Hello friends, please help to figure how to retrieve user's email using userid - thanks!

Also, if you can suggests good Linux book for commands

There are a lot of programs to use for retrieving mail. The first one that springs to my mind is Thunderbird. Can you go into a little more detail of the approach you intend?

If you want a book about the shell and the programs you can maybe start with The Linux Command Line. It's available as a free-of-charge e-book or can be purchased from your local book store. In the beginning it might be easier to buy the paper edition and use it along side your computer. If nothing else, then you are voting with your wallet that you want more such books.

About all the other "commands" on your computer, they are just programs that you have on your system, either pre-installed by your distro or added after installation by you yourself. For example, even "ls" is just a program on the system. You can string several programs together with a pipe to get an inventory of what is on your system.

Then interesting ones can be explored with "man"

so when I do this,
for i in <userid>; do getent passwd $i; done
I can get user's name but curious if I can get user's email as well...thank you.

No. To see what it can do: man getent

Tell us about your Linux system: IF it stores mail locally, look into: ls -l /var/spool/mail

Scroll down a page, to "Mail user-agents", in: http://tille.garrels.be/training/tld...#sect_10_04_02

Here's my Thread about books, where onebuck added links: http://www.linuxquestions.org/questi....php?p=5615707

Welcome & Best wishes! (in posting questions, tell more details about what you want to accomplish)

i can see many user's username mentioned here at this location - /var/spool/mail

Do you have 'root' privileges/access (to read them)?
Do you have any mail-reader [MUA] programs installed? man -k mail
(theoretically, IF you just wanted to view/look-at them, you can more/less/cat those files)
Please do some research/studying/experimenting and let us know. Best wishes on learning!