Linux commands

find - search files or directories
netstat - displays active TCP connections, ports on which the computer is listening, routing tables and network protocol statistics
diff - compare files line by line
ls - list directory and files
sed - stream editor (edit from command line)
du - disk usage
grep - search text or regexp pattern in files
nmap - network scanner
tar - tar is used for collecting many files into one archive file for distribution or backup
shred - overwrite a file to hide its contents, and optionally delete it rsync - a fast, versatile, remote (and local) file-copying tool