Grep

Grep is a command-line utility for searching plain-text data for lines matching a regular expression. Its name comes from the ed command g/re/p (globally search for a regular expression and print). It is available on Linux and all Unix-like systems. See also Sed and Awk.

Common Options #

OPTION	DESCRIPTION
`-i`	Ignore case
`-v`	Invert match (lines that do NOT match)
`-r` / `-R`	Recurse into directories (`-R` follows symlinks)
`-n`	Show line numbers
`-c`	Count matching lines
`-l` / `-L`	List files with / without matches
`-w`	Match whole words only
`-x`	Match whole lines only
`-o`	Print only the matched part
`-h`	Hide file names
`-A <N>` / `-B <N>` / `-C <N>`	Show N lines after / before / around a match
`-E`	Extended regex (same as `egrep`)
`-F`	Fixed strings, no regex (fastest)
`-P`	Perl-compatible regex (lookaheads, `\d`, `\K`)
`-q`	Quiet, only set the exit code
`-m <N>`	Stop after N matches
`--include=<GLOB>` / `--exclude=<GLOB>`	Filter files by name
`--exclude-dir=<DIR>`	Skip a directory
`--color=auto`	Highlight matches

Examples #

COMMAND	DESCRIPTION
`grep -rn "TODO" .`	Find TODOs with file and line number
`grep -rIl "password" /etc 2>/dev/null`	List text files mentioning a word, skip binaries
`grep -v '^\s#' file.conf \| grep -v '^\s$'`	Show config without comments and blank lines
`grep -E "error\|warn" app.log`	Match either word
`grep -ow "[0-9]\{1,3\}$\.[0-9]\{1,3\}$\{3\}" file`	Extract IPv4-looking strings
`grep -Po '(?<=version=)\S+' file`	Print text after a prefix (lookbehind)
`grep -rn --include="*.py" "import os" .`	Search only Python files
`grep -C 3 "Exception" app.log`	Match with 3 lines of context
`grep -c "" file`	Count lines in a file
`grep -f patterns.txt file`	Use patterns from a file
`ps aux \| grep [n]ginx`	Search processes without matching grep itself
`docker logs <CONTAINER> 2>&1 \| grep -i error`	Filter container logs (stderr must be merged)

Regex Quick Reference #

PATTERN	MEANING
`.`	Any single character
`^` / `$`	Start / end of line
`*` `+` `?`	0 or more / 1 or more / 0 or 1 (`+` and `?` need `-E`)
`{n,m}`	Between n and m repeats (`-E`)
`[abc]` / `[^abc]`	Character in / not in the set
`[[:digit:]]` `[[:space:]]` `[[:alpha:]]`	POSIX classes
`(a\|b)`	Alternation (`-E`)
`\b`	Word boundary