linux shell

gistfile1.txt

change to previous working directory:
cd -
cd $OLDPWD

backup file:
cp file{,.bak}

change file extension:
mv file{.old,.new}

ssh into remote computer:
ssh-keygen
ssh-copy-id remote-host

programs listening on ports:
netstat -nutlp

command substitution:
$ VAR=`command`
$ VAR=$(command)

automatically answer yes to any commands:
yes | command