grep

To find a string inside files in a folder, use

grep -rnw ./ -e "STRING_TO_FIND"

Or

grep -irl "STRING_TO_FIND" ./

Or

grep -ir 'STRING_TO_FIND' ./ | cat

To search for multiple strings, use

netstat -lntp | grep -E "(dovecot|speed)"

See ack better grep for programmers