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.TH PATCH 1
.SH NAME
patch - apply a diff file to an original
.SH SYNOPSIS
.B patch
[options] [origfile [patchfile]] [+ [options] [origfile]]...
.sp
but usually just
.sp
.B patch
<patchfile
.SH DESCRIPTION
.I Patch
will take a patch file containing any of the four forms of difference
listing produced by the
.I 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 \*(L".orig\*(R" (\*(L"~\*(R" on systems that do not
support long filenames), or as specified by the
.BR -b ,
.BR -B ,
or
.B -V
switches.
The extension used for making backup files may also be specified in the
.B SIMPLE_BACKUP_SUFFIX
environment variable, which is overridden by above switches.
.PP
If the backup file already exists,
.B 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.
.PP
You may also specify where you want the output to go with a
.B -o
switch; if that file already exists, it is backed up first.
.PP
If
.I patchfile
is omitted, or is a hyphen, the patch will be read from standard input.
.PP
Upon startup, patch will attempt to determine the type of the diff listing,
unless over-ruled by a
.BR -c ,
.BR -e ,
.BR -n ,
or
.B -u
switch.
Context diffs (old-style, new-style, and unified) and
normal diffs are applied by the
.I patch
program itself, while ed diffs are simply fed to the
.I ed
editor via a pipe.
.PP
.I 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
.IR patch ,
and it should work.
If the entire diff is indented by a consistent amount,
this will be taken into account.
.PP
With context diffs, and to a lesser extent with normal diffs,
.I 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,
.I patch
will scan both forwards and backwards for a set of lines matching the context
given in the hunk.
First
.I 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
.I 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 \*(L".rej\*(R" (\*(L"#\*(R" 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.
.PP
As each hunk is completed, you will be told whether the hunk succeeded or
failed, and which line (in the new file)
.I 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.
.PP
If no original file is specified on the command line,
.I 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 \*(L"***\*(R" or \*(L"---\*(R", with the shortest name of an existing
file winning.
Only context diffs have lines like that, but if there is an \*(L"Index:\*(R"
line in the leading garbage,
.I 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.
.PP
If the original file cannot be found or is read-only, but a suitable
SCCS or RCS file is handy,
.I patch
will attempt to get or check out the file.
.PP
Additionally, if the leading garbage contains a \*(L"Prereq: \*(R" line,
.I 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,
.I patch
will ask for confirmation before proceeding.
.PP
The upshot of all this is that you should be able to say, while in a news
interface, the following:
.Sp
	| patch -d /usr/src/local/blurfl
.Sp
and patch a file in the blurfl directory directly from the article containing
the patch.
.PP
If the patch file contains more than one patch,
.I 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 \*(L'+\*(R'.
(The argument list for a second or subsequent patch may not specify a new
patch file, however.)
.PP
.I Patch
recognizes the following switches:
.TP 5
.B \-b
causes the next argument to be interpreted as the backup extension, to be
used in place of \*(L".orig\*(R" or \*(L"~\*(R".
.TP 5
.B \-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 be ignored.
.TP 5
.B \-c
forces
.I patch
to interpret the patch file as a context diff.
.TP 5
.B \-d
causes
.I patch
to interpret the next argument as a directory, and cd to it before doing
anything else.
.TP 5
.B \-D
causes
.I 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
.B \-D
and the argument.
.TP 5
.B \-e
forces
.I patch
to interpret the patch file as an ed script.
.TP 5
.B \-E
causes
.I patch
to remove output files that are empty after the patches have been applied.
.TP 5
.B \-f
forces
.I 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
.B \-s
for that.
.TP 5
.B \-t
similar to
.BR \-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 \fB\-f\fP);
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.
.TP 5
.B \-F<number>
sets the maximum fuzz factor.
This switch only applies to context diffs, and causes
.I 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.
.TP 5
.B \-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.
.TP 5
.B \-n
forces
.I patch
to interpret the patch file as a normal diff.
.TP 5
.B \-N
causes
.I patch
to ignore patches that it thinks are reversed or already applied.
See also
.B \-R .
.TP 5
.B \-o
causes the next argument to be interpreted as the output file name.
.TP 5
.B \-p<number>
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
.sp
	/u/howard/src/blurfl/blurfl.c
.sp
setting
.B \-p
or
.B \-p0
gives the entire pathname unmodified,
.B \-p1
gives
.sp
	u/howard/src/blurfl/blurfl.c
.sp
without the leading slash,
.B \-p4
gives
.sp
	blurfl/blurfl.c
.sp
and not specifying
.B \-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
.B \-d
switch.
.TP 5
.B \-r
causes the next argument to be interpreted as the reject file name.
.TP 5
.B \-R
tells
.I 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.)
.I Patch
will attempt to swap each hunk around before applying it.
Rejects will come out in the swapped format.
The
.B \-R
switch will not work with ed diff scripts because there is too little
information to reconstruct the reverse operation.
.Sp
If the first hunk of a patch fails,
.I 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
.B \-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.)
.TP 5
.B \-s
makes
.I patch
do its work silently, unless an error occurs.
.TP 5
.B \-S
causes
.I patch
to ignore this patch from the patch file, but continue on looking
for the next patch in the file.
Thus
.sp
	patch -S + -S + <patchfile
.sp
will ignore the first and second of three patches.
.TP 5
.B \-u
forces
.I patch
to interpret the patch file as a unified context diff (a unidiff).
.TP 5
.B \-v
causes
.I patch
to print out its revision header and patch level.
.TP 5
.B \-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
.B VERSION_CONTROL
environment variable, which is overridden by this option.
The
.B -B
option overrides this option, causing the prefix to always be used for
making backup file names.
The value of the
.B VERSION_CONTROL
environment variable and the argument to the
.B -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):
.RS
.TP
`t' or `numbered'
Always make numbered backups.
.TP
`nil' or `existing'
Make numbered backups of files that already
have them, simple backups of the others.
This is the default.
.TP
`never' or `simple'
Always make simple backups.
.RE
.TP 5
.B \-x<number>
sets internal debugging flags, and is of interest only to
.I patch
patchers.
.SH AUTHOR
Larry Wall <lwall@netlabs.com>
.br
with many other contributors.
.SH ENVIRONMENT
.TP
.B TMPDIR
Directory to put temporary files in; default is /tmp.
.TP
.B SIMPLE_BACKUP_SUFFIX
Extension to use for backup file names instead of \*(L".orig\*(R" or
\*(L"~\*(R".
.TP
.B VERSION_CONTROL
Selects when numbered backup files are made.
.SH FILES
$TMPDIR/patch*
.SH SEE ALSO
diff(1)
.SH 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 
.B \-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.
.SH DIAGNOSTICS
Too many to list here, but generally indicative that
.I patch
couldn't parse your patch file.
.PP
The message \*(L"Hmm...\*(R" indicates that there is unprocessed text in
the patch file and that
.I patch
is attempting to intuit whether there is a patch in that text and, if so,
what kind of patch it is.
.PP
.I 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.
.SH CAVEATS
.I 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 \*(L"change\*(R" or
a \*(L"delete\*(R" 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.
.PP
.I 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.
.SH BUGS
Could be smarter about partial matches, excessively \&deviant offsets and
swapped code, but that would take an extra pass.
.PP
If code has been duplicated (for instance with #ifdef OLDCODE ... #else ...
#endif),
.I 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.
.PP
If you apply a patch you've already applied,
.I patch
will think it is a reversed patch, and offer to un-apply the patch.
This could be construed as a feature.
.rn }` ''