Append line after match

Append a setting line after the [defaults] header

sed '/^\[defaults\]/a ansible_python_interpreter=/usr/bin/python3' ansible.cfg

More sed commands

Case-insensitive substitution

sed 's/error/ERROR/gI' file.txt

GNU sed /I flag for case-insensitive match
Multiple expressions

sed -e 's/foo/bar/g' -e 's/baz/qux/g' file.txt

Apply multiple substitutions in one command
Strip leading/trailing whitespace

sed 's/^[[:space:]]*//;s/[[:space:]]*$//' file.txt

Trim leading and trailing whitespace from each line
Address range substitution

sed '5,15s/DEBUG/INFO/g' app.log

Apply substitution only to lines 5 through 15
Extract value from key=value file

sed -n 's/^DB_HOST=//p' .env

Print the value of DB_HOST from a .env file