$ Linux Command

Text

grep

Search text for lines matching a pattern.

searchtextpatternlogsregexfilter

Additional Notes

grep searches text and prints lines that match a pattern. The name comes from the old editor command for global regular-expression search and print.

It is one of the most useful Linux commands because it works with files, logs, command output, source code, configuration files, and pipelines. You can search simple words or use regular expressions for more flexible matching.

Syntax

grep [options] PATTERN [file...]
command | grep [options] PATTERN

Parameters

  • PATTERN: Text or a regular expression to search for.
  • file: One or more files to search. If no file is given, grep reads standard input.

Common Options

  • -i, --ignore-case: Match without caring about uppercase or lowercase.
  • -n, --line-number: Show line numbers.
  • -v, --invert-match: Show lines that do not match.
  • -r, -R, --recursive: Search inside directories recursively.
  • -l, --files-with-matches: Print only filenames that contain a match.
  • -L, --files-without-match: Print only filenames that do not contain a match.
  • -c, --count: Count matching lines.
  • -o: Print only the matching part of each line.
  • -w, --word-regexp: Match whole words only.
  • -x, --line-regexp: Match whole lines only.
  • -E, --extended-regexp: Use extended regular expressions.
  • -F, --fixed-strings: Treat the pattern as plain text, not regex.
  • -P, --perl-regexp: Use Perl-compatible regex when supported.
  • -A N: Show N lines after each match.
  • -B N: Show N lines before each match.
  • -C N: Show N lines before and after each match.
  • --color=auto: Highlight matching text in terminal output.

Examples

grep "error" app.log

Find lines containing error in app.log.

grep -i "timeout" /var/log/syslog

Search without case sensitivity.

grep -n "PermitRootLogin" /etc/ssh/sshd_config

Show the line number of SSH configuration matches.

grep -r "TODO" ./src

Search every file under ./src recursively.

grep -Ev "^#|^$" config.conf

Show non-comment and non-empty lines.

ps aux | grep nginx

Filter command output for lines containing nginx.

grep -C 3 "failed password" /var/log/auth.log

Show matching authentication log lines with three lines of context before and after.

grep -l "database_url" ./*.env

List only files that contain the searched text.

Regex Basics

  • ^text: Match text at the beginning of a line.
  • text$: Match text at the end of a line.
  • .: Match any single character.
  • .*: Match any amount of text.
  • [abc]: Match one character from a set.
  • [^abc]: Match one character not in the set.
  • [0-9]: Match a digit.
  • word1|word2: With -E, match either pattern.

Practical Notes

  • Quote patterns so the shell does not change them before grep receives them.
  • Use grep -F for plain strings when regex characters like . or * should not be special.
  • Use grep -r carefully in large directories; it can search many files.
  • For source code, tools like ag, ack, or ripgrep are often faster, but grep is available almost everywhere.
  • When searching logs, combine grep with tail, journalctl, or less.