the command is located.  Any
                remaining words are considered the arguments of the command.
                If no command name resulted, then the "name=value" variable
                assignments recognized in item 1 affect the current shell.

           3.   Redirections are performed, from first to last, in the order
                given, as described in the next section.

   Redirections
     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open, close, or duplicate an
     existing reference to a file.  The overall format used for redirection
     is:

           [n]redir-op file

     where redir-op is one of the redirection operators mentioned previously.
     A list of the possible redirections, and their meanings, follows.

     The [n] is an optional number, as in `3' (not `[3]'), that refers to a
     file descriptor.  If present it must occur unquoted, immediately before
     the redirection operator, with no intervening white space, and becomes a
     part of that operator.  If file descriptor n was open prior to the
     redirection, its previous use is closed.

     All redirections have a single word file argument following the operator
     (white space is allowed between the redirection operator and file),
     though it is sometimes expressed as n2.  That argument is expanded (see
     Word Expansions below) using tilde expansion, parameter expansion,
     arithmetic expansion, command substitution and quote removal to produce
     the path name (or file descriptor) to be used.  No field splitting or
     pathname expansion takes place.  In the list below, where the file is
     given as n2 the result of the expansions must be a number which refers to
     a suitable open file descriptor.

           [n]> file   Redirect standard output (or n) to file.

           [n]>| file  The same, but override the -C option.

           [n]>> file  Append standard output (or n) to file.

           [n]< file   Redirect standard input (or n) from file.

           [n1]<& n2   Redirect standard input (or n1) from a duplicate of
                       file descriptor n2.

           [n]<& -     Close standard input (or n).  Note that the `-' is
                       minus sign (or hyphen) given literally or resulting
                       from the expansion of file (or n2) for this format.
                       When given literally there is usually no space between
                       the redirection operator and the `-', though that is
                       just a convention.

           [n1]>& n2   Redirect standard output (or n1) to be a duplicate of
                       n2.

           [n]>& -     Close standard output (or n).

           [n]<> file  Open file for reading and writing on standard input (or
                       n).

     The following redirection is often called a "here-document".

           [n]<< delimiter
           ... here-doc-text ...
           delimiter

     The "here-doc-text" starts immediately after the next unquoted newline
     character following the here-document redirection operator.  If there is
     more than one here-document redirection on the same line, then the text
     for the first (from left to right) is read first, and subsequent here-
     doc-text for later here-document redirections follows immediately after,
     until all such redirections have been processed.

     All the text on successive lines up to the delimiter, which must appear
     on a line by itself, with nothing other than an immediately following
     newline, is saved away and made available to the command on standard
     input, or file descriptor n if it is specified.  If the delimiter as
     specified on the initial line is quoted, then the here-doc-text is
     treated literally; otherwise, the text is treated much like a double
     quoted string, except that `"' characters have no special meaning, and
     are not escaped by `\', and is subjected to parameter expansion, command
     substitution, and arithmetic expansion as described in the Word
     Expansions section below.  If the operator is <<- instead of <<, then
     leading tabs in all lines in the here-doc-text, including before the end
     delimiter, are stripped.  If the delimiter is not quoted, lines in here-
     doc-text that end with an unquoted \ are joined to the following line,
     the \ and following newline are simply removed while reading the here-
     document, which thus guarantees that neither of those lines can be the
     end delimiter.

     It is a syntax error for the end of the input file (or string) to be
     reached before the delimiter is encountered.

   Search and Execution
     There are three types of commands: shell functions, built-in commands,
     and normal programs -- and the command is searched for (by name) in that
     order.  A command that contains a slash `/' in its name is always a
     normal program.  They each are executed in a different way.

     When a shell function is executed, all of the shell positional parameters
     (note: excluding 0, which is a special, not positional, parameter, and
     remains unchanged) are set to the arguments of the shell function.  The
     variables which are explicitly placed in the environment of the command
     (by placing assignments to them before the function name) are made local
     to the function and are set to the values given, and exported for the
     benefit of programs executed with the function.  Then the command given
     in the function definition is executed.  The positional parameters, and
     local variables, are restored to their original values when the command
     completes.  This all occurs within the current shell, and the function
     can alter variables, or other settings, of the shell, but not the
     positional parameters nor their related special parameters.

     Shell built-ins are executed internally to the shell, without spawning a
     new process.

     Otherwise, if the command name doesn't match a function or built-in, the
     command is searched for as a normal program in the file system (as
     described in the next section).  When a normal program is executed, the
     shell runs the program, passing the arguments and the environment to the
     program.  If the program is not a normal executable file, and if it does
     not begin with the "magic number" whose ASCII representation is "#!", so
     execve(2) returns ENOEXEC then) the shell will interpret the program in a
     sub-shell.  The child shell will reinitialize itself in this case, so
     that the effect will be as if a new shell had been invoked to handle the
     ad-hoc shell script, except that the location of hashed commands located
     in the parent shell will be remembered by the child.

     Note that previous versions of this document and the source code itself
     misleadingly and sporadically refer to a shell script without a magic
     number as a "shell procedure".

   Path Search
     When locating a command, the shell first looks to see if it has a shell
     function by that name.  Then it looks for a built-in command by that
     name.  If a built-in command is not found, one of two things happen:

     1.   Command names containing a slash are simply executed without
          performing any searches.

     2.   Otherwise, the shell searches each entry in PATH in turn for the
          command.  The value of the PATH variable should be a series of
          entries separated by colons.  Each entry consists of a directory
          name.  The current directory may be indicated implicitly by an empty
          directory name, or explicitly by a single period.  If a directory
          searched contains an executable file with the same name as the
          command given, the search terminates, and that program is executed.

   Command Exit Status
     Each command has an exit status that can influence the behavior of other
     shell commands.  The paradigm is that a command exits with zero in normal
     cases, or to indicate success, and non-zero for failure, error, or a
     false indication.  The man page for each command should indicate the
     various exit codes and what they mean.  Additionally, the built-in
     commands return exit codes, as does an executed shell function.

     If a command consists entirely of variable assignments then the exit
     status of the command is that of the last command substitution if any,
     otherwise 0.

     If redirections are present, and any fail to be correctly performed, any
     command present is not executed, and an exit status of 2 is returned.

   Complex Commands
     Complex commands are combinations of simple commands with control
     operators or reserved words, together creating a larger complex command.
     Overall, a shell program is a:

     list        Which is a sequence of one or more AND-OR lists.

     AND-OR list
                 is a sequence of one or more pipelines.

     pipeline    is a sequence of one or more commands.

     command     is one of a simple command, a compound command, or a function
                 definition.

     simple command
                 has been explained above, and is the basic building block.

     compound command
                 provides mechanisms to group lists to achieve different
                 effects.

     function definition
                 allows new simple commands to be created as groupings of
                 existing commands.

     Unless otherwise stated, the exit status of a list is that of the last
     simple command executed by the list.

   Pipelines
     A pipeline is a sequence of one or more commands separated by the control
     operator `|', and optionally preceded by the "!" reserved word.  Note
     that `|' is an operator, and so is recognized anywhere it appears
     unquoted, it does not require surrounding white space or other syntax
     elements.  On the other hand "!" being a reserved word, must be separated
     from adjacent words by white space (or other operators, perhaps
     redirects) and is only recognized as the reserved word when it appears in
     a command word position (such as at the beginning of a pipeline.)

     The standard output of all but the last command in the sequence is
     connected to the standard input of the next command.  The standard output
     of the last command is inherited from the shell, as usual, as is the
     standard input of the first command.

     The format for a pipeline is:

           [!] command1 [| command2 ...]

     The standard output of command1 is connected to the standard input of
     command2.  The standard input, standard output, or both of each command
     is considered to be assigned by the pipeline before any redirection
     specified by redirection operators that are part of the command are
     performed.

     If the pipeline is not in the background (discussed later), the shell
     waits for all commands to complete.

     The commands in a pipeline can either be simple commands, or one of the
     compound commands described below.  The simplest case of a pipeline is a
     single simple command.

     If the pipefail option was set when a pipeline was started, the pipeline
     status is the status of the last (lexically last, i.e.: rightmost)
     command in the pipeline to exit with non-zero exit status, or zero, if,
     and only if, all commands in the pipeline exited with a status of zero.
     If the pipefail option was not set, which is the default state, the
     pipeline status is the exit status of the last (rightmost) command in the
     pipeline, and the exit status of any other commands in the pipeline is
     ignored.

     If the reserved word "!" precedes the pipeline, the exit status becomes
     the logical NOT of the pipeline status as determined above.  That is, if
     the pipeline status is zero, the exit status is 1; if the pipeline status
     is other than zero, the exit status is zero.  If there is no "!" reserved
     word, the pipeline status becomes the exit status.

     Because pipeline assignment of standard input or standard output or both
     takes place before redirection, it can be modified by redirection.  For
     example:

           $ command1 2>&1 | command2

     sends both the standard output and standard error of command1 to the
     standard input of command2.

     Note that unlike some other shells, each process in the pipeline is a
     child of the invoking shell (unless it is a shell built-in, in which case
     it executes in the current shell -- but any effect it has on the
     environment is wiped).

     A pipeline is a simple case of an AND-OR-list (described below.)  A ; or
     <newline> terminator causes the preceding pipeline, or more generally,
     the preceding AND-OR-list to be executed sequentially; that is, the shell
     executes the commands, and waits for them to finish before proceeding to
     following commands.  An & terminator causes asynchronous (background)
     execution of the preceding AND-OR-list (see the next paragraph below).
     The exit status of an asynchronous AND-OR-list is zero.  The actual
     status of the commands, after they have completed, can be obtained using
     the wait built-in command described later.

