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SH(1) General Commands Manual SH(1)
NAME
sh - command interpreter (shell)
SYNOPSIS
sh [-abCEeFfhIiLlmnpquVvXx] [+abCEeFfhIiLlmnpquVvXx] [-o option_name]
[+o option_name] [command_file [argument ...]]
sh -c [-s] [-abCEeFfhIiLlmnpquVvXx] [+abCEeFfhIiLlmnpquVvXx]
[-o option_name] [+o option_name] command_string
[command_name [argument ...]]
sh -s [-abCEeFfhIiLlmnpquVvXx] [+abCEeFfhIiLlmnpquVvXx] [-o option_name]
[+o option_name] [argument ...]
DESCRIPTION
sh is the standard command interpreter for the system. It is a re-
implementation and extension of the Bourne 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, if neither the -l
nor +l options were given on the command line, the shell inspects
argument 0, and if it begins with a dash `-', or if the -l option was
given, the shell is also considered a login shell. Beginning argument 0
with a dash 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
never 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, and are used with -o or +o only, either on the command line,
or with the set built-in command. Those are listed in the table below
after the options with a one letter, flag, equivalent.
Other options described are for the command line only. Specifying using
a dash (or minus) "-" turns the option on, while using a plus "+"
disables the option. This may seem counter-intuitive, but is in line
with the common practice where cmd -x runs cmd with the `x' option set.
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 had 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.
(Obsolete and 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. If
not set on the command line, this option is
set automatically at shell startup if the
shell is reading from standard input (no -c,
or command_file given at invocation of sh),
and standard input and standard error refer
to terminal type devices.
-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.
-l login When set on the command line, the shell will
be considered a login shell. When reset on
the command line (+l or +o login), the shell
will not be considered a login shell, even
if the command name parameter (argv[0])
begins with a dash (`-'). See Invocation
for the effects of this. Changing the value
of this option while the shell is running
has no effect.
-m monitor Turn on job control (set automatically at
shell startup, if not mentioned on the
command line, 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. The same applies to
effective and real GIDs. This is not set by
default to help avoid incorrect usage by
setuid root programs via system(3) or
popen(3). This option is effective only
when set on the command line, but can be
reset to drop privileges, once, at any time.
If -p is cleared, those privileges can never
be regained, however much the -p option is
manipulated.
-q quietprofile If the -v or -x options have been set,
temporarily disable them before 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, of
itself, 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 had been set).
(See the Command Line Editing section
below.)
-v verbose The shell writes its input to standard error
as it is read. Perhaps occasionally useful
for some 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.
The following options have no one letter variant, and are used only in
conjunction with -o or +o, either on the command line, or via the set
built-in command.
cdprint Make an interactive shell always print the
new directory name when changed by the cd
command. If the posix option is set, this
option also applies to non-interactive
shells. However, cdprint is an extension to
POSIX, so these two options should rarely be
set at the same time.
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, if completely unquoted, at the beginning of a line, after a
control operator, and where the syntax of a command specifically requires
a reserved word. 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, the alias word is replaced by its value in
the input stream, as if the value had been entered instead. 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, but
are not automatically added to the environment (are not
exported).
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 not explicitly present, n will be 0 (standard
input) or 1 (standard output) depending upon the redirection operator
used. 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 within 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, command names containing a slash (`/') are
simply executed without performing any searches.
If there is no slash in the name, the shell first looks to see if it is a
special built-in command, if not it looks to see if there is a shell
function by that name. If that fails it looks for an ordinary built-in
command. If a none of these searches located the command 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, except in the case where the pipeline is a
single simple command (no `|' characters appear.)
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. To be
portable, and standards compliant, the name must use the same syntax as a
variable name, (see Variables and Parameters below). As an extension,
this shell allows almost all characters in name (the exception is slash
(`/') as there is no way to invoke a function with a name containing a
slash). Including quoting, whitespace, and operator characters requires
that the word be quoted. The name is subject to quote removal, but no
other expansions. Because of implementation issues, unquoted dollar
signs (`$') and backquotes (``') are prohibited, but can be included in a
function name by use of quoting.
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 in a situation where field
splitting is never performed, such as 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 (possibly nothing if IFS has a null value), 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 the
expansion happens in other situations where field splitting
is not performed, whether double quoted or not, the results
are undefined. In most shells, including this one, $@ is
treated as $* in such a context, but this is not universally
true. 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.
- (dash, 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, and the
result of any of these expansions is not rescanned for further
instances of the expansion, or any of the others).
