patch - Unix, Linux Command

but usually just



patch -pnum <patchfile 

Upon startup, patch attempts to determine the type of the diff listing,
unless overruled by a
-c (--context),
-e (--ed),
-n (--normal),
or
-u (--unified)
option.
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(1)
editor via a pipe.


patch tries 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,
or if a context diff contains lines ending in CRLF
or is encapsulated one or more times by prepending
"- " to lines starting with "-" as specified by Internet RFC 934,
this is 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 attempts 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 scans 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 puts the
hunk out to a reject file, which normally is the name of the output file
plus a
.rej suffix, or
# if
.rej would generate a file name that is too long
(if even appending the single character
# makes the file name too long, then
# replaces the file name’s last character).
(The rejected hunk comes out in ordinary context diff form regardless of
the input patch’s form.
If the input was a normal diff, many of the contexts are simply 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 are told if the hunk
failed, and if so which line (in the new file)
patch thought the hunk should go on.
If the hunk is installed at a different line
from the line number specified in the diff you
are told the offset.
A single large offset
may indicate that a hunk was installed in the
wrong place.
You are also told if a fuzz factor was used to make the match, in which
case you should also be slightly suspicious.
If the
--verbose option is given, you are also told about hunks that match exactly.


If no original file
origfile is specified on the command line,
patch tries to figure out from the leading garbage what the name of the file
to edit is, using the following rules.


First,
patch takes an ordered list of candidate file names as follows:


Additionally, if the leading garbage contains a
Prereq: line,
patch takes the first word from the prerequisites line (normally a version
number) and checks the original file to see if that word can be found.
If not,
patch asks for confirmation before proceeding.


The upshot of all this is that you should be able to say, while in a news
interface, something like 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 tries 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
contains interesting things such as file names and revision level, as
mentioned previously.



   /u/howard/src/blurfl/blurfl.c



setting
-p0 gives the entire file name 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 option.



If the first hunk of a patch fails,
patch reverses the hunk to see if it can be applied that way.
If it can, you are asked if you want to have the
-R option set.
If it can’t, the patch continues 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 matches
anywhere.
Luckily, most patches add or change lines rather than delete them, so most
reversed normal diffs begin with a delete, which fails, triggering
the heuristic.)



The value of
method is like the GNU
Emacs ‘version-control’ variable;
patch also recognizes synonyms that
are more descriptive.  The valid values for
method are (unique abbreviations are
accepted):



The
-Z or
--set-utc and
-T or
--set-time options normally refrain from setting a file’s time if the file’s original time
does not match the time given in the patch header, or if its
contents do not match the patch exactly.  However, if the
-f or
--force option is given, the file time is set regardless.



Due to the limitations of
diff output format, these options cannot update the times of files whose
contents have not changed.  Also, if you use these options, you should remove
(e.g. with
make clean) all files that depend on the patched files, so that later invocations of
make do not get confused by the patched files’ times.

diff (1)
diff (1)
ed (1)
ed (1)

Create your patch systematically.
A good method is the command
diff -Naur old new where
old and
new identify the old and new directories.
The names
old and
new should not contain any slashes.
The
diff command’s headers should have dates
and times in Universal Time using traditional Unix format,
so that patch recipients can use the
-Z or
--set-utc option.
Here is an example command, using Bourne shell syntax:



   LC_ALL=C TZ=UTC0 diff -Naur gcc-2.7 gcc-2.8


Tell your recipients how to apply the patch
by telling them which directory to
cd to, and which
patch options to use.  The option string
-Np1 is recommended.
Test your procedure by pretending to be a recipient and applying
your patch to a copy of the original files.


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.


You can create a file by sending out a diff that compares
/dev/null or an empty file dated the Epoch (1970-01-01 00:00:00 UTC)
to the file you want to create.
This only works if the file you want to create doesn’t exist already in
the target directory.
Conversely, you can remove a file by sending out a context diff that compares
the file to be deleted with an empty file dated the Epoch.
The file will be removed unless
patch is conforming to POSIX and the
-E or
--remove-empty-files option is not given.
An easy way to generate patches that create and remove files
is to use GNU
diff’s -N or
--new-file option.


If the recipient is supposed to use the
-pN option, do not send output that looks like this:




   diff -Naur v2.0.29/prog/README prog/README

   --- v2.0.29/prog/README   Mon Mar 10 15:13:12 1997

   +++ prog/README   Mon Mar 17 14:58:22 1997



because the two file names have different numbers of slashes,
and different versions of
patch interpret the file names differently.
To avoid confusion, send output that looks like this instead:




   diff -Naur v2.0.29/prog/README v2.0.30/prog/README

   --- v2.0.29/prog/README   Mon Mar 10 15:13:12 1997

   +++ v2.0.30/prog/README   Mon Mar 17 14:58:22 1997





Avoid sending patches that compare backup file names like
README.orig, since this might confuse
patch into patching a backup file instead of the real file.
Instead, send patches that compare the same base file names
in different directories, e.g.
old/README and
new/README. 

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


Try not to have your patch modify derived files
(e.g. the file
configure where there is a line
configure: configure.in in your makefile), since the recipient should be
able to regenerate the derived files anyway.
If you must send diffs of derived files,
generate the diffs using UTC,
have the recipients apply the patch with the
-Z or
--set-utc option, and have them remove any unpatched files that depend on patched files
(e.g. with
make clean). 

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


If the
--verbose option is given, 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’s exit status is
0 if all hunks are applied successfully,
1 if some hunks cannot be applied,
and 2 if there is more serious trouble.
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 detect
bad line numbers in a normal diff only when it finds a change or deletion.
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.



Also,
traditional
patch simply counted slashes when stripping path prefixes;
patch now counts pathname components.
That is, a sequence of one or more adjacent slashes
now counts as a single slash.
For maximum portability, avoid sending patches containing
// in file names.



Conversely, in POSIX
patch, backups are never made, even when there is a mismatch.
In GNU
patch, this behavior is enabled with the
--no-backup-if-mismatch option, or by conforming to POSIX with the
--posix option or by setting the
POSIXLY_CORRECT environment variable.



The
-b suffix option
of traditional
patch is equivalent to the
-b -z suffix options of GNU
patch. 





-c 
-d dir 
-D define 
-e 
-l 
-n 
-N 
-o outfile 
-pnum 
-R 
-r rejectfile 



patch 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 thinks it is a reversed patch, and offers to un-apply the patch.
This could be construed as a feature.


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are preserved on all copies.


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