   Background Commands -- &
     If a command, pipeline, or AND-OR-list is terminated by the control
     operator ampersand (&), the shell executes the command asynchronously --
     that is, the shell does not wait for the command to finish before
     executing the next command.

     The format for running a command in background is:

           command1 & [command2 & ...]

     If the shell is not interactive, the standard input of an asynchronous
     command is set to /dev/null.  The process identifier of the most recent
     command started in the background can be obtained from the value of the
     special parameter "!" (see Special Parameters) provided it is accessed
     before the next asynchronous command is started.

   Lists -- Generally Speaking
     A list is a sequence of one or more commands separated by newlines,
     semicolons, or ampersands, and optionally terminated by one of these
     three characters.  A shell program, which includes the commands given to
     an interactive shell, is a list.  Each command in such a list is executed
     when it is fully parsed.  Another use of a list is as a complete-command,
     which is parsed in its entirety, and then later the commands in the list
     are executed only if there were no parsing errors.

     The commands in a list are executed in the order they are written.  If
     command is followed by an ampersand, the shell starts the command and
     immediately proceeds to the next command; otherwise it waits for the
     command to terminate before proceeding to the next one.  A newline is
     equivalent to a `;' when no other operator is present, and the command
     being input could syntactically correctly be terminated at the point
     where the newline is encountered, otherwise it is just whitespace.

   AND-OR Lists (Short-Circuit List Operators)
     "&&" and "||" are AND-OR list operators.  After executing the commands
     that precede the "&&" the subsequent command is executed if and only if
     the exit status of the preceding command(s) is zero.  "||" is similar,
     but executes the subsequent command if and only if the exit status of the
     preceding command is nonzero.  If a command is not executed, the exit
     status remains unchanged and the following AND-OR list operator (if any)
     uses that status.  "&&" and "||" both have the same priority.  Note that
     these operators are left-associative, so
           true || echo bar && echo baz
     writes "baz" and nothing else.  This is not the way it works in C.

   Flow-Control Constructs -- if, while, until, for, case
     These commands are instances of compound commands.  The syntax of the if
     command is

           if list
           then list
           [elif list
           then list] ...
           [else list]
           fi

     The first list is executed, and if the exit status of that list is zero,
     the list following the then is executed.  Otherwise the list after an
     elif (if any) is executed and the process repeats.  When no more elif
     reserved words, and accompanying lists, appear, the list after the else
     reserved word, if any, is executed.

     The syntax of the while command is

           while list
           do list
           done

     The two lists are executed repeatedly while the exit status of the first
     list is zero.  The until command is similar, but has the word until in
     place of while, which causes it to repeat until the exit status of the
     first list is zero.

     The syntax of the for command is

           for variable [in word ...]
           do list
           done

     The words are expanded, or "$@" if in (and the following words) is not
     present, and then the list is executed repeatedly with the variable set
     to each word in turn.  If in appears after the variable, but no words are
     present, the list is not executed, and the exit status is zero.  do and
     done may be replaced with `{' and `}', but doing so is non-standard and
     not recommended.

     The syntax of the break and continue commands is

           break [num]
           continue [num]

     break terminates the num innermost for, while, or until loops.  continue
     breaks execution of the num-1 innermost for, while, or until loops, and
     then continues with the next iteration of the enclosing loop.  These are
     implemented as special built-in commands.  The parameter num, if given,
     must be an unsigned positive integer (greater than zero).  If not given,
     1 is used.

     The syntax of the case command is

           case word in
           [(] pattern) [list] ;&
           [(] pattern) [list] ;;
           ...
           esac

     The pattern can actually be one or more patterns (see Shell Patterns
     described later), separated by "|" characters.

     word is expanded and matched against each pattern in turn, from first to
     last, with each pattern being expanded just before the match is
     attempted.  When a match is found, pattern comparisons cease, and the
     associated list, if given, is evaluated.  If the list is terminated with
     ";&" execution then falls through to the following list, if any, without
     evaluating its pattern, or attempting a match.  When a list terminated
     with ";;" has been executed, or when esac is reached, execution of the
     case statement is complete.  The exit status is that of the last command
     executed from the last list evaluated, if any, or zero otherwise.

   Grouping Commands Together
     Commands may be grouped by writing either
           (list)
     or
           { list; }
     These also form compound commands.

     Note that while parentheses are operators, and do not require any extra
     syntax, braces are reserved words, so the opening brace must be followed
     by white space (or some other operator), and the closing brace must occur
     in a position where a new command word might otherwise appear.

     The first of these executes the commands in a sub-shell.  Built-in
     commands grouped into a (list) will not affect the current shell.  The
     second form does not fork another shell so is slightly more efficient,
     and allows for commands which do affect the current shell.  Grouping
     commands together this way allows you to redirect their output as though
     they were one program:

           { echo -n "hello " ; echo "world" ; } > greeting

     Note that "}" must follow a control operator (here, ";") so that it is
     recognized as a reserved word and not as another command argument.

   Functions
     The syntax of a function definition is

           name() command [redirect ...]

     A function definition is an executable statement; when executed it
     installs a function named name and returns an exit status of zero.  The
     command is normally a list enclosed between "{" and "}".  The standard
     syntax also allows the command to be any of the other compound commands,
     including a sub-shell, all of which are supported.  As an extension, this
     shell also allows a simple command (or even another function definition)
     to be used, though users should be aware this is non-standard syntax.
     This means that
           l() ls "$@"
     works to make "l" an alternative name for the ls command.

     If the optional redirect, (see Redirections), which may be of any of the
     normal forms, is given, it is applied each time the function is called.
     This means that a simple "Hello World" function might be written (in the
     extended syntax) as:

           hello() cat <<EOF
           Hello World!
           EOF

     To be correctly standards conforming this should be re-written as:

           hello() { cat; } <<EOF
           Hello World!
           EOF

     Note the distinction between those forms, and

           hello() { cat <<EOF
           Hello World!
           EOF
           }

     which reads and processes the here-document each time the shell executes
     the function, and which applies that input only to the cat command, not
     to any other commands that might appear in the function.

     Variables may be declared to be local to a function by using the local
     command.  This should usually appear as the first statement of a
     function, though local is an executable command which can be used
     anywhere in a function.  See Built-ins below for its definition.

     The function completes after having executed command with exit status set
     to the status returned by command.  If command is a compound-command it
     can use the return command (see Built-ins below) to finish before
     completing all of command.

   Variables and Parameters
     The shell maintains a set of parameters.  A parameter denoted by a name
     is called a variable.  When starting up, the shell turns all the
     environment variables into shell variables, and exports them.  New
     variables can be set using the form

           name=value

     Variables set by the user must have a name consisting solely of
     alphabetics, numerics, and underscores -- the first of which must not be
     numeric.  A parameter can also be denoted by a number or a special
     character as explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a number (n > 0).  The
     shell sets these initially to the values of its command line arguments
     that follow the name of the shell script.  The set built-in can also be
     used to set or reset them, and shift can be used to manipulate the list.

     To refer to the 10th (and later) positional parameters, the form ${n}
     must be used.  Without the braces, a digit following "$" can only refer
     to one of the first 9 positional parameters, or the special parameter 0.
     The word "$10" is treated identically to "${1}0".

   Special Parameters
     A special parameter is a parameter denoted by one of the following
     special characters.  The value of the parameter is listed next to its
     character.

     *            Expands to the positional parameters, starting from one.
                  When the expansion occurs within a double-quoted string it
                  expands to a single field with the value of each parameter
                  separated by the first character of the IFS variable, or by
                  a <space> if IFS is unset.

     @            Expands to the positional parameters, starting from one.
                  When the expansion occurs within double quotes, each
                  positional parameter expands as a separate argument.  If
                  there are no positional parameters, the expansion of @
                  generates zero arguments, even when $@ is double-quoted.
                  What this basically means, for example, is if $1 is "abc"
                  and $2 is "def ghi", then "$@" expands to the two arguments:

                        "abc" "def ghi"

     #            Expands to the number of positional parameters.

     ?            Expands to the exit status of the most recent pipeline.

     - (hyphen, or minus)
                  Expands to the current option flags (the single-letter
                  option names concatenated into a string) as specified on
                  invocation, by the set built-in command, or implicitly by
                  the shell.

     $            Expands to the process ID of the invoked shell.  A sub-shell
                  retains the same value of $ as its parent.

     !            Expands to the process ID of the most recent background
                  command executed from the current shell.  For a pipeline,
                  the process ID is that of the last command in the pipeline.
                  If no background commands have yet been started by the
                  shell, then "!" will be unset.  Once set, the value of "!"
                  will be retained until another background command is
                  started.

     0 (zero)     Expands to the name of the shell or shell script.

   Word Expansions
     This section describes the various expansions that are performed on
     words.  Not all expansions are performed on every word, as explained
     later.

     Tilde expansions, parameter expansions, command substitutions, arithmetic
     expansions, and quote removals that occur within a single word expand to
     a single field.  It is only field splitting or pathname expansion that
     can create multiple fields from a single word.  The single exception to
     this rule is the expansion of the special parameter @ within double
     quotes, as was described above.

     The order of word expansion is:

     1.   Tilde Expansion, Parameter Expansion, Command Substitution,
          Arithmetic Expansion (these all occur at the same time).

     2.   Field Splitting is performed on fields generated by step (1) unless
          the IFS variable is null.

     3.   Pathname Expansion (unless set -f is in effect).

     4.   Quote Removal.

     The $ character is used to introduce parameter expansion, command
     substitution, or arithmetic evaluation.

   Tilde Expansion (substituting a user's home directory)
     A word beginning with an unquoted tilde character (~) is subjected to
     tilde expansion.  Provided all of the subsequent characters in the word
     are unquoted up to an unquoted slash (/) or when in an assignment or not
     in posix mode, an unquoted colon (:), or if neither of those appear, the
     end of the word, they are treated as a user name and are replaced with
     the pathname of the named user's home directory.  If the user name is
     missing (as in ~/foobar), the tilde is replaced with the value of the
     HOME variable (the current user's home directory).

