$ grep [OPTIONS] PATTERN [FILE...]

Key Flags

-rRecursive — search all files in a directory -nShow line numbers with each match -iCase-insensitive matching -vInvert — show lines that do NOT match -cCount matching lines instead of printing them -EExtended regex (alternation, +, ?, groups) -lList only filenames that contain a match

Examples

$ grep -rn 'ERROR' /var/log/

/var/log/syslog:4821: kernel: ERROR unable to enumerate USB device
/var/log/auth.log:1203: pam_unix: ERROR conversation failed

Recursive search with line numbers — great for log triage

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

Port 22
PermitRootLogin no
PasswordAuthentication yes
PubkeyAuthentication yes

Strip comments — shows only active config lines

$ grep -c 'Failed password' /var/log/auth.log

347

Count brute-force attempts without reading every line