Searching files grep (r - recursive, s - no messages, n -line number, i - case insensitve, H -with filename)
grep -rsniH your_string
match lines containing the string "I am a cat" or the string "I am a dog".
grep "I am a \(cat\|dog\)"
Search application to find PID (process ID) of a certain application or process,
find application port
netstat -a | grep ftp