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PATCH(1)                    General Commands Manual                   PATCH(1)

     patch - apply a diff file to an original

     patch [-bCcEeflNnRstuv] [-B backup-prefix] [-D symbol] [-d directory]
           [-F max-fuzz] [-i patchfile] [-o out-file] [-p strip-count]
           [-r rej-name] [-V t | nil | never | none] [-x number]
           [-z backup-ext] [--backup-if-mismatch] [--no-backup-if-mismatch]
           [--posix] [origfile [patchfile]]
     patch <patchfile

     patch will take a patch file containing any of the four forms of
     difference listing produced by the diff(1) program and apply those
     differences to an original file, producing a patched version.  If
     patchfile is omitted, or is a hyphen, the patch will be read from the
     standard input.

     patch will attempt to determine the type of the diff listing, unless
     over-ruled by a -c, -e, -n, or -u option.  Context diffs (old-style, new-
     style, and unified) and normal diffs are applied directly by the patch
     program itself, whereas ed diffs are simply fed to the ed(1) editor via a

     If the patchfile contains more than one patch, patch will try to apply
     each of them as if they came from separate patch files.  This means,
     among other things, that it is assumed that the name of the file to patch
     must be determined for each diff listing, and that the garbage before
     each diff listing will be examined for interesting things such as file
     names and revision level (see the section on Filename Determination

     The options are as follows:

     -B backup-prefix, --prefix backup-prefix
             Causes the next argument to be interpreted as a prefix to the
             backup file name.  If this argument is specified, any argument to
             -z will be ignored.

     -b, --backup
             Save a backup copy of the file before it is modified.  By default
             the original file is saved with a backup extension of ".orig"
             unless the file already has a numbered backup, in which case a
             numbered backup is made.  This is equivalent to specifying "-V
             existing".  This option is currently the default, unless --posix
             is specified.

             Create a backup file if the patch doesn't apply cleanly.  This
             option only makes sense when --backup is disabled, i.e. when in
             --posix mode.

     -C, --check
             Checks that the patch would apply cleanly, but does not modify

     -c, --context
             Forces patch to interpret the patch file as a context diff.

     -D symbol, --ifdef symbol
             Causes patch to use the "#ifdef...#endif" construct to mark
             changes.  The argument following will be used as the
             differentiating symbol.  Note that, unlike the C compiler, there
             must be a space between the -D and the argument.

     -d directory, --directory directory
             Causes patch to interpret the next argument as a directory, and
             change the working directory to it before doing anything else.

     -E, --remove-empty-files
             Causes patch to remove output files that are empty after the
             patches have been applied.  This option is useful when applying
             patches that create or remove files.

     -e, --ed
             Forces patch to interpret the patch file as an ed(1) script.

     -F max-fuzz, --fuzz max-fuzz
             Sets the maximum fuzz factor.  This option only applies to
             context diffs, and causes patch to ignore up to that many lines
             in looking for places to install a hunk.  Note that a larger fuzz
             factor increases the odds of a faulty patch.  The default fuzz
             factor is 2, and it may not be set to more than the number of
             lines of context in the context diff, ordinarily 3.

     -f, --force
             Forces patch to assume that the user knows exactly what he or she
             is doing, and to not ask any questions.  It assumes the
             following: skip patches for which a file to patch can't be found;
             patch files even though they have the wrong version for the
             "Prereq:" line in the patch; and assume that patches are not
             reversed even if they look like they are.  This option does not
             suppress commentary; use -s for that.

     -i patchfile, --input patchfile
             Causes the next argument to be interpreted as the input file name
             (i.e., a patchfile).  This option may be specified multiple

     -l, --ignore-whitespace
             Causes the pattern matching to be done loosely, in case the tabs
             and spaces have been munged in your input file.  Any sequence of
             whitespace in the pattern line will match any sequence in the
             input file.  Normal characters must still match exactly.  Each
             line of the context must still match a line in the input file.

     -N, --forward
             Causes patch to ignore patches that it thinks are reversed or
             already applied.  See also -R.

