Patch will take a patch file containing any of the four 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 (~ on systems that do not support long file names), or as specified by the -b (--suffix), -B (--prefix), or -V (--version-control) options. The extension used for making backup files may also be specified in the SIMPLE_BACKUP_SUFFIX environment variable, which is overridden by the above options.
If the backup file already exists, patch creates a new backup file name by changing the first lowercase letter in the last component of the files name into uppercase. If there are no more lowercase letters in the name, it removes the first character from the name. It repeats this process until it comes up with a backup file that does not already exist.
You may also specify where you want the output to go with a -o (--output) option; if that file already exists, it is backed up first.
If patchfile is omitted, or is a hyphen, the patch will be read from standard input. If a -i argument is specified, the filename following it will be used, instead of standard input. You may specify only one -i directive.
Upon startup, patch will attempt to determine the type of the diff listing, unless over-ruled 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 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 consistent 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 its 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 (# on systems that do not support long file names). (Note that the rejected hunk will 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 location 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 file name is found from lines beginning with *** or ---, with the shortest name of an existing file winning. Only context diffs have lines like that, but if there is an Index: line in the leading garbage, patch will try to use the file name from that line. The context diff header takes precedence over an Index line. If no file name 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 or is read-only, 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 determined for each diff listing, and that the garbage before each diff listing will be examined for interesting things such as file names and revision level, as mentioned previously. You can give options (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 options: