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What's the difference

Table of Contents


cmp

$ cmp --version | head -n1
cmp (GNU diffutils) 3.3

$ man cmp
CMP(1)                           User Commands                          CMP(1)

NAME
       cmp - compare two files byte by byte

SYNOPSIS
       cmp [OPTION]... FILE1 [FILE2 [SKIP1 [SKIP2]]]

DESCRIPTION
       Compare two files byte by byte.

       The optional SKIP1 and SKIP2 specify the number of bytes to skip at the
       beginning of each file (zero by default).
...

Useful to compare binary files. If the two files are same, no output is displayed (exit status 0)
If there is a difference, it prints the first difference - line number and byte location (exit status 1)
Option -s allows to suppress the output, useful in scripts

$ cmp /bin/grep /bin/fgrep
/bin/grep /bin/fgrep differ: byte 25, line 1
  • More examples here

diff

$ diff --version | head -n1
diff (GNU diffutils) 3.3

$ man diff
DIFF(1)                          User Commands                         DIFF(1)

NAME
       diff - compare files line by line

SYNOPSIS
       diff [OPTION]... FILES

DESCRIPTION
       Compare FILES line by line.
...

Useful to compare old and new versions of text files
All the differences are printed, which might not be desirable if files are too long

Options

  • -s convey message when two files are same
  • -y two column output
  • -i ignore case while comparing
  • -w ignore white-spaces
  • -r recursively compare files between the two directories specified
  • -q report if files differ, not the details of difference

Examples

  • diff -s test_list_mar2.txt test_list_mar3.txt compare two files
  • diff -s report.log bkp/mar10/ no need to specify second filename if names are same
  • diff -qr report/ bkp/mar10/report/ recursively compare files between report and bkp/mar10/report directories, filenames not matching are also specified in output
    • see this link for detailed analysis and corner cases
  • diff report/ bkp/mar10/report/ | grep -w '^diff' useful trick to get only names of mismatching files (provided no mismatches contain the whole word diff at start of line)

Further Reading