     -n, --normal
             Forces patch to interpret the patch file as a normal diff.

             Turn off --backup-if-mismatch.  This option exists mostly for
             compatibility with GNU patch.

     -o out-file, --output out-file
             Causes the next argument to be interpreted as the output file

     -p strip-count, --strip strip-count
             Sets the pathname strip count, which controls how pathnames found
             in the patch file are treated, in case you keep your files in a
             different directory than the person who sent out the patch.  The
             strip count specifies how many slashes are to be stripped from
             the front of the pathname.  (Any intervening directory names also
             go away.)  For example, supposing the file name in the patch file
             was /u/howard/src/blurfl/blurfl.c:

             Setting -p0 gives the entire pathname unmodified.

             -p1 gives


             without the leading slash.

             -p4 gives


             Not specifying -p at all just gives you blurfl.c, unless all of
             the directories in the leading path (u/howard/src/blurfl) exist
             and that path is relative, in which case you get the entire
             pathname unmodified.  Whatever you end up with is looked for
             either in the current directory, or the directory specified by
             the -d option.

     -R, --reverse
             Tells patch that this patch was created with the old and new
             files swapped.  (Yes, I'm afraid that does happen occasionally,
             human nature being what it is.)  patch will attempt to swap each
             hunk around before applying it.  Rejects will come out in the
             swapped format.  The -R option will not work with ed diff scripts
             because there is too little information to reconstruct the
             reverse operation.

             If the first hunk of a patch fails, patch will reverse the hunk
             to see if it can be applied that way.  If it can, you will be
             asked if you want to have the -R option set.  If it can't, the
             patch will continue to be applied normally.  (Note: this method
             cannot detect a reversed patch if it is a normal diff and if the
             first command is an append (i.e., it should have been a delete)
             since appends always succeed, due to the fact that a null context
             will match anywhere.  Luckily, most patches add or change lines
             rather than delete them, so most reversed normal diffs will begin
             with a delete, which will fail, triggering the heuristic.)

     -r rej-name, --reject-file rej-name
             Causes the next argument to be interpreted as the reject file

     -s, --quiet, --silent
             Makes patch do its work silently, unless an error occurs.

     -t, --batch
             Similar to -f, in that it suppresses questions, but makes some
             different assumptions: skip patches for which a file to patch
             can't be found (the same as -f); skip patches for which the file
             has the wrong version for the "Prereq:" line in the patch; and
             assume that patches are reversed if they look like they are.

     -u, --unified
             Forces patch to interpret the patch file as a unified context
             diff (a unidiff).

     -V t | nil | never | none, --version-control t | nil | never | none
             Causes the next argument to be interpreted as a method for
             creating backup file names.  The type of backups made can also be
             given in the PATCH_VERSION_CONTROL or VERSION_CONTROL environment
             variables, which are overridden by this option.  The -B option
             overrides this option, causing the prefix to always be used for
             making backup file names.  The values of the
             PATCH_VERSION_CONTROL and VERSION_CONTROL environment variables
             and the argument to the -V option are like the GNU Emacs
             "version-control" variable; they also recognize synonyms that are
             more descriptive.  The valid values are (unique abbreviations are

                   t, numbered
                           Always make numbered backups.

                   nil, existing
                           Make numbered backups of files that already have
                           them, simple backups of the others.

                   never, simple
                           Always make simple backups.

                   none    No backups are created.

     -v, --version
             Causes patch to print out its revision header and patch level.

     -x number, --debug number
             Sets internal debugging flags, and is of interest only to patch

     -z backup-ext, --suffix backup-ext
             Causes the next argument to be interpreted as the backup
             extension, to be used in place of ".orig".

             Enables strict IEEE Std 1003.1-2004 ("POSIX.1") conformance,

             1.   Backup files are not created unless the -b option is

             2.   If unspecified, the file name used is the first of the old,
                  new and index files that exists.

   Patch Application
     patch will try to skip any leading garbage, apply the diff, and then skip
     any trailing garbage.  Thus you could feed an article or message
     containing a diff listing to patch, and it should work.  If the entire
     diff is indented by a consistent amount, this will be taken into account.

