Where does "getent group A" get the information on group A?

Question

I have some questions about getent group.

1. Where does getent group A get the information on group A?
2. Does it only get it from /etc/group?
3. Is it possible that I can find a user (belonging to group A) by getent group A while I can not find the user from group A in /etc/group? If yes, how could this happen?

The third one is actually the problem I met.

Example:

1st. [root@*** test]# getent group A | grep A

2nd. [root@*** test]# cat $INSTALLROOT/etc/group | grep A

I can find a user a from the first command while I cannot find the user a from the second command.

Answer 1

Where does "getent group A" get the information on group A?

It is configured in /etc/nsswitch.conf. man getent tells this.

Does it only get from /etc/group?

It depends on your config files. On my machine it does not get this information only from /etc/group since it is configured in this way:

group:      files ldap

is it possible that i can find a user (belong group A) by "getent group A" while can NOT find the user from group A in /etc/group?

Yes it is possible if there are at least two sources of data for groups. By the way, take a look at how getent group work - run it under strace.

strace -s 256 -o getent.strace.txt -v getent group

And see what system calls it does. In my case it first reade from /etc/group and loads an ldap module and starts reading from socket