How to use the diff command in Linux

You can use the diff command to show differences between two files, or each corresponding file in two directories. diff outputs differences between files line by line in any of several formats, selectable by command line options. This set of differences is often 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.

1,2c1
< abcd
< dxxxf
---
> hihiih

#diff -v
diff (GNU diffutils) 3.3

To view differences in context mode, use the -c option.

*** fileone	2021-07-04 17:49:47.618438022 +0800
--- filetwo	2021-07-04 17:49:31.594436803 +0800
***************
*** 1,2 ****
! abcd
! dxxxf
--- 1 ----
! hihiih

To view differences in unified mode, use the -u option.

--- fileone	2021-07-04 17:49:47.618438022 +0800
+++ filetwo	2021-07-04 17:49:31.594436803 +0800
@@ -1,2 +1 @@
-abcd
-dxxxf
+hihiih

The syntax for the diff command is as follows:

Usage: 
  diff [OPTION]... FILES

OPTIONS:
--normal
output a normal diff (the default)
-q, --brief
report only when files differ
-s, --report-identical-files
report when two files are the same
-c, -C NUM, --context[=NUM]
output NUM (default 3) lines of copied context
-u, -U NUM, --unified[=NUM]
output NUM (default 3) lines of unified context
-e, --ed
output an ed script
-n, --rcs
output an RCS format diff
-y, --side-by-side
output in two columns
-W, --width=NUM
output at most NUM (default 130) print columns
--left-column
output only the left column of common lines
--suppress-common-lines
do not output common lines
-p, --show-c-function
show which C function each change is in
-F, --show-function-line=RE
show the most recent line matching RE
--label LABEL
use LABEL instead of file name (can be repeated)
-t, --expand-tabs
expand tabs to spaces in output
-T, --initial-tab
make tabs line up by prepending a tab
--tabsize=NUM
tab stops every NUM (default 8) print columns
--suppress-blank-empty
suppress space or tab before empty output lines
-l, --paginate
pass output through 'pr' to paginate it
-r, --recursive
recursively compare any subdirectories found
--no-dereference
don't follow symbolic links
-N, --new-file
treat absent files as empty
--unidirectional-new-file
treat absent first files as empty
--ignore-file-name-case
ignore case when comparing file names
--no-ignore-file-name-case
consider case when comparing file names
-x, --exclude=PAT
exclude files that match PAT
-X, --exclude-from=FILE
exclude files that match any pattern in FILE
-S, --starting-file=FILE
start with FILE when comparing directories
--from-file=FILE1
compare FILE1 to all operands; FILE1 can be a directory
--to-file=FILE2
compare all operands to FILE2; FILE2 can be a directory
-i, --ignore-case
ignore case differences in file contents
-E, --ignore-tab-expansion
ignore changes due to tab expansion
-Z, --ignore-trailing-space
ignore white space at line end
-b, --ignore-space-change
ignore changes in the amount of white space
-w, --ignore-all-space
ignore all white space
-B, --ignore-blank-lines
ignore changes where lines are all blank
-I, --ignore-matching-lines=RE
ignore changes where all lines match RE
-a, --text
treat all files as text
--strip-trailing-cr
strip trailing carriage return on input
-D, --ifdef=NAME
output merged file with '#ifdef NAME' diffs
--GTYPE-group-format=GFMT
format GTYPE input groups with GFMT
--line-format=LFMT
format all input lines with LFMT
--LTYPE-line-format=LFMT
format LTYPE input lines with LFMT
-d, --minimal
try hard to find a smaller set of changes
--horizon-lines=NUM
keep NUM lines of the common prefix and suffix
--speed-large-files
assume large files and many scattered small changes
--help
display this help and exit
-v, --version
output version information and exit

FILES are 'FILE1 FILE2' or 'DIR1 DIR2' or 'DIR FILE...' or 'FILE... DIR'. If --from-file or --to-file is given, there are no restrictions on FILE(s). If a FILE is '-', read standard input. Exit status is 0 if inputs are the same, 1 if different, 2 if trouble.