2. Unless the IFS variable has an empty value, Field Splitting is
performed on the text resulting from the expansions in step (1)
except for Tilde Expansion.
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.
Note that the special rules for @ can result in multiple fields
being produced, but this is not because of field-splitting. If
unquoted, each field produced by $@ is subject to field splitting.
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). Most built-in commands will
exit with status 2 if used incorrectly (bad options, excess or
insufficient number of arguments, etc). Otherwise, unless stated
differently, the built-in commands exit with status 0, unless some error
occurs, which would be reported to standard error.
: [arg ...]
A null command that returns a 0 (true) exit value. Any arguments
or redirects are evaluated just as for any other command, 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, but not necessarily any others.
-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 [-Pe] [directory [replace]]
Switch to the specified directory (default $HOME). If replace is
specified, then the new directory name is generated by replacing
the first occurrence of the string directory in the current
working directory name with replace. Otherwise if directory is
`-', then the current working directory is changed to the previous
current working directory as set in OLDPWD. Otherwise if an entry
for CDPATH appears in the environment of the cd command or the
shell variable CDPATH is set and the directory name does not begin
with a slash, and its first (or only) component isn't dot or dot
dot, then the directories listed in CDPATH will be searched for
the specified directory. The format of CDPATH is the same as that
of PATH.
The -P option (which is the unalterable default in this sh)
instructs the shell to change to the directory specified (or
determined) and if successful update PWD with the new physical
directory path. That is the path name, not traversing any
symbolic links, of the altered working directory of the shell.
The -e option alters the interpretation of the exit status. cd
will exit with status 0 if successful. If the directory was
successfully changed, but PWD was unable to be updated, cd will
exit with status 1 if the -e option was given, and status 0
otherwise. Upon any other error, including usage errors, and
failing to successfully change directory, cd will exit with status
2.
When the directory changes, and PWD is updated, the variable
OLDPWD is set to the working directory ($PWD) as it was before the
change.
Some shells also support a -L option, which instructs the shell to
update PWD with the logical path using string manipulation, and
then to change the current directory accordingly. This is not
supported.
In an interactive shell, or if the posix option is set, the cd
command will print out the name of the directory that it actually
switched to; (that is, the pathname passed to the successful
chdir(2) system call) if this is different from the name that the
user gave, or if the cdprint option is set. The destination may
be different because a non-empty element of the CDPATH mechanism
was used, or because the replace argument was used, or because the
directory parameter was specified as "-".
eval string ...
Concatenate all the string arguments with intervening spaces.
Then parse and execute the resulting command. The exit status
from eval is the exit status of the command executed, or 0 if
there was none.
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 $?) is used.
export [-nx] name[=value] ...
export [-x] [-p [name ...]]
export -q [-x] name ...
With no options, but one or more names, the specified names are
exported so that they will appear in the environment of subsequent
commands. With -n the specified names are un-exported. Variables
can also be un-exported using the unset built in command. With -x
(exclude) the specified names are marked not to be exported, and
any that had been exported, will be un-exported. Later attempts
to export the variable will be refused. Note this does not
prevent explicitly exporting a variable to a single command,
script or function by preceding that command invocation by a
variable assignment to that variable, provided the variable is not
also read-only. That is
export -x FOO # FOO will now not be able to be exported
export FOO # this command will fail (non-fatally)
But with
FOO=some_value my_command
sh still passes the value (FOO=some_value) to my_command through
the environment.
The shell allows the value of a variable to be set at the same
time it is exported (or unexported, etc) by writing
export [-nx] name=value
Note that in such a usage, the "name=value" argument often needs
to be quoted, more often than is required of an assignment
statement, as, like with any other command, the command name and
arguments are all subject to the various expansions, including
filename expansion and field splitting, before the export command
is invoked. With the default value for IFS:
X='a b c'
export Y=$X
the command invoked would be
export Y=a b c
exporting Y, with the value "a" and also exporting the variables
named "b" and "c", which is probably not as intended.
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 the option by whitespace. If an option character is not
found where expected, getopts will set the variable var to `?';
getopts will then unset OPTARG and write an error to standard
error.
By specifying a colon (:) as the first character of optstring, the
error handling behavior changes: no errors will be written to
standard error; unknown option characters will set var to `?' and
set OPTARG to the unknown option character (instead of unset
OPTARG); and missing option arguments will set var to `:' and set
OPTARG to the option character with the missing argument.
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 [-befqrsuv] [command ...]