     In variable assignments, an unquoted tilde immediately after the
     assignment operator (=), and each unquoted tilde immediately after an
     unquoted colon in the value to be assigned is also subject to tilde
     expansion as just stated.

   Parameter Expansion
     The format for parameter expansion is as follows:

           ${expression}

     where expression consists of all characters until the matching `}'.  Any
     `}' escaped by a backslash or within a quoted string, and characters in
     embedded arithmetic expansions, command substitutions, and variable
     expansions, are not examined in determining the matching `}'.

     The simplest form for parameter expansion is:

           ${parameter}

     The value, if any, of parameter is substituted.

     The parameter name or symbol can be enclosed in braces, which are
     optional in this simple case, except for positional parameters with more
     than one digit or when parameter is followed by a character that could be
     interpreted as part of the name.  If a parameter expansion occurs inside
     double quotes:

     1.   pathname expansion is not performed on the results of the expansion;

     2.   field splitting is not performed on the results of the expansion,
          with the exception of the special rules for @.

     In addition, a parameter expansion where braces are used, can be modified
     by using one of the following formats.  If the `:' is omitted in the
     following modifiers, then the test in the expansion applies only to unset
     parameters, not null ones.

     ${parameter:-word}    Use Default Values. If parameter is unset or null,
                           the expansion of word is substituted; otherwise,
                           the value of parameter is substituted.

     ${parameter:=word}    Assign Default Values. If parameter is unset or
                           null, the expansion of word is assigned to
                           parameter.  In all cases, the final value of
                           parameter is substituted.  Only variables, not
                           positional parameters or special parameters, can be
                           assigned in this way.

     ${parameter:?[word]}  Indicate Error if Null or Unset. If parameter is
                           unset or null, the expansion of word (or a message
                           indicating it is unset if word is omitted) is
                           written to standard error and a non-interactive
                           shell exits with a nonzero exit status.  An
                           interactive shell will not exit, but any associated
                           command(s) will not be executed.  If the parameter
                           is set, its value is substituted.

     ${parameter:+word}    Use Alternative Value. If parameter is unset or
                           null, null is substituted; otherwise, the expansion
                           of word is substituted.  The value of parameter is
                           not used in this expansion.

     ${#parameter}         String Length. The length in characters of the
                           value of parameter.

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell
     Patterns), rather than regular expression notation, is used to evaluate
     the patterns.  If parameter is * or @, the result of the expansion is
     unspecified.  Enclosing the full parameter expansion string in double
     quotes does not cause the following four varieties of pattern characters
     to be quoted, whereas quoting characters within the braces has this
     effect.

     ${parameter%word}     Remove Smallest Suffix Pattern. The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in parameter, with the
                           smallest portion of the suffix matched by the
                           pattern deleted.  If the word is to start with a
                           `%' character, it must be quoted.

     ${parameter%%word}    Remove Largest Suffix Pattern. The word is expanded
                           to produce a pattern.  The parameter expansion then
                           results in parameter, with the largest portion of
                           the suffix matched by the pattern deleted.  The
                           "%%" pattern operator only produces different
                           results from the "%" operator when the pattern
                           contains at least one unquoted `*'.

     ${parameter#word}     Remove Smallest Prefix Pattern. The word is
                           expanded to produce a pattern.  The parameter
                           expansion then results in parameter, with the
                           smallest portion of the prefix matched by the
                           pattern deleted.  If the word is to start with a
                           `#' character, it must be quoted.

     ${parameter##word}    Remove Largest Prefix Pattern. The word is expanded
                           to produce a pattern.  The parameter expansion then
                           results in parameter, with the largest portion of
                           the prefix matched by the pattern deleted.  This
                           has the same relationship with the "#" pattern
                           operator as "%%" has with "%".

   Command Substitution
     Command substitution allows the output of a command to be substituted in
     place of the command (and surrounding syntax).  Command substitution
     occurs when a word contains a command list enclosed as follows:

           $(list)

     or the older ("backquoted") version, which is best avoided:

           `list`

     See the section Complex Commands above for the definition of list.

     The shell expands the command substitution by executing the list in a
     sub-shell environment and replacing the command substitution with the
     standard output of the list after removing any sequence of one or more
     <newline>s from the end of the substitution.  (Embedded <newline>s before
     the end of the output are not removed; however, during field splitting,
     they may be used to separate fields (as spaces usually are) depending on
     the value of IFS and any quoting that is in effect.)

     Note that if a command substitution includes commands to be run in the
     background, the sub-shell running those commands will only wait for them
     to complete if an appropriate wait command is included in the command
     list.  However, the shell in which the result of the command substitution
     will be used will wait for both the sub-shell to exit and for the file
     descriptor that was initially standard output for the command
     substitution sub-shell to be closed.  In some circumstances this might
     not happen until all processes started by the command substitution have
     finished.

   Arithmetic Expansion
     Arithmetic expansion provides a mechanism for evaluating an arithmetic
     expression and substituting its value.  The format for arithmetic
     expansion is as follows:

           $((expression))

     The expression in an arithmetic expansion is treated as if it were in
     double quotes, except that a double quote character inside the expression
     is just a normal character (it quotes nothing.)  The shell expands all
     tokens in the expression for parameter expansion, command substitution,
     and quote removal (the only quoting character is the backslash `\', and
     only when followed by another `\', a dollar sign `$', a backquote ``' or
     a newline.)

     Next, the shell evaluates the expanded result as an arithmetic expression
     and substitutes the calculated value of that expression.

     Arithmetic expressions use a syntax similar to that of the C language,
     and are evaluated using the `intmax_t' data type (this is an extension to
     POSIX, which requires only `long' arithmetic.)  Shell variables may be
     referenced by name inside an arithmetic expression, without needing a "$"
     sign.  Variables that are not set, or which have an empty (null string)
     value, used this way evaluate as zero (that is, "x" in arithmetic, as an
     R-Value, is evaluated as "${x:-0}") unless the sh -u flag is set, in
     which case a reference to an unset variable is an error.  Note that unset
     variables used in the ${var} form expand to a null string, which might
     result in syntax errors.  Referencing the value of a variable which is
     not numeric is an error.

     All of the C expression operators applicable to integers are supported,
     and operate as they would in a C expression.  Use white space, or
     parentheses, to disambiguate confusing syntax, otherwise, as in C, the
     longest sequence of consecutive characters which make a valid token
     (operator, variable name, or number) is taken to be that token, even if
     the token designated cannot be used and a different interpretation could
     produce a successful parse.  This means, as an example, that "a+++++b" is
     parsed as the gibberish sequence "a ++ ++ + b", rather than as the valid
     alternative "a ++ + ++ b".  Similarly, separate the `,' operator from
     numbers with white space to avoid the possibility of confusion with the
     decimal indicator in some locales (though fractional, or floating-point,
     numbers are not supported in this implementation.)

     It should not be necessary to state that the C operators which operate
     on, or produce, pointer types, are not supported.  Those include unary
     "*" and "&" and the struct and array referencing binary operators: ".",
     "->" and "[".

   White Space Splitting (Field Splitting)
     After parameter expansion, command substitution, and arithmetic expansion
     the shell scans the results of expansions and substitutions that did not
     occur in double quotes, and "$@" even if it did, for field splitting and
     multiple fields can result.

     The shell treats each character of the IFS as a delimiter and uses the
     delimiters to split the results of parameter expansion and command
     substitution into fields.

     Non-whitespace characters in IFS are treated strictly as parameter
     separators.  So adjacent non-whitespace IFS characters will produce empty
     parameters.  On the other hand, any sequence of whitespace characters
     that occur in IFS (known as IFS whitespace) can occur, leading and
     trailing IFS whitespace, and any IFS whitespace surrounding a non
     whitespace IFS delimiter, is removed.  Any sequence of IFS whitespace
     characters without a non-whitespace IFS delimiter acts as a single field
     separator.

     If IFS is unset it is assumed to contain space, tab, and newline, all of
     which are IFS whitespace characters.  If IFS is set to a null string,
     there are no delimiters, and no field splitting occurs.

   Pathname Expansion (File Name Generation)
     Unless the -f flag is set, file name generation is performed after word
     splitting is complete.  Each word is viewed as a series of patterns,
     separated by slashes.  The process of expansion replaces the word with
     the names of all existing files whose names can be formed by replacing
     each pattern with a string that matches the specified pattern.  There are
     two restrictions on this: first, a pattern cannot match a string
     containing a slash, and second, a pattern cannot match a string starting
     with a period unless the first character of the pattern is a period.  The
     next section describes the patterns used for both Pathname Expansion and
     the case command.

   Shell Patterns
     A pattern consists of normal characters, which match themselves, and
     meta-characters.  The meta-characters are "!", "*", "?", and "[".  These
     characters lose their special meanings if they are quoted.  When command
     or variable substitution is performed and the dollar sign or backquotes
     are not double-quoted, the value of the variable or the output of the
     command is scanned for these characters and they are turned into meta-
     characters.

     An asterisk ("*") matches any string of characters.  A question mark
     ("?") matches any single character.  A left bracket ("[") introduces a
     character class.  The end of the character class is indicated by a right
     bracket ("]"); if this "]" is missing then the "[" matches a "[" rather
     than introducing a character class.  A character class matches any of the
     characters between the square brackets.  A named class of characters (see
     wctype(3)) may be specified by surrounding the name with ("[:") and
     (":]").  For example, ("[[:alpha:]]") is a shell pattern that matches a
     single letter.  A range of characters may be specified using a minus sign
     ("-").  The character class may be complemented by making an exclamation
     mark ("!") the first character of the character class.

