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 This option is obsolete.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  dif-
              fering  lines  from both files in if-then-else for-
              mat.

       -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 subdi-
              rectories whose basenames match pattern.

       --exclude-from=file
              When comparing directories, ignore files and subdi-
              rectories  whose  basenames  match any pattern con-
              tained 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 lines of the common suf-
              fix.

       -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 an aborted compar-
              ison.

       --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 an aborted compar-
              ison.

       --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 subdi-
              rectories whose basenames match pattern.

       -X file
              When comparing directories, ignore files and subdi-
              rectories whose basenames match  any  pattern  con-
              tained in file.

       -y     Use the side by side output format.

Notes
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 may 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:

diff man page - Apple.com
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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