The shell maintains a hash table which remembers the locations and
types of commands. With the -r option given, the hash command
begins by clearing all commands, except special built-in commands
and functions, from this table. Commands, other than functions,
are added to the table as described below, or as they are
encountered through normal execution, or for functions, when they
are defined. Functions are removed with the unset built-in
command. Special built-in commands are added at shell startup,
and never removed. Utilities can also be removed when PATH is
altered.
With no command arguments the hash command then prints out the
contents of this table. Note that this is a hash table, the order
of the contents is unpredictable, and meaningless.
The -b, -f, -s, and -u options control which entries are printed.
With -f functions are printed; with -b or -s regular, or special,
built-in commands are listed; and with -u normal utilities (those
commands found in the filesystem by searching PATH) are printed.
For compatibility with some older versions of the hash command, -c
is accepted as an alternative of -u.
Some normal command entries which have not been verified since the
last cd command are marked with an asterisk; it is possible for
these entries to be invalid.
The -v option causes more verbose output to be included,
indicating the type of the command, rather than simply its name.
For functions, the body of the function is included.
If none of the above options is given, the default is to show
normal commands only. With -v and no other options, the whole
table (all types) will be shown.
Unless there is an error writing the output, the hash command will
exit with status 0 in this usage.
With command arguments, the hash command removes the specified
commands from the hash table (unless they are functions or special
built-in commands) and then locates and reinstalls them. With the
-v option, hash prints the locations of the commands as it finds
them. The -bfsu options control which types of commands will be
affected. If any of those options is given, and a command found
to already be in the hash table is not one of the designated
types, that entry, and the command argument, will simply be
silently skipped. If none of those flags is given, any command
type can be affected.
If a command is not located, then unless -q was given, a "not
found" error message will be printed.
The -e option implies -q if that option was not given, and also
causes the exit status of the hash command to ignore the unfound
command. Otherwise if any command is not found, the hash command
will exit with status 1.
To allow a method to permit backwards compatibility with the way
that the hash command worked before NetBSD 10.0, if both the -e
and -q options are given, then an error message will be printed
about commands unable to be found, but the exit status will remain
0. This is not considered useful.
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 ...]
jobs -Z [title]
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 (which is also the process group identifier).
Note that this is not necessarily the same process identifier as
reported in the special parameter ! when a background job is
started.
With the -Z flag, the process command line is set using
setproctitle(3). If title is omitted or a null string,
setproctitle(3) is called with a NULL format.
These options are mutually exclusive, the last specified is used.
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'
See the description of the export built-in command for notes on
why quoting the value is required.
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 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 [-d delim] [-p prompt] [-r] variable [...]
The prompt is printed on standard error if the -p option is
specified and the standard input is a terminal. Then a record,
terminated by the first character of delim if the -d option was
given, or a newline character otherwise, is read from the standard
input. The ending delimiter is deleted from the record which is
then split as described in the field splitting section of the Word
Expansions section above. 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 all 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,
or a read error, 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, when
that character is the escape character, or end delimiter
character, to be treated literally when reading the record. This
is the only form of quoting that applies. If an unescaped
backslash is followed by a newline, the backslash and the newline
will be deleted, and replaced by the contents from the following
line, which is processed as if it had been part of the original
line. This includes reading yet more input if necessary, until a
line is read that contains or ends with an unescaped copy of the
delimiter character. If the end delimiter (when it is not a
newline) is escaped, it is treated as a normal character, and read
continues looking for an unescaped end delimiter character. No
other escape sequences are meaningful, the escape character is
simply ignored. This happens as the record is read, before field
splitting occurs. When -r is used, no escaping occurs, no line
joining happens, any input backslash is simply an input character.
Note that if delim is given as an empty string, the nul character
(`\0') is used as the delimiter. Other than this use, any nul
characters in the input stream are silently 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
where the value often needs to be quoted, as explained for the
export command.
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
set { -o | +o }
set { -options | +options | -o opt | +o opt } ... [--] [arg ...]
set -- [arg ...]
The set command performs four different functions.
With no arguments, set lists the names and values of all set 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, sh sets the specified option flags, or
clears them as described in the Argument List Processing section.
Note that not all options available on the command line are
available to the set built-in command. However, in addition to
the options listed there, when the "option name" (opt) 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 with no possibility of changing
any options, use "--" as the first argument to set. If no
following args are present, the set command will clear all the
positional parameters (equivalent to executing "shift $#".)
