Well, what do you mean by a user's existence?
If a user acct exists, it's in /etc/passwd, a plain text file.
The user's name is the first field, the unique id ( 'uid' ) is the 3rd field of each record.
What kind of backup did you have in mind?
There's no (need to) chk; it's either there or its not..
Usually home for a user is
For a newbie, read & digest http://rute.2038bug.com/index.html.gz
to start with.
For more bash cmds http://tldp.org/LDP/Bash-Beginners-G...tml/index.html http://www.tldp.org/LDP/abs/html/
HTH & Welcome to LQ