diff

Display the differences between two files, or each corresponding file in two directories.
Each set of differences is called a "diff" or "patch".
For files that are identical, 'diff' normally produces no output; for binary (non-text) files, 'diff' normally reports only that they are different.

Syntax
      diff [options] from-file to-file 
Options
    Multiple single letter options (unless they take an argument) can be combined into a single command-line word: so `-ac' is equivalent to `-a -c'.

      -lines  Show lines lines of context. This option is obsolete.

       -a     Treat all files as text and compare them  line-by-line, even if they do not seem to be text.

       -b     Ignore changes in amount of white space.

       -B     Ignore  changes  that  just  insert or delete blank lines.

       --brief
              Report only  whether  the  files  differ,  not  the details of the differences.

       -c     Use the context output format.

       -C lines
       --context[=lines]
              Use  the  context  output format, showing lines (an integer) lines of context,
              or three if lines is not given.  For proper operation, patch typically needs
              at least two lines of context.

       --changed-group-format=format
              Use format to output a line group  containing  differing  lines  from both files
              in if-then-else format.

       -d     Change the algorithm to perhaps find a smaller  set of changes.
              This makes diff slower (sometimes much slower).

       -D name
              Make merged if-then-else format output, conditional on the preprocessor macro name.

       -e
       --ed   Make output that is a valid ed script.

       --exclude=pattern
              When comparing directories, ignore files and subdirectories whose basenames match pattern.

       --exclude-from=file
              When comparing directories, ignore files and subdirectories  whose  basenames
              match any pattern contained in file.

       --expand-tabs
              Expand tabs to spaces in the output,  to  preserve the alignment of tabs in the input files.

       -f     Make output that looks vaguely like an ed script but has changes in the order
              they appear in the file.

       -F regexp
              In  context  and  unified  format, for each hunk of differences, show some of the last
              preceding line that matches regexp.

       --forward-ed
              Make output that looks vaguely like an ed script but has changes in the order they appear
              in the file.

       -h     This option currently has no effect; it is present for Unix compatibility.

       -H     Use heuristics to speed handling of large files that have numerous scattered small changes.

       --horizon-lines=lines
              Do not discard the last lines lines of the common prefix and the first lines
              of the common suffix.

       -i     Ignore  changes in case; consider upper- and lower-case letters equivalent.

       -I regexp
              Ignore changes that just insert or delete lines that match regexp.

       --ifdef=name
              Make merged if-then-else format output, conditional on the preprocessor macro name.

       --ignore-all-space
              Ignore white space when comparing lines.

       --ignore-blank-lines
              Ignore changes that just insert or delete blank lines.

       --ignore-case
              Ignore  changes in case; consider upper- and lower-case to be the same.

       --ignore-matching-lines=regexp
              Ignore changes that just insert or delete lines that match regexp.

       --ignore-space-change
              Ignore changes in amount of white space.

       --initial-tab
              Output a tab rather than a space before the text of a line in normal or context  format.
              This  causes the alignment of tabs in the line to look normal.

       -l     Pass the output through pr to paginate it.

       -L label
       --label=label
              Use label instead of the file name in the context format and unified format headers.

       --left-column
              Print only the left column of two common lines in side by side format.

       --line-format=format
              Use  format  to  output all input lines in in-then-else format.

       --minimal
              Change the algorithm to perhaps find a smaller  set of changes.
              This makes diff slower (sometimes much slower).

       -n     Output RCS-format diffs; like -f except that each command specifies the number of
              lines affected.

       -N
       --new-file
              In directory comparison, if a file is found in only one directory, treat it as
              present but empty in the other directory.

       --new-group-format=format
              Use format to output a group of lines taken from just the second file in
              if-then-else format.

       --new-line-format=format
              Use format to output a line taken from just the second file in if-then-else format.

       --old-group-format=format
              Use format to output a group of lines taken from just the first file in
              if-then-else format.

