Process ID, User ID

[Process ID]
echo $$


[User ID]
echo $UID
id
id -nu
id -u

----------------------------

echo $_ # Just called /bin/bash to run the script.

ls >/dev/null # no output from command.
echo $_ # ls

ls -al >/dev/null # no output from command.
echo $_ # -al (last argument)


$ !! # last command
$ ![any char] #last command with that character