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GNU patch and Traditional patch

The current version of GNU patch normally follows the POSIX standard. See patch and POSIX, for the few exceptions to this general rule.

Unfortunately, POSIX redefined the behavior of patch in several important ways. You should be aware of the following differences if you must interoperate with traditional patch, or with GNU patch version 2.1 and earlier.

• In traditional patch, the -p option's operand was optional, and a bare -p was equivalent to -p0. The -p option now requires an operand, and -p 0 is now equivalent to -p0. For maximum compatibility, use options like -p0 and -p1.

  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.

• In traditional patch, backups were enabled by default. This behavior is now enabled with the -b or --backup option.

  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.

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

• Traditional patch used a complicated (and incompletely documented) method to intuit the name of the file to be patched from the patch header. This method did not conform to POSIX, and had a few gotchas. Now patch uses a different, equally complicated (but better documented) method that is optionally POSIX-conforming; we hope it has fewer gotchas. The two methods are compatible if the file names in the context diff header and the Index: line are all identical after prefix-stripping. Your patch is normally compatible if each header's file names all contain the same number of slashes.
• When traditional patch asked the user a question, it sent the question to standard error and looked for an answer from the first file in the following list that was a terminal: standard error, standard output, /dev/tty, and standard input. Now patch sends questions to standard output and gets answers from /dev/tty. Defaults for some answers have been changed so that patch never goes into an infinite loop when using default answers.
• Traditional patch exited with a status value that counted the number of bad hunks, or with status 1 if there was real trouble. Now patch exits with status 1 if some hunks failed, or with 2 if there was real trouble.
• Limit yourself to the following options when sending instructions meant to be executed by anyone running GNU patch, traditional patch, or a patch that conforms to POSIX. Spaces are significant in the following list, and operands are required.
  

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