diff Command on Linux: Compare Two Files and Directories

The diff command on Linux is still how developers and ops answer “what changed between these two configs?” before a deploy or after a bad edit. Unified output (-u) is the default language of code reviews and patch files.

Related: cut,
awk & sed,
grep.

Compare two files

Shows line differences with < / > prefixes or unified hunks with -u.

diff -u /etc/nginx/nginx.conf /etc/nginx/nginx.conf.bak

Exit codes

diff returns 0 if identical, 1 if different, 2 on errors — useful in CI scripts.

if diff -q a.txt b.txt; then echo same; else echo different; fi

Directories

-r walks trees — pair with --exclude for .git or build dirs.

diff -rq /etc/nginx/sites-available /backup/nginx/sites-available

Ignore noise

Whitespace-only changes disappear with -w; use -I REGEX to skip version headers.

Quick Reference

• Unified: diff -u a b
• Quick: diff -q a b
• Dirs: diff -r dir1 dir2

Related tutorials

• cut command
• awk & sed
• find

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