grep "pattern" file

Examples:

grep "error" log.txt
grep -i "error" log.txt
grep -n "main" source.c

• -i → ignore case
• -r → recursive
• -n → line numbers
• -c → count
• -v → invert match
• -w → whole word

grep "^root" /etc/passwd
grep "[0-9]\{3\}" file
grep -E "error|fail|panic" log.txt

Find failed SSH logins:

grep "Failed password" /var/log/auth.log

Find function references:

grep -r "initDatabase" src/

• Insanely fast (written in C)
• Handles massive files
• Full regex engine
• Always installed
• Foundation of shell pipelines

grep is the backbone of Unix search.