PATCH(1) USER COMMANDS PATCH(1)
NAME
patch a program for applying a diff file to an original
SYNOPSIS
patch [options] orig patchfile [+ [options] orig]
but usually just
patch <patchfile
DESCRIPTION
Patch will take a patch file containing any of the three
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", or as specified by
the b switch. You may also specify where you want the out-
put to go with a o switch. If patchfile is omitted, or is
a hyphen, the patch will be read from standard input.
Upon startup, patch will attempt to determine the type of
the diff listing, unless over ruled by a c, e, or n
switch. Context diffs and normal diffs are applied by the
patch program itself, while ed diffs are simply fed to the
ed editor via a pipe.
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 con-
sistent 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
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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 loca-
tion 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 slightly suspicious.
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 win-
ning. 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, 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 deter-
mined for each diff listing, and that the garbage before
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each diff listing will be examined for interesting things
such as filenames and revision level, as mentioned previ-
ously. 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".
-c forces patch to interpret the patch file as a context
diff.
-d causes patch to interpret the next argument as a direc-
tory, 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.
-f forces patch to assume that the user knows exactly what
he or she is doing, and to not ask any questions. It
does not suppress commentary, however. Use -s for
that.
-F<number>
sets the maximum fuzz factor. This switch only applied
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.
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-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 out-
put file name.
-p<number>
sets the pathname strip count, which controls how path-
names 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 speci-
fies how many backslashes 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
/u/howard/src/blurfl/blurfl.c
setting -p or -p0 gives the entire pathname unmodified,
-p1 gives
u/howard/src/blurfl/blurfl.c
without the leading slash, -p4 gives
blurfl/blurfl.c
and not specifying -p at all just gives you "blurfl.c".
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 hap-
pen 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 for-
mat. The -R switch 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
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
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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.
-v causes patch to print out it's revision header and
patch level.
-x<number>
sets internal debugging flags, and is of interest only
to patch patchers.
ENVIRONMENT
No environment variables are used by patch.
FILES
/tmp/patch*
SEE ALSO
diff(1)
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.
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DIAGNOSTICS
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.
CAVEATS
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 con-
text 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 gen-
erated from.
BUGS
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 OLD-
CODE ... #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.
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