     To include a "]" in a character class, make it the first character listed
     (after the "!", if any).  To include a "-", make it the first (after !)
     or last character listed.  If both "]" and "-" are to be included, the
     "]" must be first (after !) and the "-" last, in the character class.

   Built-ins
     This section lists the built-in commands which are built-in because they
     need to perform some operation that can't be performed by a separate
     process.  Or just because they traditionally are.  In addition to these,
     there are several other commands that may be built in for efficiency
     (e.g.  printf(1), echo(1), test(1), etc).

     : [arg ...]
            A null command that returns a 0 (true) exit value.  Any arguments
            or redirects are evaluated, then ignored.

     . file
            The dot command reads and executes the commands from the specified
            file in the current shell environment.  The file does not need to
            be executable and is looked up from the directories listed in the
            PATH variable if its name does not contain a directory separator
            (`/').  The return command (see below) can be used for a premature
            return from the sourced file.

            The POSIX standard has been unclear on how loop control keywords
            (break and continue) behave across a dot command boundary.  This
            implementation allows them to control loops surrounding the dot
            command, but obviously such behavior should not be relied on.  It
            is now permitted by the standard, but not required.

     alias [name[=string ...]]
            If name=string is specified, the shell defines the alias name with
            value string.  If just name is specified, the value of the alias
            name is printed.  With no arguments, the alias built-in prints the
            names and values of all defined aliases (see unalias).

     bg [job ...]
            Continue the specified jobs (or the current job if no jobs are
            given) in the background.

     command [-pVv] command [arg ...]
            Execute the specified command but ignore shell functions when
            searching for it.  (This is useful when you have a shell function
            with the same name as a command.)

            -p     search for command using a PATH that guarantees to find all
                   the standard utilities.

            -V     Do not execute the command but search for the command and
                   print the resolution of the command search.  This is the
                   same as the type built-in.

            -v     Do not execute the command but search for the command and
                   print the absolute pathname of utilities, the name for
                   built-ins or the expansion of aliases.

     cd [-P] [directory [replace]]
            Switch to the specified directory (default $HOME).  If replace is
            specified, then the new directory name is generated by replacing
            the first occurrence of the string directory in the current
            directory name with replace.  Otherwise if directory is `-', then
            the current working directory is changed to the previous current
            working directory as set in OLDPWD.  Otherwise if an entry for
            CDPATH appears in the environment of the cd command or the shell
            variable CDPATH is set and the directory name does not begin with
            a slash, and its first (or only) component isn't dot or dot dot,
            then the directories listed in CDPATH will be searched for the
            specified directory.  The format of CDPATH is the same as that of
            PATH.

            The -P option instructs the shell to update PWD with the specified
            physical directory path and change to that directory.  This is the
            default.

            When the directory changes, the variable OLDPWD is set to the
            working directory before the change.

            Some shells also support a -L option, which instructs the shell to
            update PWD with the logical path and to change the current
            directory accordingly.  This is not supported.

            In an interactive shell, the cd command will print out the name of
            the directory that it actually switched to if this is different
            from the name that the user gave, or always if the cdprint option
            is set.  The destination may be different either because the
            CDPATH mechanism was used or if the replace argument was used.

     eval string ...
            Concatenate all the arguments with spaces.  Then re-parse and
            execute the command.

     exec [command [arg ...]]
            Unless command is omitted, the shell process is replaced with the
            specified program (which must be a real program, not a shell
            built-in or function).  Any redirections on the exec command are
            marked as permanent, so that they are not undone when the exec
            command finishes.  When the posix option is not set, file
            descriptors created via such redirections are marked close-on-exec
            (see open(2) O_CLOEXEC or fcntl(2) F_SETFD / FD_CLOEXEC), unless
            the descriptors refer to the standard input, output, or error
            (file descriptors 0, 1, 2).  Traditionally Bourne-like shells
            (except ksh(1)), made those file descriptors available to exec'ed
            processes.  To be assured the close-on-exec setting is off,
            redirect the descriptor to (or from) itself, either when invoking
            a command for which the descriptor is wanted open, or by using
            exec (perhaps the same exec as opened it, after the open) to leave
            the descriptor open in the shell and pass it to all commands
            invoked subsequently.  Alternatively, see the fdflags command
            below, which can set, or clear, this, and other, file descriptor
            flags.

     exit [exitstatus]
            Terminate the shell process.  If exitstatus is given it is used as
            the exit status of the shell; otherwise the exit status of the
            preceding command (the current value of $?) is used.

     export [-nx] name[=value] ...

     export [-x] [-p [name ...]]

     export -q [-x] name ...
            With no options, but one or more names, the specified names are
            exported so that they will appear in the environment of subsequent
            commands.  With -n the specified names are un-exported.  Variables
            can also be un-exported using the unset built in command.  With -x
            (exclude) the specified names are marked not to be exported, and
            any that had been exported, will be un-exported.  Later attempts
            to export the variable will be refused.  Note this does not
            prevent explicitly exporting a variable to a single command,
            script or function by preceding that command invocation by a
            variable assignment to that variable, provided the variable is not
            also read-only.  That is

                  export -x FOO # FOO will now not be exported by default
                  export FOO    # this command will fail (non-fatally)
                  FOO=some_value my_command

            still passes the value (FOO=some_value) to my_command through the
            environment.

            The shell allows the value of a variable to be set at the same
            time it is exported (or unexported, etc) by writing

                  export [-nx] name=value

            With no arguments the export command lists the names of all set
            exported variables, or if -x was given, all set variables marked
            not for export.  With the -p option specified, the output will be
            formatted suitably for non-interactive use, and unset variables
            are included.  When -p is given, variable names, but not values,
            may also be given, in which case output is limited to the
            variables named.

            With -q and a list of variable names, the export command will exit
            with status 0 if all the named variables have been marked for
            export, or 1 if any are not so marked.  If -x is also given, the
            test is instead for variables marked not to be exported.

            Other than with -q, the export built-in exits with status 0,
            unless an attempt is made to export a variable which has been
            marked as unavailable for export, in which cases it exits with
            status 1.  In all cases if an invalid option, or option
            combination, is given, or an invalid variable name is present,
            export will write a message to the standard error output, and exit
            with a non-zero status.  A non-interactive shell will terminate.

            Note that there is no restriction upon exporting, or un-exporting,
            read-only variables.  The no-export flag can be reset by unsetting
            the variable and creating it again - provided the variable is not
            also read-only.

     fc [-e editor] [first [last]]

     fc -l [-nr] [first [last]]

     fc -s [old=new] [first]
            The fc built-in lists, or edits and re-executes, commands
            previously entered to an interactive shell.

            -e editor
                   Use the editor named by editor to edit the commands.  The
                   editor string is a command name, subject to search via the
                   PATH variable.  The value in the FCEDIT variable is used as
                   a default when -e is not specified.  If FCEDIT is null or
                   unset, the value of the EDITOR variable is used.  If EDITOR
                   is null or unset, ed(1) is used as the editor.

            -l (ell)
                   List the commands rather than invoking an editor on them.
                   The commands are written in the sequence indicated by the
                   first and last operands, as affected by -r, with each
                   command preceded by the command number.

            -n     Suppress command numbers when listing with -l.

            -r     Reverse the order of the commands listed (with -l) or
                   edited (with neither -l nor -s).

            -s     Re-execute the command without invoking an editor.

            first

            last   Select the commands to list or edit.  The number of
                   previous commands that can be accessed are determined by
                   the value of the HISTSIZE variable.  The value of first or
                   last or both are one of the following:

                   [+]number
                          A positive number representing a command number;
                          command numbers can be displayed with the -l option.

                   -number
                          A negative decimal number representing the command
                          that was executed number of commands previously.
                          For example, -1 is the immediately previous command.

            string
                   A string indicating the most recently entered command that
                   begins with that string.  If the old=new operand is not
                   also specified with -s, the string form of the first
                   operand cannot contain an embedded equal sign.

            The following environment variables affect the execution of fc:

            FCEDIT    Name of the editor to use.

            HISTSIZE  The number of previous commands that are accessible.

     fg [job]
            Move the specified job or the current job to the foreground.  A
            foreground job can interact with the user via standard input, and
            receive signals from the terminal.

     fdflags [-v] [fd ...]

     fdflags [-v] -s flags fd [...]
            Get or set file descriptor flags.  The -v argument enables verbose
            printing, printing flags that are also off, and the flags of the
            file descriptor being set after setting.  The -s flag interprets
            the flags argument as a comma separated list of file descriptor
            flags, each preceded with a "+" or a "-" indicating to set or
            clear the respective flag.  Valid flags are: append, async, sync,
            nonblock, fsync, dsync, rsync, direct, nosigpipe, and cloexec.
            Unique abbreviations of these names, of at least 2 characters, may
            be used on input.  See fcntl(2) and open(2) for more information.

     getopts optstring var
            The POSIX getopts command, not to be confused with the Bell
            Labs-derived getopt(1).

            The first argument should be a series of letters, each of which
            may be optionally followed by a colon to indicate that the option
            requires an argument.  The variable specified is set to the parsed
            option.

            The getopts command deprecates the older getopt(1) utility due to
            its handling of arguments containing whitespace.

            The getopts built-in may be used to obtain options and their
            arguments from a list of parameters.  When invoked, getopts places
            the value of the next option from the option string in the list in
            the shell variable specified by var and its index in the shell
            variable OPTIND.  When the shell is invoked, OPTIND is initialized
            to 1.  For each option that requires an argument, the getopts
            built-in will place it in the shell variable OPTARG.  If an option
            is not allowed for in the optstring, then OPTARG will be unset.

            optstring is a string of recognized option letters (see
            getopt(3)).  If a letter is followed by a colon, the option is
            expected to have an argument which may or may not be separated
            from it by whitespace.  If an option character is not found where
            expected, getopts will set the variable var to a `?'; getopts will
            then unset OPTARG and write output to standard error.  By
            specifying a colon as the first character of optstring all errors
            will be ignored.

