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grep searches input for lines that match a pattern and prints the selected lines. Use a literal search with -F, regular expressions with the default basic mode or -E, and recursive options such as -r when you need to search a directory tree.
What grep does
GNU describes grep as a pattern-matching program: given one or more patterns, it searches input files for matches. In its normal mode, the result is each complete input line containing a match—not just the matching word or substring.
The general form is:
grep [option]... [patterns] [file]...
The pattern and file names are separate operands. If you omit a file, grep reads standard input, so it can filter another command’s output. Quote patterns in shell commands when they contain spaces, wildcard characters, dollar signs, parentheses, brackets, or other characters that the shell could interpret before grep sees them.
Essential grep examples
Search one file
grep 'error' app.log prints every line in app.log containing error. Matching is case-sensitive unless you request otherwise.
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Show line numbers
grep -n 'error' app.log prefixes each selected line with its line number, which is useful when opening the file in an editor or reporting a defect.
Ignore case
grep -i 'warning' app.log matches warning, Warning, and other case variants.
Search command output
ps aux | grep 'sshd' sends the output of ps aux to grep through a pipe. The same pattern can be used with logs, directory listings, package-manager output, or any command that writes text to standard output.
Include surrounding context
grep -C 2 'failed' app.log prints two lines before and two lines after every matching line. Use -B for only preceding context or -A for only following context.
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Search literal text
grep -F 'a.b' file.txt treats the period as an ordinary character. Without -F, the period is a regular-expression metacharacter that can match any single character.
Choose the matching mode
| Mode | Option | Use it when | Example |
|---|---|---|---|
| Basic regular expressions (BRE) | Default | You need regular-expression matching with the traditional grep syntax. | grep 'colou?r' file |
| Extended regular expressions (ERE) | -E |
You want operators such as +, ?, |, and parentheses without the older escaping rules. |
grep -E 'error|failed' app.log |
| Fixed strings | -F |
The pattern is exact text and must not be interpreted as a regular expression. | grep -F 'a.b' file.txt |
| Perl-compatible regular expressions | -P |
You specifically need PCRE syntax and your installed GNU grep supports it. | grep -P '(?<=ID:)d+' file.txt |
-E and -F are specified by POSIX. -P is an implementation-specific GNU option and is not a portable assumption across Unix systems. The exact BRE and ERE details also follow the implementation’s regular-expression rules.
Options that solve common search tasks
-i— ignore case distinctions.-n— print input line numbers.-v— invert the match and print lines that do not match.-w— select only whole-word matches where supported.-x— select only lines whose entire content matches.-l— print the names of files with at least one match instead of the matching lines.-L— print names of files with no match.-c— print a count for each file rather than matching lines.-q— quiet mode; stop after determining whether a match exists.-H— include file names in output.-h— suppress file names when several files are searched.-o— print only the matching portions of selected lines where supported.-C N,-B N,-A N— show context around matches.
Options can be combined. For example, grep -Rni --include='*.py' 'TODO' src/ searches Python files below src/, follows all symbolic links, ignores case, and prints line numbers.
Recursive searches
GNU grep: -r versus -R
grep -r 'TODO' src/ recursively searches directories below src/. GNU grep follows symbolic links supplied explicitly on the command line but skips symbolic links encountered while traversing directories. grep -R 'TODO' src/ follows all symbolic links, which can cause a search to leave the apparent tree or revisit files through links.
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Limit the files you inspect
GNU grep supports filters such as --include='*.js' and --exclude='*.min.js'. These long options are GNU extensions; check your platform’s man grep or info grep before relying on them in portable scripts.
Search hidden or binary content deliberately
Recursive searches may encounter generated, binary, or very large files. Add explicit include/exclude rules, or use the implementation’s binary-file options, when those files would create noise. Do not assume that every grep implementation handles binary data identically.
Patterns, quoting, and regular expressions
Single quotes usually provide the safest shell quoting: grep 'user$' file passes the dollar sign to grep instead of allowing the shell to expand it. Double quotes still allow shell substitutions such as $HOME and command substitution.
Shell filename globbing and regular-expression syntax are different mechanisms. In grep '*.log' file, the asterisk is a regex operator, not a shell filename wildcard, because it is inside quotes; it means “zero or more of the preceding item.” To search for literal wildcard characters, use -F or escape the metacharacters.
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Using grep in shell scripts
GNU grep normally returns status 0 when at least one line is selected, 1 when no line is selected, and 2 when an error occurs. That makes it suitable for conditions:
if grep -q '^enabled=true$' settings.conf; then
echo 'Feature is enabled'
fi
Do not treat every nonzero status as “no match”: status 2 indicates a problem such as an unreadable file or invalid pattern. GNU’s quiet mode has a documented exception: a found match returns 0 even if an error also occurred.
When you need to distinguish all outcomes explicitly:
grep -q 'ready' service.log
case $? in
0) echo 'found' ;;
1) echo 'not found' ;;
2) echo 'grep error' >&2; exit 2 ;;
esac
Portability: GNU, BSD, and POSIX grep
The basic command, BRE matching, and the POSIX -E and -F modes are broadly portable. GNU long options, -P, some binary-file controls, and certain context or filename-filter conveniences are not guaranteed by POSIX.
GNU -r and -R also have defined symlink behavior that may differ from another implementation’s recursive options. If a script must run on Linux, macOS, BSD, and other Unix systems, use the POSIX synopsis where possible and verify extensions with the local manual page. GNU documents both man grep and info grep as local references.
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- Searching for a literal dot or bracket: use
-F, for examplegrep -F '[INFO]' app.log. - Getting no results because of case: add
-ior use the exact capitalization. - Expecting only the matched word: ordinary grep prints the whole selected line; use
-owhere your implementation supports it, or a tool designed for field extraction. - Accidentally searching filenames through the shell: quote the pattern and remember that shell globs are not grep regexes.
- Missing files in a recursive search: check permissions, excluded paths, symlink behavior, and whether the files are binary.
- Breaking a script on status 1: distinguish “no match” (1) from an actual grep error (2).
Where grep fits
Use grep when your output is line-oriented and you need filtering, counting, filenames, or nearby context. Use -F for exact text, BRE or -E for regular expressions, and recursive options for directory trees. For structured data such as JSON or CSV, a parser is usually safer than treating the data as unstructured lines. Grep is free software, and the GNU Project publishes its source and documentation.
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