patch - apply a diff file to an original
patch [options] [origfile [patchfile]] [+ [options] [origfile]]...
but usually just
Patch will take a patch file containing any of the four forms of
difference listing produced by the diff program and apply those
differences to an original file, producing a patched version. 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''
(``~'' on systems that do not support long filenames), or as specified by
the -b, -B, or -V switches. The extension used for making backup files
may also be specified in the SIMPLE_BACKUP_SUFFIX environment variable,
which is overridden by above switches.
If the backup file already exists, 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.
You may also specify where you want the output to go with a -o switch; if
that file already exists, it is backed up first.
If patchfile is omitted, or is a hyphen, the patch will be read from
Upon startup, patch will attempt to determine the type of the diff
listing, unless over-ruled by a -c, -e, -n, or -u switch. Context diffs
(old-style, new-style, and unified) and normal diffs are applied by the
patch program itself, while ed diffs are simply fed to the ed editor via
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''
(``#'' on systems that do not support long filenames). (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 one.
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
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.
In the header of a context diff, the filename is found from lines
beginning with ``***'' or ``---'', with the shortest name of an existing
file winning. Only context diffs have lines like that, but if there is
an ``Index:'' line in the leading garbage, patch will try to use the
filename from that line. The context diff header takes precedence over
an Index line. If no filename can be intuited from the leading garbage,
you will be asked for the name of the file to patch.
If the original file cannot be found or is read-only, but a suitable SCCS
or RCS file is handy, patch will attempt to get or check out the file.
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.
If the patch file 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
filenames and revision level, as mentioned previously. You can give
switches (and another original file name) for the second and subsequent
patches by separating the corresponding argument lists by a `+'. (The
argument list for a second or subsequent patch may not specify a new
patch file, however.)
Patch recognizes the following switches:
-b causes the next argument to be interpreted as the backup extension,
to be used in place of ``.orig'' or ``~''.
-B causes the next argument to be interpreted as a prefix to the backup
file name. If this argument is specified any argument from -b will
-c forces patch to interpret the patch file as a context diff.
-d causes patch to interpret the next argument as a directory, and cd
to it before doing anything else.
-D 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.
-e forces patch to interpret the patch file as an ed script.
-E causes patch to remove output files that are empty after the patches
have been applied.
-f 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
-t 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.
sets the maximum fuzz factor. This switch 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.
-l 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 forces patch to interpret the patch file as a normal diff.
-N causes patch to ignore patches that it thinks are reversed or
already applied. See also -R .
-o causes the next argument to be interpreted as the output file name.
sets the pathname strip count, which controls how pathnames found in
the patch file are treated, in case the 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 filename in the patch file was
setting -p or -p0 gives the entire pathname unmodified, -p1 gives
without the leading slash, -p4 gives
and 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 switch.
-r causes the next argument to be interpreted as the reject file name.
-R 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 switch will not work with ed diff scripts because
there is too little information to reconstruct the reverse
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 switch 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.)
-s makes patch do its work silently, unless an error occurs.
-S causes patch to ignore this patch from the patch file, but continue
on looking for the next patch in the file. Thus
patch -S + -S + <patchfile
will ignore the first and second of three patches.
-u forces patch to interpret the patch file as a unified context diff
-v causes patch to print out its revision header and patch level.
-V 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 VERSION_CONTROL environment variable, which is overridden by
this option. The -B option overrides this option, causing the
prefix to always be used for making backup file names. The value of
the VERSION_CONTROL environment variable 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 accepted):
`t' or `numbered'
Always make numbered backups.
`nil' or `existing'
Make numbered backups of files that already have them, simple
backups of the others. This is the default.
`never' or `simple'
Always make simple backups.
sets internal debugging flags, and is of interest only to patch
Larry Wall <firstname.lastname@example.org>
with many other contributors.
Directory to put temporary files in; default is /tmp.
Extension to use for backup file names instead of ``.orig'' or
Selects when numbered backup files are made.
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 filenames
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 switch 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.
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.
Patch will exit with a non-zero status if any reject files were created.
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.
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.
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.