            A nonzero value is returned when the last option is reached.  If
            there are no remaining arguments, getopts will set var to the
            special option, "--", otherwise, it will set var to `?'.

            The following code fragment shows how one might process the
            arguments for a command that can take the options -a and -b, and
            the option -c, which requires an argument.

                  while getopts abc: f
                  do
                          case $f in
                          a | b)  flag=$f;;
                          c)      carg=$OPTARG;;
                          \?)     echo $USAGE; exit 1;;
                          esac
                  done
                  shift $((OPTIND - 1))

            This code will accept any of the following as equivalent:

                  cmd -acarg file file
                  cmd -a -c arg file file
                  cmd -carg -a file file
                  cmd -a -carg -- file file

     hash [-rv] [command ...]
            The shell maintains a hash table which remembers the locations of
            commands.  With no arguments whatsoever, the hash command prints
            out the contents of this table.  Entries which have not been
            looked at since the last cd command are marked with an asterisk;
            it is possible for these entries to be invalid.

            With arguments, the hash command removes the specified commands
            from the hash table (unless they are functions) and then locates
            them.  With the -v option, hash prints the locations of the
            commands as it finds them.  The -r option causes the hash command
            to delete all the entries in the hash table except for functions.

     inputrc file
            Read the file to set key bindings as defined by editrc(5).

     jobid [-g|-j|-p] [job]
            With no flags, print the process identifiers of the processes in
            the job.  If the job argument is omitted, the current job is used.
            Any of the ways to select a job may be used for job, including the
            `%' forms, or the process id of the job leader ("$!" if the job
            was created in the background.)

            If one of the flags is given, then instead of the list of process
            identifiers, the jobid command prints:

            -g     the process group, if one was created for this job, or
                   nothing otherwise (the job is in the same process group as
                   the shell.)

            -j     the job identifier (using "%n" notation, where n is a
                   number) is printed.

            -p     only the process id of the process group leader is printed.

            These flags are mutually exclusive.

            jobid exits with status 2 if there is an argument error, status 1,
            if with -g the job had no separate process group, or with -p there
            is no process group leader (should not happen), and otherwise
            exits with status 0.

     jobs [-l|-p] [job ...]
            Without job arguments, this command lists out all the background
            processes which are children of the current shell process.  With
            job arguments, the listed jobs are shown instead.  Without flags,
            the output contains the job identifier (see Job Control below), an
            indicator character if the job is the current or previous job, the
            current status of the job (running, suspended, or terminated
            successfully, unsuccessfully, or by a signal) and a (usually
            abbreviated) command string.

            With the -l flag the output is in a longer form, with the process
            identifiers of each process (run from the top level, as in a
            pipeline), and the status of each process, rather than the job
            status.

            With the -p flag, the output contains only the process identifier
            of the lead process.

            In an interactive shell, each job shown as completed in the output
            from the jobs command is implicitly waited for, and is removed
            from the jobs table, never to be seen again.  In an interactive
            shell, when a background job terminates, the jobs command (with
            that job as an argument) is implicitly run just before outputting
            the next PS1 command prompt, after the job terminated.  This
            indicates that the job finished, shows its status, and cleans up
            the job table entry for that job.  Non-interactive shells need to
            execute wait commands to clean up terminated background jobs.

     local [-INx] [variable | -] ...
            Define local variables for a function.  Local variables have their
            attributes, and values, as they were before the local declaration,
            restored when the function terminates.

            With the -N flag, variables made local, are unset initially inside
            the function.  Unless the -x flag is also given, such variables
            are also unexported.  The -I flag, which is the default in this
            shell, causes the initial value and exported attribute of local
            variables to be inherited from the variable with the same name in
            the surrounding scope, if there is one.  If there is not, the
            variable is initially unset, and not exported.  The -N and -I
            flags are mutually exclusive, if both are given, the last
            specified applies.  The read-only and unexportable attributes are
            always inherited, if a variable with the same name already exists.

            The -x flag (lower case) causes the local variable to be exported,
            while the function runs, unless it has the unexportable attribute.
            This can also be accomplished by using the export command, giving
            the same variable names, after the local command.

            Making an existing read-only variable local is possible, but
            pointless.  If an attempt is made to assign an initial value to
            such a variable, the local command fails, as does any later
            attempted assignment.  If the readonly command is applied to a
            variable that has been declared local, the variable cannot be
            (further) modified within the function, or any other functions it
            calls, however when the function returns, the previous status (and
            value) of the variable is returned.

            Values may be given to local variables on the local command line
            in a similar fashion as used for export and readonly.  These
            values are assigned immediately after the initialization described
            above.  Note that any variable references on the command line will
            have been expanded before local is executed, so expressions like

                  local -N X=${X}

            are well defined, first $X is expanded, and then the command run
            is

                  local -N X=old-value-of-X

            After arranging to preserve the old value and attributes, of X
            ("old-value-of X") local unsets X, unexports it, and then assigns
            the "old-value-of-X" to X.

            The shell uses dynamic scoping, so that if you make the variable x
            local to function f, which then calls function g, references to
            the variable x made inside g will refer to the variable x declared
            inside f, not to the global variable named x.

            Another way to view this, is as if the shell just has one flat,
            global, namespace, in which all variables exist.  The local
            command conceptually copies the variable(s) named to unnamed
            temporary variables, and when the function ends, copies them back
            again.  All references to the variables reference the same global
            variables, but while the function is active, after the local
            command has run, the values and attributes of the variables might
            be altered, and later, when the function completes, be restored.

            Note that the positional parameters 1, 2, ... (see Positional
            Parameters), and the special parameters #, * and @ (see Special
            Parameters), are always made local in all functions, and are reset
            inside the function to represent the options and arguments passed
            to the function.  Note that $0 however retains the value it had
            outside the function, as do all the other special parameters.

            The only special parameter that can optionally be made local is
            "-".  Making "-" local causes any shell options that are changed
            via the set command inside the function to be restored to their
            original values when the function returns.  If -X option is
            altered after "-" has been made local, then when the function
            returns, the previous destination for xtrace output (as of the
            time of the local command) will also be restored.  If any of the
            shell's magic variables (those which return a value which may vary
            without the variable being explicitly altered, e.g.: SECONDS or
            HOSTNAME) are made local in a function, they will lose their
            special properties when set within the function, including by the
            local command itself (if not to be set in the function, there is
            little point in making a variable local) but those properties will
            be restored when the function returns.

            It is an error to use local outside the scope of a function
            definition.  When used inside a function, it exits with status 0,
            unless an undefined option is used, or an attempt is made to
            assign a value to a read-only variable.

            Note that either -I or -N should always be used, or variables made
            local should always be given a value, or explicitly unset, as the
            default behavior (inheriting the earlier value, or starting unset
            after local) differs amongst shell implementations.  Using "local
            -" is an extension not implemented by most shells.

            See the section LINENO below for details of the effects of making
            the variable LINENO local.

     pwd [-LP]
            Print the current directory.  If -L is specified the cached value
            (initially set from PWD) is checked to see if it refers to the
            current directory; if it does the value is printed.  Otherwise the
            current directory name is found using getcwd(3).  The environment
            variable PWD is set to the printed value.

            The default is pwd -L, but note that the built-in cd command
            doesn't support the -L option and will cache (almost) the absolute
            path.  If cd is changed (as unlikely as that is), pwd may be
            changed to default to pwd -P.

            If the current directory is renamed and replaced by a symlink to
            the same directory, or the initial PWD value followed a symbolic
            link, then the cached value may not be the absolute path.

            The built-in command may differ from the program of the same name
            because the program will use PWD and the built-in uses a
            separately cached value.

     read [-p prompt] [-r] variable [...]
            The prompt is printed if the -p option is specified and the
            standard input is a terminal.  Then a line is read from the
            standard input.  The trailing newline is deleted from the line and
            the line is split as described in the field splitting section of
            the Word Expansions section above, and the pieces are assigned to
            the variables in order.  If there are more pieces than variables,
            the remaining pieces (along with the characters in IFS that
            separated them) are assigned to the last variable.  If there are
            more variables than pieces, the remaining variables are assigned
            the null string.  The read built-in will indicate success unless
            EOF is encountered on input, in which case failure is returned.

            By default, unless the -r option is specified, the backslash "\"
            acts as an escape character, causing the following character to be
            treated literally.  If a backslash is followed by a newline, the
            backslash and the newline will be deleted.

     readonly name[=value] ...

     readonly [-p [name ...]]

     readonly -q name ...
            With no options, the specified names are marked as read only, so
            that they cannot be subsequently modified or unset.  The shell
            allows the value of a variable to be set at the same time it is
            marked read only by writing

                  readonly name=value

            With no arguments the readonly command lists the names of all set
            read only variables.  With the -p option specified, the output
            will be formatted suitably for non-interactive use, and unset
            variables are included.  When the -p option is given, a list of
            variable names (without values) may also be specified, in which
            case output is limited to the named variables.

            With the -q option, the readonly command tests the read-only
            status of the variables listed and exits with status 0 if all
            named variables are read-only, or with status 1 if any are not
            read-only.

            Other than as specified for -q the readonly command normally exits
            with status 0.  In all cases, if an unknown option, or an invalid
            option combination, or an invalid variable name, is given; or a
            variable which was already read-only is attempted to be set; the
            exit status will not be zero, a diagnostic message will be written
            to the standard error output, and a non-interactive shell will
            terminate.

     return [n]
            Stop executing the current function or a dot command with return
            value of n or the value of the last executed command, if not
            specified.  For portability, n should be in the range from 0 to
            255.