     With context diffs, and to a lesser extent with normal diffs, patch can
     detect when the line numbers mentioned in the patch are incorrect, and
     will attempt to find the correct place to apply each hunk of the patch.
     As a first guess, it takes the line number mentioned for the hunk, plus
     or minus any offset used in applying the previous hunk.  If that is not
     the correct place, patch will scan both forwards and backwards for a set
     of lines matching the context given in the hunk.  First patch looks for a
     place where all lines of the context match.  If no such place is found,
     and it's a context diff, and the maximum fuzz factor is set to 1 or more,
     then another scan takes place ignoring the first and last line of
     context.  If that fails, and the maximum fuzz factor is set to 2 or more,
     the first two and last two lines of context are ignored, and another scan
     is made.  (The default maximum fuzz factor is 2.)

     If patch cannot find a place to install that hunk of the patch, it will
     put the hunk out to a reject file, which normally is the name of the
     output file plus ".rej".  (Note that the rejected hunk will come out in
     context diff form whether the input patch was a context diff or a normal
     diff.  If the input was a normal diff, many of the contexts will simply
     be null.)  The line numbers on the hunks in the reject file may be
     different than in the patch file: they reflect the approximate location
     patch thinks the failed hunks belong in the new file rather than the old

     As each hunk is completed, you will be told whether the hunk succeeded or
     failed, and which line (in the new file) patch thought the hunk should go
     on.  If this is different from the line number specified in the diff, you
     will be told the offset.  A single large offset MAY be an indication that
     a hunk was installed in the wrong place.  You will also be told if a fuzz
     factor was used to make the match, in which case you should also be
     slightly suspicious.

   Filename Determination
     If no original file is specified on the command line, patch will try to
     figure out from the leading garbage what the name of the file to edit is.
     When checking a prospective file name, pathname components are stripped
     as specified by the -p option and the file's existence and writability
     are checked relative to the current working directory (or the directory
     specified by the -d option).

     If the diff is a context or unified diff, patch is able to determine the
     old and new file names from the diff header.  For context diffs, the
     "old" file is specified in the line beginning with "***" and the "new"
     file is specified in the line beginning with "---".  For a unified diff,
     the "old" file is specified in the line beginning with "---" and the
     "new" file is specified in the line beginning with "+++".  If there is an
     "Index:" line in the leading garbage (regardless of the diff type), patch
     will use the file name from that line as the "index" file.

     patch will choose the file name by performing the following steps, with
     the first match used:

     1.   If patch is operating in strict IEEE Std 1003.1-2004 ("POSIX.1")
          mode, the first of the "old", "new" and "index" file names that
          exist is used.  Otherwise, patch will examine either the "old" and
          "new" file names or, for a non-context diff, the "index" file name,
          and choose the file name with the fewest path components, the
          shortest basename, and the shortest total file name length (in that

     2.   If no file exists, patch checks for the existence of the files in an
          RCS directory using the criteria specified above.  If found, patch
          will attempt to get or check out the file.

     3.   If no suitable file was found to patch, the patch file is a context
          or unified diff, and the old file was zero length, the new file name
          is created and used.

     4.   If the file name still cannot be determined, patch will prompt the
          user for the file name to use.

     Additionally, if the leading garbage contains a "Prereq: " line, patch
     will take the first word from the prerequisites line (normally a version
     number) and check the input file to see if that word can be found.  If
     not, patch will ask for confirmation before proceeding.

     The upshot of all this is that you should be able to say, while in a news
     interface, the following:

           | patch -d /usr/src/local/blurfl

     and patch a file in the blurfl directory directly from the article
     containing the patch.

   Backup Files
     By default, the patched version is put in place of the original, with the
     original file backed up to the same name with the extension ".orig", or
     as specified by the -B, -V, or -z options.  The extension used for making
     backup files may also be specified in the SIMPLE_BACKUP_SUFFIX
     environment variable, which is overridden by the options above.

