Show context lines

Print 3 lines before and 5 lines after each match

grep -B3 -A5 'FATAL' app.log

More grep commands

Count matches

grep -c 'POST /api' /var/log/nginx/access.log

Print only the number of matching lines
Print only matched part

grep -oP '\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}' access.log | sort -u

Extract IP addresses from log using Perl regex
Invert match

grep -v '^#' /etc/nginx/nginx.conf | grep -v '^\s*$'

Remove comment and blank lines from a config file
Search only in file type

grep -r --include='*.py' 'import os' /srv/app/

Limit recursive search to .py files only
Exclude directories

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

Search recursively skipping vcs and dependency dirs