            The POSIX standard says that the results of return outside a
            function or a dot command are unspecified.  This implementation
            treats such a return as a no-op with a return value of 0 (success,
            true).  Use the exit command instead, if you want to return from a
            script or exit your shell.

     set [{ -options | +options | -- }] arg ...
            The set command performs four different functions.

            With no arguments, it lists the values of all shell variables.

            With a single option of either "-o" or "+o" set outputs the
            current values of the options.  In the -o form, all options are
            listed, with their current values.  In the +o form, the shell
            outputs a string that can later be used as a command to reset all
            options to their current values.

            If options are given, it sets the specified option flags, or
            clears them as described in the Argument List Processing section.
            In addition to the options listed there, when the "option name"
            given to set -o is default all of the options are reset to the
            values they had immediately after sh initialization, before any
            startup scripts, or other input, had been processed.  While this
            may be of use to users or scripts, its primary purpose is for use
            in the output of "set +o", to avoid that command needing to list
            every available option.  There is no +o default.

            The fourth use of the set command is to set the values of the
            shell's positional parameters to the specified arguments.  To
            change the positional parameters without changing any options, use
            "--" as the first argument to set.  If no following arguments are
            present, the set command will clear all the positional parameters
            (equivalent to executing "shift $#".) Otherwise the following
            arguments become $1, $2, ..., and $# is set to the number of
            arguments present.

     setvar variable value
            Assigns value to variable.  (In general it is better to write
            variable=value rather than using setvar.  setvar is intended to be
            used in functions that assign values to variables whose names are
            passed as parameters.)

     shift [n]
            Shift the positional parameters n times.  If n is omitted, 1 is
            assumed.  Each shift sets the value of $1 to the previous value of
            $2, the value of $2 to the previous value of $3, and so on,
            decreasing the value of $# by one.  The shift count must be less
            than or equal to the number of positional parameters ( "$#")
            before the shift.

     specialvar variable ...
            For each variable name given, if the variable named is one which,
            in this sh, could be treated as a special variable, then cause
            that variable to be made special, undoing any effects of an
            earlier unset or assignment to the variable.  If all variables
            given are recognized special variables in this sh the specialvar
            command will exit with status 0, otherwise 1.  Invalid usage will
            result in an exit status of 2.

            Note that all variables capable of being special are created that
            way, this command is not required to cause that to happen.
            However should such a variable be imported from the environment,
            that will cause (for those special variables so designated) the
            special effects for that variable to be lost.  Consequently, as
            the contents of the environment cannot be controlled, any script
            which desires to make use of the properties of most of the special
            variables should use this command, naming the variables required,
            to ensure that their special properties are available.

     times  Prints two lines to standard output.  Each line contains two
            accumulated time values, expressed in minutes and seconds
            (including fractions of a second.)  The first value gives the user
            time consumed, the second the system time.

            The first output line gives the CPU and system times consumed by
            the shell itself.  The second line gives the accumulated times for
            children of this shell (and their descendants) which have exited,
            and then been successfully waited for by the relevant parent.  See
            times(3) for more information.

            times has no parameters, and exits with an exit status of 0 unless
            an attempt is made to give it an option.

     trap action signal ...

     trap -

     trap [-l]

     trap [-p] signal ...

     trap N signal ...

            Cause the shell to parse and execute action when any of the
            specified signals are received.  The signals are specified by
            signal number or as the name of the signal.  If signal is 0 or its
            equivalent, EXIT, the action is executed when the shell exits.
            The action may be a null (empty) string, which causes the
            specified signals to be ignored.  With action set to `-' the
            specified signals are set to their default actions.  If the first
            signal is specified in its numeric form, then action can be
            omitted to achieve the same effect.  This archaic, but still
            standard, form should not be relied upon, use the explicit `-'
            action.  If no signals are specified with an action of `-', all
            signals are reset.

            When the shell forks off a sub-shell, it resets trapped (but not
            ignored) signals to the default action.  On non-interactive
            shells, the trap command has no effect on signals that were
            ignored on entry to the shell.  On interactive shells, the trap
            command will catch or reset signals ignored on entry.

            Issuing trap with option -l will print a list of valid signal
            names.  trap without any arguments causes it to write a list of
            signals and their associated non-default actions to the standard
            output in a format that is suitable as an input to the shell that
            achieves the same trapping results.  With the -p flag, trap prints
            the same information for the signals specified, or if none are
            given, for all signals, including those where the action is the
            default.  These variants of the trap command may be executed in a
            sub-shell (such as in a command substitution), provided they
            appear as the sole, or first, command in that sub-shell, in which
            case the state of traps from the parent of that sub-shell is
            reported.

            Examples:

                  trap

            List trapped signals and their corresponding actions.

                  trap -l

            Print a list of valid signals.

                  trap '' INT QUIT tstp 30

            Ignore signals INT QUIT TSTP USR1.

                  trap date INT

            Run the "date" command (print the date) upon receiving signal INT.

                  trap HUP INT

            Run the "HUP" command, or function, upon receiving signal INT.

                  trap 1 2

            Reset the actions for signals 1 (HUP) and 2 (INT) to their
            defaults.

                  traps=$(trap -p)
                     # more commands ...
                  trap 'action' SIG
                     # more commands ...
                  eval "$traps"

            Save the trap status, execute commands, changing some traps, and
            then reset all traps to their values at the start of the sequence.
            The -p option is required in the first command here, or any
            signals that were previously untrapped (in their default states)
            and which were altered during the intermediate code, would not be
            reset by the final eval.

     type [name ...]
            Interpret each name as a command and print the resolution of the
            command search.  Possible resolutions are: shell keyword, alias,
            shell built-in, command, tracked alias and not found.  For aliases
            the alias expansion is printed; for commands and tracked aliases
            the complete pathname of the command is printed.

     ulimit [-H|-S] [-a | -btfdscmlrpnv [value]]
            Inquire about or set the hard or soft limits on processes or set
            new limits.  The choice between hard limit (which no process is
            allowed to violate, and which may not be raised once it has been
            lowered) and soft limit (which causes processes to be signaled but
            not necessarily killed, and which may be raised) is made with
            these flags:

            -H          set or inquire about hard limits

            -S          set or inquire about soft limits.

            If neither -H nor -S is specified, the soft limit is displayed or
            both limits are set.  If both are specified, the last one wins.

            The limit to be interrogated or set, then, is chosen by specifying
            any one of these flags:

            -a          show all the current limits

            -b          the socket buffer size of a process (bytes)

            -c          the largest core dump size that can be produced
                        (512-byte blocks)

            -d          the data segment size of a process (kilobytes)

            -f          the largest file that can be created (512-byte blocks)

            -l          how much memory a process can lock with mlock(2)
                        (kilobytes)

            -m          the total physical memory that can be in use by a
                        process (kilobytes)

            -n          the number of files a process can have open at once

            -p          the number of processes this user can have at one time

            -r          the number of threads this user can have at one time

            -s          the stack size of a process (kilobytes)

            -t          CPU time (seconds)

            -v          how large a process address space can be

            If none of these is specified, it is the limit on file size that
            is shown or set.  If value is specified, the limit is set to that
            number; otherwise the current limit is displayed.

            Limits of an arbitrary process can be displayed or set using the
            sysctl(8) utility.

     umask [-S] [mask]
            Set the value of umask (see umask(2)) to the specified octal
            value.  If the argument is omitted, the umask value is printed.
            With -S a symbolic form is used instead of an octal number.

     unalias [-a] [name]
            If name is specified, the shell removes that alias.  If -a is
            specified, all aliases are removed.

     unset [-efvx] name ...
            If -v is specified, the specified variables are unset and
            unexported.  Readonly variables cannot be unset.  If -f is
            specified, the specified functions are undefined.  If -e is given,
            the specified variables are unexported, but otherwise unchanged,
            alternatively, if -x is given, the exported status of the variable
            will be retained, even after it is unset.

            If no flags are provided -v is assumed.  If -f is given with one
            of the other flags, then the named variables will be unset, or
            unexported, and functions of the same names will be undefined.
            The -e and -x flags both imply -v.  If -e is given, the -x flag is
            ignored.

            The exit status is 0, unless an attempt was made to unset a
            readonly variable, in which case the exit status is 1.  It is not
            an error to unset (or undefine) a variable (or function) that is
            not currently set (or defined.)

     wait [-n] [-p var] [job ...]
            Wait for the specified jobs to complete and return the exit status
            of the last job in the parameter list, or 127 if that job is not a
            current child of the shell.

            If no job arguments are given, wait for all jobs to complete and
            then return an exit status of zero (including when there were no
            jobs, and so nothing exited.)

            With the -n option, wait instead for any one of the given jobs, or
            if none are given, any job, to complete, and return the exit
            status of that job.  If none of the given job arguments is a
            current child of the shell, or if no job arguments are given and
            the shell has no unwaited for children, then the exit status will
            be 127.

            The -p var option allows the process (or job) identifier of the
            job for which the exit status is returned to be obtained.  The
            variable named (which must not be readonly) will be unset
            initially, then if a job has exited and its status is being
            returned, set to the identifier from the arg list (if given) of
            that job, or the lead process identifier of the job to exit when
            used with -n and no job arguments.  Note that -p with neither -n
            nor job arguments is useless, as in that case no job status is
            returned, the variable named is simply unset.

            If the wait is interrupted by a signal, its exit status will be
            greater than 128, and var, if given, will remain unset.

            Once waited upon, by specific process number or job-id, or by a
            wait with no arguments, knowledge of the child is removed from the
            system, and it cannot be waited upon again.