       --old-line-format=format
              Use format to output a line taken from just the first file in if-then-else format.

       -p     Show which C function each change is in.

       -P     When  comparing directories, if a file appears only in the second directory of
              the two, treat it as present but empty in the other.

       --paginate
              Pass the output through pr to paginate it.

       -q     Report only whether the files differ, not the details of the differences.

       -r     When comparing directories, recursively compare any subdirectories found.

       --rcs  Output  RCS-format diffs; like -f except that each command specifies the
              number of lines affected.

       --recursive
              When comparing directories, recursively compare any subdirectories found.

       --report-identical-files
       -s     Report when two files are the same.

       -S file
              When  comparing  directories,  start  with the file file.
              This is used for resuming a cancelled comparison.

       --sdiff-merge-assist
              Print extra information to help sdiff.  sdiff uses this option when it runs diff.
              This option is  not intended for users to use directly.

       --show-c-function
              Show which C function each change is in.

       --show-function-line=regexp
              In  context  and  unified  format, for each hunk of differences, show some of the
              last  preceding  line that matches regexp.

       --side-by-side
              Use the side by side output format.

       --speed-large-files
              Use heuristics to speed handling of large files that have numerous scattered small changes.

       --starting-file=file
              When comparing directories,  start  with  the  file file.
              This is used for resuming a cancelled comparison.

       --suppress-common-lines
              Do not print common lines in side by side format.

       -t     Expand tabs to spaces in the output, to preserve the alignment of tabs in the input files.

       -T     Output a tab rather than a space before the text of a line in normal or context format.
              This causes the alignment of tabs in the line to look normal.

       --text Treat all files as text and compare them line-by-line, even if they do not appear to be text.

       -u     Use the unified output format.

       --unchanged-group-format=format
              Use format to output a group of common lines taken from both files in if-then-else format.

       --unchanged-line-format=format
              Use format to output a line common to both files in if-then-else format.

       --unidirectional-new-file
              When comparing directories, if a file appears only in the second directory of
              the two, treat it as present but empty in the other.

       -U lines
       --unified[=lines]
              Use the unified output format, showing lines (an integer) lines of context, or
              three if lines is not given.
              For proper operation, patch typically needs at least two lines of context.

       -v
       --version
              Output the version number of diff.

       -w     Ignore white space when comparing lines.

       -W columns
       --width=columns
              Use an output width of columns in side by side for mat.

       -x pattern
              When comparing directories, ignore files and subdirectories whose basenames match pattern.

       -X file
              When comparing directories, ignore files and subdirectories whose basenames match
              any pattern contained in file.

       -y     Use the side by side output format.

In the simplest case, diff compares the contents of the two files from-file and to-file. A file name of - stands for text read from the standard input.

If from-file is a directory and to-file is not, diff compares the file in from-file whose file name is that of to-file, and vice versa.
The non-directory file must not be -.

If both from-file and to-file are directories, diff compares corresponding files in both directories, in alphabetical order; this comparison is not recursive unless the -r or --recursive option is given.

As a special case, diff - - compares a copy of standard input to itself.

diff never compares the actual contents of a directory as if it were a file. The file that is fully specified can not be standard input, because standard input is nameless and the notion of 'file with the same name' does not apply.

diff options begin with -, so normally from-file and to-filemay not begin with -.
However, -- as an argument by itself treats the remaining arguments as file names even if they begin with -.

“In order to really enjoy a dog, one doesn't merely try to train him to be semihuman. The point of it is to open oneself to the possibility of becoming partly a dog” - Edward Hoagland

Related macOS commands:

patch - apply a diff file to an original.
comm - Compare two sorted files line by line.
ed - Line editor.
cmp - Compare two files.
diff3 - Show differences among three files.
FileMerge - GUI tool to compare two ASCII files/folders (Developer tools).
pr - Convert text files for printing.
sdiff - Merge two files interactively.


 
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