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GNU Grep Cheat Sheet

A comprehensive cheat sheet for GNU grep, covering essential options, patterns, and usage examples for efficient text searching in files and streams.

Grep Basics and Usage

Basic Syntax

grep [OPTIONS] PATTERN [FILE...]

Searches for PATTERN in each FILE. If no files are specified, grep searches standard input. PATTERN can be a string or a regular expression.

Example:
grep 'hello' file.txt - Searches for ‘hello’ in file.txt

grep 'error' server.log - Searches for the word error in the server.log file.

grep -i 'warning' config.txt - Searches for the word warning case-insensitively in the config.txt file.

Common Options

-i, --ignore-case

Ignore case distinctions in both the PATTERN and the input files.

-v, --invert-match

Select non-matching lines.

-c, --count

Print only a count of matching lines per file.

-n, --line-number

Prefix each line of output with the line number within its input file.

-r, --recursive

Recursively search directories.

-l, --files-with-matches

Print only the names of files containing matches.

Examples with Options

grep -i 'error' *.log - Searches for ‘error’ case-insensitively in all .log files.

grep -v 'success' app.log - Shows lines that do NOT contain ‘success’ in app.log.

grep -c '404' access.log - Counts lines containing ‘404’ in access.log.

grep -n 'function' script.js - Shows lines containing ‘function’ with line numbers in script.js.

grep -r 'TODO' . - Recursively searches for ‘TODO’ in the current directory.

Regular Expressions in Grep

Basic Regular Expressions (BRE)

^

Matches the beginning of a line. Example: ^hello matches lines starting with ‘hello’.

$

Matches the end of a line. Example: world$ matches lines ending with ‘world’.

.

Matches any single character. Example: a.c matches ‘abc’, ‘aec’, etc.

*

Matches zero or more occurrences of the preceding character. Example: ab*c matches ‘ac’, ‘abc’, ‘abbc’, etc.

[]

Matches any single character within the brackets. Example: [aeiou] matches any vowel.

[^]

Matches any single character NOT within the brackets. Example: [^0-9] matches any non-digit.

Extended Regular Expressions (ERE)

+

Matches one or more occurrences of the preceding character. Example: ab+c matches ‘abc’, ‘abbc’, but not ‘ac’.

?

Matches zero or one occurrence of the preceding character. Example: ab?c matches ‘ac’ or ‘abc’.

|

Specifies an alternative. Example: cat|dog matches either ‘cat’ or ‘dog’.

()

Groups regular expressions. Example: (ab)+c matches ‘abc’, ‘ababc’, etc.

{n}

Matches exactly n occurrences of the preceding character/group. Example: a{3} matches ‘aaa’.

{n,m}

Matches between n and m occurrences of the preceding character/group. Example: a{1,3} matches ‘a’, ‘aa’, or ‘aaa’.

ERE Examples

grep -E '^(cat|dog)' file.txt - Finds lines starting with ‘cat’ or ‘dog’.

grep -E '[0-9]+$' data.txt - Finds lines ending with one or more digits.

grep -E 'a(bc)+d' file.txt - Finds lines containing ‘a’ followed by one or more ‘bc’ and then ‘d’.

grep -E 'colou?r' text.txt - Finds lines containing ‘color’ or ‘colour’.

Advanced Grep Usage

Context Control

-A NUM, --after-context=NUM

Print NUM lines of trailing context after matching lines.

-B NUM, --before-context=NUM

Print NUM lines of leading context before matching lines.

-C NUM, --context=NUM

Print NUM lines of output context.

--group-separator=SEP

Use SEP as a group separator. The default is --.

File and Directory Options

-d ACTION, --directories=ACTION

How to handle directories; ACTION can be read, skip, or recurse.

--exclude=GLOB

Skip files matching GLOB.

--include=GLOB

Search only files matching GLOB.

--exclude-dir=GLOB

Skip directories matching GLOB for recursive searches.

Examples of Context and File Options

grep -A 2 'error' logfile.txt - Shows ‘error’ lines and 2 lines after each match.

grep -B 1 'warning' code.txt - Shows ‘warning’ lines and 1 line before each match.

grep -C 3 'exception' debug.log - Shows ‘exception’ lines and 3 lines of context around each match.

grep --exclude='*.o' 'main' * - Searches for ‘main’ in all files except those ending with ‘.o’.

grep --include='*.txt' 'data' . - Searches for ‘data’ only in ‘.txt’ files in the current directory.

More Grep Pattern Options

Pattern Control Options

-e PATTERN, --regexp=PATTERN

Use PATTERN as the pattern; useful to protect patterns beginning with -.

-f FILE, --file=FILE

Obtain PATTERN from FILE, one per line.

-w, --word-regexp

Select only those lines containing matches that form whole words.

-x, --line-regexp

Select only those matches that exactly match the whole line.

Output Control Options

-m NUM, --max-count=NUM

Stop reading a file after NUM matching lines.

-o, --only-matching

Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.

-q, --quiet, --silent

Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected.

--color[=WHEN], --colour[=WHEN]

Surround the matching string with escape sequences to display it in color; WHEN is always, never, or auto.

Pattern Option Examples

grep -e '^abc' file.txt - Searches for lines starting with ‘abc’.

grep -f patterns.txt data.txt - Uses patterns from patterns.txt to search data.txt.

grep -w 'error' logfile.txt - Searches for the whole word ‘error’ in logfile.txt.

grep -x 'exact match' file.txt - Finds lines that exactly match ‘exact match’.

grep -m 10 'keyword' bigfile.txt - Stops after finding 10 lines containing ‘keyword’.

grep -o '[0-9]+' data.txt - Prints only the matching numbers in data.txt.
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