            Note than when a list of jobs are given, more that one argument
            might refer to the same job.  In that case, if the final argument
            represents a job that is also given earlier in the list, it is not
            defined whether the status returned will be the exit status of the
            job, or 127 indicating that the child no longer existed when the
            wait command reached the later argument in the list.  In this sh
            the exit status will be that from the job.  sh waits for each job
            exactly once, regardless of how many times (or how many different
            ways) it is listed in the arguments to wait.  That is
                  wait 100 100 100
            is identical to
                  wait 100

   Job Control
     Each process (or set of processes) started by sh is created as a "job"
     and added to the jobs table.  When enabled by the -m option (aka -o
     monitor) when the job is created, sh places each job (if run from the top
     level shell) into a process group of its own, which allows control of the
     process(es), and its/their descendants, as a unit.  When the -m option is
     off, or when started from a sub-shell environment, jobs share the same
     process group as the parent shell.  The -m option is enabled by default
     in interactive shells with a terminal as standard input and standard
     error.

     Jobs with separate process groups may be stopped, and then later resumed
     in the foreground (with access to the terminal) or in the background
     (where attempting to read from the terminal will result in the job
     stopping.)  A list of current jobs can be obtained using the jobs built-
     in command.  Jobs are identified using either the process identifier of
     the lead process of the job (the value available in the special parameter
     "!" if the job is started in the background), or using percent notation.
     Each job is given a "job number" which is a small integer, starting from
     1, and can be referenced as "%n" where n is that number.  Note that this
     applies to jobs both with and without their own process groups.  Job
     numbers are shown in the output from the jobs command enclosed in
     brackets (`[' and `]').  Whenever the job table becomes empty, the
     numbers begin at one again.  In addition, there is the concept of a
     current, and a previous job, identified by "%+" (or "%%" or even just
     "%"), and a previous job, identified by "%-".  Whenever a background job
     is started, or a job is resumed in the background, it becomes the current
     job.  The job that was the current job (prepare for a big surprise here,
     drum roll..., wait for it...) becomes the previous job.  When the current
     job terminates, the previous job is promoted to be the current job.  In
     addition the form "%string" finds the job for which the command starts
     with string and the form "%?string" finds the job which contains the
     string in its command somewhere.  Both forms require the result to be
     unambiguous.  For this purpose the "command" is that shown in the output
     from the jobs command, not the original command line.

     The bg, fg, jobid, jobs, kill, and wait commands all accept job
     identifiers as arguments, in addition to process identifiers (larger
     integers).  See the Built-ins section above, and kill(1), for more
     details of those commands.  In addition, a job identifier (using one of
     the "% forms") issued as a command, without arguments, is interpreted as
     if it had been given as the argument to the fg command.

     To cause a foreground process to stop, enter the terminal's stop
     character (usually control-Z).  To cause a background process to stop,
     send it a STOP signal, using the kill command.  A useful function to
     define is

           stop() { kill -s STOP "${@:-%%}"; }

     The fg command resumes a stopped job, placing it in the foreground, and
     bg resumes a stopped job in the background.  The jobid command provides
     information about process identifiers, job identifiers, and the process
     group identifier, for a job.

     Whenever a sub-shell is created, the jobs table becomes invalid (the sub-
     shell has no children.)  However, to enable uses like

           PID=$(jobid -p %1)

     the table is only actually cleared in a sub-shell when needed to create
     the first job there (built-in commands run in the foreground do not
     create jobs.)  Note that in this environment, there is no useful current
     job ("%%" actually refers to the sub-shell itself, but is not accessible)
     but the job which is the current job in the parent can be accessed as
     "%-".

   Command Line Editing
     When sh is being used interactively from a terminal, the current command
     and the command history (see fc in the Built-ins section) can be edited
     using emacs-mode or vi-mode command-line editing.  The command `set -o
     emacs' (or -E option) enables emacs-mode editing.  The command `set -o
     vi' (or -V option) enables vi-mode editing and places the current shell
     process into vi insert mode.  (See the Argument List Processing section
     above.)

     The vi-mode uses commands similar to a subset of those described in the
     vi(1) man page.  With vi-mode enabled, sh can be switched between insert
     mode and command mode.  It's similar to vi: pressing the <ESC> key will
     throw you into vi command mode.  Pressing the <return> key while in
     command mode will pass the line to the shell.

     The emacs-mode uses commands similar to a subset available in the emacs
     editor.  With emacs-mode enabled, special keys can be used to modify the
     text in the buffer using the control key.

     sh uses the editline(3) library.  See editline(7) for a list of the
     possible command bindings, and the default settings in emacs and vi
     modes.  Also see editrc(5) for the commands that can be given to
     configure editline(7) in the file named by the EDITRC parameter, or a
     file used with the inputrc built-in command, or using editline(7)'s
     configuration command line.

     When command line editing is enabled, the editline(7) functions control
     printing of the PS1 and PS2 prompts when required.  As, in this mode, the
     command line editor needs to keep track of what characters are in what
     position on the command line, care needs to be taken when setting the
     prompts.  Normal printing characters are handled automatically, however
     mode setting sequences, which do not actually display on the terminal,
     need to be identified to editline(7).  This is done, when needed, by
     choosing a character that is not needed anywhere in the prompt, including
     in the mode setting sequences, any single character is acceptable, and
     assigning it to the shell parameter PSlit.  Then that character should be
     used, in pairs, in the prompt string.  Between each pair of PSlit
     characters are mode setting sequences, which affect the printing
     attributes of the following (normal) characters of the prompt, but do not
     themselves appear visibly, nor change the terminal's cursor position.

     Each such sequence, that is PSlit character, mode setting character
     sequence, and another PSlit character, must currently be followed by at
     least one following normal prompt character, or it will be ignored.  That
     is, a PSlit character cannot be the final character of PS1 or PS2, nor
     may two PSlit delimited sequences appear adjacent to each other.  Each
     sequence can contain as many mode altering sequences as are required
     however.  Only the first character from PSlit will be used.  When set
     PSlit should usually be set to a string containing just one character,
     then it can simply be embedded in PS1 (or PS2) as in

           PS1="${PSlit}mset${PSlit}XYZ${PSlit}mclr${PSlit}ABC"

     The prompt visible will be "XYZABC" with the "XYZ" part shown according
     as defined by the mode setting characters mset, and then cleared again by
     mclr.  See tput(1) for one method to generate appropriate mode sequences.
     Note that both parts, XYZ and ABC, must each contain at least one
     character.

     If PSlit is unset, which is its initial state, or set to a null string,
     no literal character will be defined, and all characters of the prompt
     strings will be assumed to be visible characters (which includes spaces
     etc.)  To allow smooth use of prompts, without needing redefinition, when
     editline(7) is disabled, the character chosen should be one which will be
     ignored by the terminal if received, as when editline(7) is not in use,
     the prompt strings are simply written to the terminal.  For example,
     setting:

       PSlit="$(printf '\1')"
       PS1="${PSlit}$(tput bold blink)${PSlit}\$${PSlit}$(tput sgr0)${PSlit} "

     will arrange for the primary prompt to be a bold blinking dollar sign, if
     supported by the current terminal, followed by an (ordinary) space, and,
     as the SOH (control-A) character (`\1') will not normally affect a
     terminal, this same prompt will usually work with editline(7) enabled or
     disabled.

ENVIRONMENT
     CDPATH     The search path used with the cd built-in.

     EDITRC     Gives the name of the file containing commands for
                editline(7).  See editrc(5) for possible content and format.
                The file is processed, when in interactive mode with command
                line editing enabled, whenever EDITRC is set (even with no
                actual value change,) and if command line editing changes from
                disabled to enabled, or the editor style used is changed.
                (See the -E and -V options of the set built-in command,
                described in Built-ins above, which are documented further
                above in Argument List Processing.) If unset "$HOME/.editrc"
                is used.

     ENV        Names the file sourced at startup by the shell.  Unused by
                this shell after initialization, but is usually passed through
                the environment to descendant shells.  See the Invocation
                section above for details of how ENV is processed and used.

     EUSER      Set to the login name of the effective user id running the
                shell, as returned by
                      getpwuid(geteuid())->pw_name
                (See getpwuid(3) and geteuid(2) for more details.) This is
                obtained each time EUSER is expanded, so changes to the
                shell's execution identity cause updates without further
                action.  If unset, it returns nothing.  If set it loses its
                special properties, and is simply a variable.  See the
                specialvar built-in command for remedial action.

     HISTSIZE   The number of lines in the history buffer for the shell.

     HOME       Set automatically by login(1) from the user's login directory
                in the password file (passwd(5)).  This environment variable
                also functions as the default argument for the cd built-in.

     HOSTNAME   Set to the current hostname of the system, as returned by
                gethostname(3).  This is obtained each time HOSTNAME is
                expanded, so changes to the system's name are reflected
                without further action.  If unset, it returns nothing.  If set
                it loses its special properties, and is simply a variable.
                See the specialvar built-in command for remedial action.

     IFS        Input Field Separators.  This is normally set to <space>,
                <tab>, and <newline>.  White Space Splitting section for more
                details.

     LANG       The string used to specify localization information that
                allows users to work with different culture-specific and
                language conventions.  See nls(7).

     LINENO     The current line number in the script or function.  See the
                section LINENO below for more details.

     MAIL       The name of a mail file, that will be checked for the arrival
                of new mail.  Overridden by MAILPATH.  The check occurs just
                before PS1 is written, immediately after reporting jobs which
                have changed status, in interactive shells only.  New mail is
                considered to have arrived if the monitored file has increased
                in size since the last check.

     MAILPATH   A colon ":" separated list of file names, for the shell to
                check for incoming mail.  This environment setting overrides
                the MAIL setting.  There is a maximum of 10 mailboxes that can
                be monitored at once.

