grep

Overview

grep searches input lines for a pattern. GNU grep supports basic/extended/Perl-like regexes, recursive directory search, and rich context controls. Still the default everywhere; rg (ripgrep) is a popular faster alternative when installed.

Syntax

grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] -e PATTERN ... [FILE...]
command | grep [OPTIONS] PATTERN

Common Options

Option	Description
-E	Extended regex (ERE)
-F	Fixed strings (no regex)
-P	Perl-compatible regex (PCRE) if built-in
-i	Case-insensitive
-v	Invert match
-n	Line numbers
-H / -h	With / without filename
-r / -R	Recursive (follow / include symlinks differently)
-I	Skip binary
-l / -L	Files with / without matches
-c	Count matches per file
-A/-B/-C n	Context after/before/both
-w	Whole word
-o	Only the match
-m n	Stop after n matches per file
--color=auto	Highlight
-q	Silent (exit status only)

Key Use Cases

1. Filter logs and command output
   
2. Code/config search in trees
   
3. Scripting success/failure via exit status
   
4. Extract substrings with -o

Examples with Explanations

Basic

grep error /var/log/syslog
grep -i error /var/log/syslog

Extended regex

grep -E 'error|warn|fail' app.log

Fixed string (fast, safe)

grep -F 'https://example.com/path?x=1' urls.txt

Recursive code search

grep -RIn --exclude-dir=.git --exclude-dir=node_modules 'TODO' .

Context

grep -n -C3 'OutOfMemory' heap.log

Files that match

grep -rl 'Listen 80' /etc/apache2 2>/dev/null

Exit status in scripts

if grep -q 'ready' /tmp/status; then
  echo ok
fi

Only matching part

grep -oE '[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+' access.log | sort -u

Invert

grep -v '^#' /etc/ssh/sshd_config | grep -v '^$'

Understanding Output

Default: matching lines. With multiple files, lines are prefixed by filename. Exit codes: 0 match, 1 no match, 2 error.

Common Usage Patterns

Pipe from journalctl

journalctl -u nginx --since today | grep -i error

Count HTTP status codes

grep -oE 'HTTP/[0-9.]+" [0-9]{3}' access.log | awk '{print $NF}' | sort | uniq -c

Notes & Pitfalls

• Quote patterns to stop shell globbing.
  
• Basic regex treats +/| differently than -E.
  
• Binary matches print “Binary file … matches”; use -a or -I.
  
• For huge repos, consider rg or git grep.

Related Commands

• rg — ripgrep (if installed)
  
• ack / ag — other code search tools
  
• sed / awk — transform, not just filter
  
• find … -exec grep — older recursive pattern
  
• journalctl / dmesg — log sources

Additional Resources

• man grep
  
• GNU grep manual