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Apply a patch

To apply a patch to a single file, do that in it's directory:
$ patch < foo.patch

Assuming that the patch is distributed in unified format identifying the file the patch should be applied to, the above will work. Otherwise:
$ patch foo.txt < bar.patch

You can apply a patch to an entire directory, but note the "p level". What this means is that inside patch files will be the files that you intend to patch, identified by path names that might be different when the files ane located on your own computer instead of on the computer where the patch was created. 'p' level instructs the 'patch' utility to ignore parts of the path name to identify the files correctly. Usually a p level of 1 will work, so you would use:
$ patch -p1 < baz.patch

Change to the top level directory before running this. If a patch level of 1 cannot identify the files to patch, then inspect the patch file for file names. For example:
/home/user/do/not/panic/yet.c

and you are working in a directory that contains panic/yet.c, use:
$ patch -p5 < baz.patch

You usually count one up for each path separator (forward slash) removed from the beginning of the path, until you are left with a path that exists in the current working directory. The count is the p level.

Removing a patch using the -R flag
$ patch -p5 -R < baz.patch

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Create a patch with diff

Diff can create a patch for a single file:
$ diff -u original.c new.c > original.patch

For diff'ing a source tree:
$ cp -R original new

Do whatever you want in new/ and then diff it:
$ diff -rupN original/ new/ > original.patch

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git diff

git is something special.

Just make whatever changes you want to a git clone and then:
$ git diff > patch.git

Note the git revision that you did this with:
$ git log

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git apply

it really is.

Now to apply that patch in the future, just git clone it again and do with the git revision you found from above:
$ git reset --hard REVISIONNUMBER

Now put patch.git in the git clone directory and do:
$ git apply patch.git

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Copyright © 2014 Francis Rowe <info@gluglug.org.uk>
This document is released under the Creative Commons Attribution-ShareAlike 4.0 International Public License and all future versions. A copy of the license can be found at license.txt.

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See license.txt for more information.