     If the backup file is a symbolic or hard link to the original file, patch
     creates a new backup file name by changing the first lowercase letter in
     the last component of the file's name into uppercase.  If there are no
     more lowercase letters in the name, it removes the first character from
     the name.  It repeats this process until it comes up with a backup file
     that does not already exist or is not linked to the original file.

     You may also specify where you want the output to go with the -o option;
     if that file already exists, it is backed up first.

   Notes For Patch Senders
     There are several things you should bear in mind if you are going to be
     sending out patches:

     First, you can save people a lot of grief by keeping a patchlevel.h file
     which is patched to increment the patch level as the first diff in the
     patch file you send out.  If you put a "Prereq:" line in with the patch,
     it won't let them apply patches out of order without some warning.

     Second, make sure you've specified the file names right, either in a
     context diff header, or with an "Index:" line.  If you are patching
     something in a subdirectory, be sure to tell the patch user to specify a
     -p option as needed.

     Third, you can create a file by sending out a diff that compares a null
     file to the file you want to create.  This will only work if the file you
     want to create doesn't exist already in the target directory.

     Fourth, take care not to send out reversed patches, since it makes people
     wonder whether they already applied the patch.

     Fifth, while you may be able to get away with putting 582 diff listings
     into one file, it is probably wiser to group related patches into
     separate files in case something goes haywire.

     POSIXLY_CORRECT        When set, patch behaves as if the --posix option
                            has been specified.
     SIMPLE_BACKUP_SUFFIX   Extension to use for backup file names instead of
     TMPDIR                 Directory to put temporary files in; default is
     PATCH_VERSION_CONTROL  Selects when numbered backup files are made.

     $TMPDIR/patch*  patch temporary files
     /dev/tty        used to read input when patch prompts the user

     Too many to list here, but generally indicative that patch couldn't parse
     your patch file.

     The message "Hmm..." indicates that there is unprocessed text in the
     patch file and that patch is attempting to intuit whether there is a
     patch in that text and, if so, what kind of patch it is.

     The patch utility exits with one of the following values:

           0       Successful completion.
           1       One or more lines were written to a reject file.
           >1      An error occurred.

     When applying a set of patches in a loop it behooves you to check this
     exit status so you don't apply a later patch to a partially patched file.


     The patch utility is compliant with the IEEE Std 1003.1-2004 ("POSIX.1")
     specification (except as detailed above for the --posix option), though
     the presence of patch itself is optional.

     The flags [-CEfstuvBFVxz] and [--posix] are extensions to that

     Larry Wall with many other contributors.

     patch cannot tell if the line numbers are off in an ed script, and can
     only detect bad line numbers in a normal diff when it finds a "change" or
     a "delete" command.  A context diff using fuzz factor 3 may have the same
     problem.  Until a suitable interactive interface is added, you should
     probably do a context diff in these cases to see if the changes made
     sense.  Of course, compiling without errors is a pretty good indication
     that the patch worked, but not always.

     patch usually produces the correct results, even when it has to do a lot
     of guessing.  However, the results are guaranteed to be correct only when
     the patch is applied to exactly the same version of the file that the
     patch was generated from.

     Could be smarter about partial matches, excessively deviant offsets and
     swapped code, but that would take an extra pass.

     Check patch mode (-C) will fail if you try to check several patches in
     succession that build on each other.  The entire patch code would have to
     be restructured to keep temporary files around so that it can handle this

     If code has been duplicated (for instance with #ifdef OLDCODE ... #else
     ...  #endif), patch is incapable of patching both versions, and, if it
     works at all, will likely patch the wrong one, and tell you that it
     succeeded to boot.

     If you apply a patch you've already applied, patch will think it is a
     reversed patch, and offer to un-apply the patch.  This could be construed
     as a feature.

NetBSD 10.99                     June 16, 2023                    NetBSD 10.99