grep

Quick Reference

grep [options] "pattern" file
command | grep [options] "pattern"

Flag	Purpose	Example
`-i`	Case-insensitive	`grep -i "error" log.txt`
`-r` / `-R`	Recursive (search directories)	`grep -r "TODO" src/`
`-n`	Show line numbers	`grep -n "main" app.py`
`-c`	Count matching lines	`grep -c "404" access.log`
`-l`	List filenames with matches only	`grep -rl "password" /etc/`
`-L`	List filenames without matches	`grep -rL "license" src/`
`-v`	Invert match (show non-matching lines)	`grep -v "^#" config.conf`
`-w`	Match whole words only	`grep -w "port" config.txt`
`-o`	Print only matched part	`grep -oP '\d+\.\d+\.\d+\.\d+' log.txt`
`-A N`	Show N lines after match	`grep -A 3 "error" log.txt`
`-B N`	Show N lines before match	`grep -B 2 "error" log.txt`
`-C N`	Show N lines around match	`grep -C 5 "panic" syslog`
`-E`	Extended regex (same as `egrep`)	`grep -E “(error
`-P`	Perl-compatible regex	`grep -P "\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}" file`
`-q`	Quiet mode (exit code only)	`grep -q "root" /etc/passwd && echo "found"`
`--include`	Filter file types in recursive search	`grep -r --include="*.py" "import" .`
`--exclude-dir`	Skip directories	`grep -r --exclude-dir=node_modules "TODO" .`
`--color`	Highlight matches	`grep --color=auto "error" log.txt`

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

grep -oP '\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b' access.log

grep -rn --include="*.js" "addEventListener" src/

grep -rc "error" /var/log/ | grep -v ":0$"

ps aux | grep nginx
cat /etc/passwd | grep -c "/bin/bash"
dmesg | grep -i "usb"
history | grep "ssh"

Command	Equivalent	Regex type
`grep`	—	Basic regex (BRE)
`egrep`	`grep -E`	Extended regex (ERE) — `+`, `?`, `
`fgrep`	`grep -F`	Fixed strings (no regex, fastest)

egrep and fgrep are deprecated aliases but still work everywhere.