     PATH       The default search path for executables.  See the Path Search
                section above.

     POSIXLY_CORRECT
                If set in the environment upon initialization of the shell,
                then the shell option posix will be set.  (See the description
                of the set command in the Built-ins section.) After
                initialization it is unused by the shell, but is usually
                passed through the environment to descendant processes,
                including other instances of the shell, which may interpret it
                in a similar way.

     PPID       The process identified of the parent process of the current
                shell.  This value is set at shell startup, ignoring any value
                in the environment, and then made readonly.

     PS1        The primary prompt string, which defaults to "$ ", unless you
                are the superuser, in which case it defaults to "# ".  This
                string is subject to parameter, arithmetic, and if enabled by
                setting the promptcmds option, command substitution before
                being output.  During execution of commands used by command
                substitution, execution tracing, the xtrace (set -x) option is
                temporarily disabled.  If promptcmds is not set and the prompt
                string uses command substitution, the prompt used will be an
                appropriate error string.  For other expansion errors, the
                prompt will become an empty string, without an error message.
                To verify parsing of PS1, the method suggested for ENV can be
                used.

     PS2        The secondary prompt string, which defaults to "> ".  After
                expansion (as for PS1) it is written whenever more input is
                required to complete the current command.

     PS4        is Output, after expansion like PS1, as a prefix for each line
                when execution trace (set -x) is enabled.  PS4 defaults to "+
                ".

     PSc        Initialized by the shell, ignoring any value from the
                environment, to a single character string, either `#' or `$',
                depending upon whether the current user is the superuser or
                not.  This is intended for use when building a custom PS1.

     PSlit      Defines the character which may be embedded in pairs, in PS1
                or PS2 to indicate to editline(7) that the characters between
                each pair of occurrences of the PSlit character will not
                appear in the visible prompt, and will not cause the
                terminal's cursor to change position, but rather set terminal
                attributes for the following prompt character(s) at least one
                of which must be present.  See Command Line Editing above for
                more information.

     RANDOM     Returns a different pseudo-random integer, in the range
                [0,32767] each time it is accessed.  RANDOM can be assigned an
                integer value to seed the PRNG.  If the value assigned is a
                constant, then the sequence of values produces on subsequent
                references of RANDOM will repeat after the next time the same
                constant is assigned.  Note, this is not guaranteed to remain
                constant from one version of the shell to another - the PRNG
                algorithm, or seeding method is subject to change.  If RANDOM
                is assigned an empty value (null string) then the next time
                RANDOM is accessed, it will be seeded from a more genuinely
                random source.  The sequence of pseudo-random numbers
                generated will not be able to be generated again (except by
                luck, whether good or bad, depends!)  This is also how the
                initial seed is generated, if none has been assigned before
                RANDOM is first accessed after shell initialization.  Should
                the error message "RANDOM initialisation failed" appear on
                standard error, it indicates that the source of good random
                numbers was not available, and RANDOM has instead been seeded
                with a more predictable value.  The following sequence of
                random numbers will not be as unpredictable as they otherwise
                would be.

     SECONDS    Returns the number of seconds since the current shell was
                started.  If unset, it remains unset, and returns nothing,
                unless set again.  If set, it loses its special properties,
                and becomes a normal variable.  See the specialvar built-in
                command for remedial action.

     START_TIME
                Initialized by the shell to the number of seconds since the
                Epoch (see localtime(3)) when the shell was started.  The
                value of
                      $((START_TIME + SECONDS))
                represents the current time, if START_TIME has not been
                modified, and SECONDS has not been set or unset.

     TERM       The default terminal setting for the shell.  This is inherited
                by children of the shell, and is used in the history editing
                modes.

     ToD        When referenced, uses the value of ToD_FORMAT (or "%T" if
                ToD_FORMAT is unset) as the format argument to strftime(3) to
                encode the current time of day, in the time zone defined by TZ
                if set, or current local time if not, and returns the result.
                If unset ToD returns nothing.  If set, it loses its special
                properties, and becomes a normal variable.  See the specialvar
                built-in command for remedial action.

     ToD_FORMAT
                Can be set to the strftime(3) format string to be used when
                expanding ToD.  Initially unset.

     TZ         If set, gives the time zone (see localtime(3), environ(7)) to
                use when formatting ToD and if exported, other utilities that
                deal with times.  If unset, the system's local wall clock time
                zone is used.

     NETBSD_SHELL
                Unlike the variables previously mentioned, this variable is
                somewhat strange, in that it cannot be set, inherited from the
                environment, modified, or exported from the shell.  If set, by
                the shell, it indicates that the shell is the sh defined by
                this manual page, and gives its version information.  It can
                also give information in additional space separated words,
                after the version string.  If the shell was built as part of a
                reproducible build, the relevant date that was used for that
                build will be included.  Finally, any non-standard compilation
                options, which may affect features available, that were used
                when building the shell will be listed.  NETBSD_SHELL behaves
                like any other variable that has the read-only and un-
                exportable attributes set.

   LINENO
     LINENO is in many respects a normal shell variable, containing an integer
     value. and can be expanded using any of the forms mentioned above which
     can be used for any other variable.

     LINENO can be exported, made readonly, or unset, as with any other
     variable, with similar effects.  Note that while being readonly prevents
     later attempts to set, or unset, LINENO, it does not prevent its value
     changing.  References to LINENO (when not unset) always obtain the
     current line number.  However, LINENO should normally not ever be set or
     unset.  In this shell setting LINENO reverses the effect of an earlier
     unset, but does not otherwise affect the value obtained.  If unset,
     LINENO should not normally be set again, doing so is not portable.  If
     LINENO is set or unset, different shells act differently.  The value of
     LINENO is never imported from the environment when the shell is started,
     though if present there, as with any other variable, LINENO will be
     exported by this shell.

     LINENO is set automatically by the shell to be the number of the source
     line on which it occurs.  When exported, LINENO is exported with its
     value set to the line number it would have had had it been referenced on
     the command line of the command to which it is exported.  Line numbers
     are counted from 1, which is the first line the shell reads from any
     particular file.  For this shell, standard input, including in an
     interactive shell, the user's terminal, is just another file and lines
     are counted there as well.  However note that not all shells count
     interactive lines this way, it is not wise to rely upon LINENO having a
     useful value, except in a script, or a function.

     The role of LINENO in functions is less clear.  In some shells, LINENO
     continues to refer to the line number in the script which defines the
     function, in others lines count from one within the function, always (and
     resume counting normally once the function definition is complete) and
     others count in functions from one if the function is defined
     interactively, but otherwise just reference the line number in the script
     in which the function is defined.  This shell gives the user the option
     to choose.  If the -L flag (the local_lineno option, see Argument List
     Processing) is set, when the function is defined, then the function
     defaults to counting lines with one being the first line of the function.
     When the -L flag is not set, the shell counts lines in a function
     definition in the same continuous sequence as the lines that surround the
     function definition.  Further, if LINENO is made local (see Built-ins
     above) inside the function, the function can decide which behavior it
     prefers.  If LINENO is made local and inherited, and not given a value,
     as in
           local -I LINENO
     then from that point in the function, LINENO will give the line number as
     if lines are counted in sequence with the lines that surround the
     function definition (and any other function definitions in which this is
     nested.)  If LINENO is made local, and in that same command, given a
     value, as
           local [-I|-N] LINENO=value
     then LINENO will give the line number as if lines are counted from one
     from the beginning of the function.  The value nominally assigned in this
     case is irrelevant, and ignored.  For completeness, if lineno is made
     local and unset, as in
           local -N LINENO
     then LINENO is simply unset inside the function, and gives no value at
     all.

     Now for some technical details.  The line on which LINENO occurs in a
     parameter expansion, is the line that contains the `$' that begins the
     expansion of LINENO.  In the case of nested expansions, that `$' is the
     one that actually has LINENO as its parameter.  In an arithmetic
     expansion, where no `$' is used to evaluate LINENO but LINENO is simply
     referenced as a variable, then the value is the line number of the line
     that contains the `L' of LINENO.  For functions line one of the function
     definition (when relevant) is the line that contains the first character
     of the function name in the definition.  When exported, the line number
     of the command is the line number where the first character of the word
     which becomes the command name occurs.

     When the shell opens a new file, for any reason, it counts lines from one
     in that file, and then resumes its original counting once it resumes
     reading the previous input stream.  When handling a string passed to eval
     the line number starts at the line on which the string starts, and then
     if the string contains internal newline characters, those characters
     increase the line number.  This means that references to LINENO in such a
     case can produce values larger than would be produced by a reference on
     the line after the eval.

FILES
     $HOME/.profile

     /etc/profile

EXIT STATUS
     Errors that are detected by the shell, such as a syntax error, will cause
     the shell to exit with a non-zero exit status.  If the shell is not an
     interactive shell, the execution of the shell file will be aborted.
     Otherwise the shell will return the exit status of the last command
     executed, or if the exit built-in is used with a numeric argument, it
     will return the argument.

SEE ALSO
     csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
     editline(3), getopt(3), editrc(5), passwd(5), editline(7), environ(7),
     nls(7), sysctl(8)

HISTORY
     A sh command appeared in Version 1 AT&T UNIX.  It was replaced in
     Version 7 AT&T UNIX with a version that introduced the basis of the
     current syntax.  That was, however, unmaintainable so we wrote this one.

BUGS
     Setuid shell scripts should be avoided at all costs, as they are a
     significant security risk.

     The characters generated by filename completion should probably be quoted
     to ensure that the filename is still valid after the input line has been
     processed.

     Job control of compound statements (loops, etc) is a complete mess.

     Many, many, more.  (But less than there were...)

NetBSD 8.99.34                  April 22, 2019                  NetBSD 8.99.34