Otherwise the following args become $1, $2, ..., and $# is set to
the number of args present. The third and fourth forms may be
combined, to set options, and the positional parameters, in one
operation. Note that if it is possible that no arguments might be
present, or if the first arg might begin with a minus (`-') then
the "--" is required to distinguish this case from the first and
third variants of this command, and an arg beginning with `-' from
being an attempt to set options.
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 -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. The -P flag is similar, but prints only the action(s)
associated with the named signals, at least (and usually only) one
of which must be given. Nothing is printed if the action is the
default, an empty line is printed for ignored signals. These
variants of the trap command may be executed in a sub-shell (such
as in a command substitution), provided they appear as the initial
sequence of commands 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.
eval "$( trap -P QUIT )"
Parse and execute the action that would be invoked were a SIGQUIT
received.
trap 1 2
Reset the actions for signals 1 (HUP) and 2 (INT) to their
defaults.
traps=$(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, then with -a both are
displayed, the soft followed by the hard limit, otherwise for
setting, both limits are set, and for interrogating the soft limit
is displayed.
The limit to be interrogated or set, then, is chosen by specifying
any one of these flags:
-a show all the current limits (it is an error to attempt
to set the limits by giving a value)
-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.
NBSH_INVOCATION
When sh starts, after it has processed its arguments, and
imported variables from the environment, this variable is set
to a string of one or more characters which indicate the way
the command line was processed. This is intended to be used
in the startup scripts (see Invocation) to allow them to
determine what actions are appropriate to take.
NBSH_INVOCATION is marked "not to be exported". Apart from
the way it is initialized, and that it overrides any value
that may have been set in the environment, there is nothing
special about it. It can be unset, or altered, with no
ramifications, other than whatever effect this might have on
its use in the startup scripts.
When the value of this variable remains as set at startup by
sh the following characters may appear in the value, in the
circumstances described. Any present will always appear in
ASCII lexical order, as they appear below (to make testing the
value easier to code).
! Always present when set by sh, and is always first.
No specific meaning is attributed to this
character.
- Set when the first character of argv[0] as set when
the shell was invoked was a dash (`-').
0 Set when at startup, the special parameter $# has
the value 0. That is, no arguments were given to
the script in the case that there is a script.
c The -c option was given on the command line.
f Neither the -c nor -s options were present on the
command line, but there is at least one non-option
argument, which will then be interpreted as the
name of the command_file to process.
i The shell is interactive. At startup this
indicates that `i' will appear in the value of the
special parameter $-. However, the special
parameter will alter as the -i option is
manipulated by the set built-in command, but
NBSH_INVOCATION is never subsequently altered by
the shell itself (unless manipulated by a regular
variable operations).
l The shell is a login shell. As with `i' (the same
operational conditions apply) this character will
be present if the `l' is present in $- when the
shell is starting. Note that if `l' is present,
and `-' is not, then the shell was invoked with the
-l option on the command line (or the equivalent -o
login). On the other hand, if `-' appears, and `l'
does not, then the shell was invoked with the +l
option (or its equivalent) on the command line. If
both `-' and `l' appear, then the shell is a normal
login shell, the -l option might have been given,
but had no effect. If neither `-' nor `l' appear,
then the shell is not a login shell, and was never
intended to be. The +l option might have been
given, but had no effect.
p The shell was started as a privileged (set user id)
process. This indicates that the -p option must
have been given on the command line, or privileges
would have been dropped.
s The shell will read commands from standard input.
The -s option was given, or implied. This does not
imply that the shell is interactive.
PATH The default search path for executables. See the Path Search
section above.
POSIXLY_CORRECT
If set in the environment upon initialization of the shell,
then the shell option posix will be set. (See the description
of the set command in the Built-ins section.) After
initialization it is unused by the shell, but is usually
passed through the environment to descendant processes,
including other instances of the shell, which may interpret it
in a similar way.
PPID The process identified of the parent process of the current
shell. This value is set at shell startup, ignoring any value
in the environment, and then made readonly.
PS1 The primary prompt string, which defaults to "$ ", 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 as described for 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 `$',
depending upon whether the current user is the superuser or
not. This is intended for use when building a custom PS1. If
a privileged shell has its privileges removed by clearing the
-p option, an attempt will be made to be reset PSc to "#" or
"$", as appropriate for its new privilege level.
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.
This NetBSD sh is a much modified descendant of the ash shell written by
Ken Almquist.
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.
The -Z option to the jobs built-in command is bizarre, but is implemented
this way to be compatible with the similar option in zsh(1).
Many, many, more. (But less than there were...)
NetBSD 10.99 December 9, 2022 NetBSD 10.99