sed

Substitute first match per line Replace the first occurrence of 'foo' with 'bar' per line sed 's/foo/bar/' file.txt #sed#linux Substitute all matches (global) Replace every occurrence per line with the /g flag sed 's/foo/bar/g' file.txt #sed#linux In-place edit Modify file in-place (no backup); on macOS add empty string: -i '' sed -i 's/old/new/g' config.cfg #sed#linux In-place edit with backup Edit in-place and save original as file.txt.bak sed -i.bak 's/old/new/g' file.txt #sed#linux Delete matching lines Delete all comment lines starting with # sed '/^#/d' config.txt #sed#linux Delete blank lines Remove all empty or whitespace-only lines sed '/^[[:space:]]*$/d' file.txt #sed#linux Print specific line numbers Print lines 10 through 20 only (suppress default output with -n) sed -n '10,20p' file.txt #sed#linux Print lines matching pattern Print only lines that match ERROR sed -n '/ERROR/p' app.log #sed#linux#debug Insert line before match Insert a comment line before every server_name directive sed '/^server_name/i # Added by deploy script' nginx.conf #sed#linux Append line after match Append a setting line after the [defaults] header sed '/^\[defaults\]/a ansible_python_interpreter=/usr/bin/python3' ansible.cfg #sed#linux Case-insensitive substitution GNU sed /I flag for case-insensitive match sed 's/error/ERROR/gI' file.txt #sed#linux Multiple expressions Apply multiple substitutions in one command sed -e 's/foo/bar/g' -e 's/baz/qux/g' file.txt #sed#linux Strip leading/trailing whitespace Trim leading and trailing whitespace from each line sed 's/^[[:space:]]*//;s/[[:space:]]*$//' file.txt #sed#linux Address range substitution Apply substitution only to lines 5 through 15 sed '5,15s/DEBUG/INFO/g' app.log #sed#linux Extract value from key=value file Print the value of DB_HOST from a .env file sed -n 's/^DB_HOST=//p' .env #sed#linux