When patching what's the difference between arguments -p0 and -p1?

The most common way to create a patch is to run the diff command or some version control's built-in diff-like command. Sometimes, you're just comparing two files, and you run diff like this:

diff -u version_by_alice.txt version_by_bob.txt >alice_to_bob.patch

Then you get a patch that contains changes for one file and doesn't contain a file name at all. When you apply that patch, you need to specify which file you want to apply it to:

patch <alice_to_bob.patch version2_by_alice.txt

Often, you're comparing two versions of a whole multi-file project contained in a directory. A typical invocation of diff looks like this:

diff -ru old_version new_version >some.patch

Then the patch contains file names, given in header lines like diff -ru old_version/dir/file new_version/dir/file. You need to tell patch to strip the prefix (old_version or new_version) from the file name. That's what -p1 means: strip one level of directory.

Sometimes, the header lines in the patch contain the file name directly with no lead-up. This is common with version control systems; for example cvs diff produces header lines that look like diff -r1.42 foo. Then there is no prefix to strip, so you must specify -p0.

In the special case when there are no subdirectories in the trees that you're comparing, no -p option is necessary: patch will discard all the directory part of the file names. But most of the time, you do need either -p0 or -p1, depending on how the patch was produced.

From the man:

-pnum or --strip=num Strip the smallest prefix containing num leading slashes from each file name found in the patch file. A sequence of one or more adjacent slashes is counted as a single slash. This controls how file names found in the patch file are treated, in case you keep your files in a different directory than the person who sent out the patch. For example, supposing the file name in the patch file was:

 /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

The difference is that the number after -p would specify the number of path components that would be removed.

Say we have a path /Barack/Obama. Executing a patch on it with the -p0 argument will treat the path as is:

/Barack/Obama

But we can trim the path while patching:

-p1 will remove the root slash (note that it will just be Barack now, without a slash left to it):

Barack/Obama

-p2 will remove Barack (and adjacent right slash):

 Obama

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To expand on the "why" of this patch behavior, read this thread.