Use "getent group" instead of reading "/etc/group" to get group information

[Dim-P]

Summary

SSIA, Fixes #13711 .

Test Plan

1. Install Netdata using netdata-installer.sh.
2. Uninstall Netdata using netdata-uninstaller.sh
3. Observe group creation, user assignment and group/user deletion runs as expected.

Additional Information

For users: How does this change affect me?

[ilyam8]

I suggest dev null stdout and stderr in this function so a caller doesn't need to do it

[Dim-P]

I will leave it as is due to the following call that needs to evaluate the function return value:
if get_group "${groupname}" | cut -d ':' -f 4 | grep -wq "${username}"; then

But I renamed group_exists() to get_group() to make the function purpose clearer.

[ilyam8]

As it is now, get_group == getent group. Do we need cat /etc/group (also can be just grep "^${1}:" /etc/group) at all? I see we used getent group before this PR and it wasn't a problem.

[Dim-P]

It's a fallback, see the other thread. You are right about cat, I am just used to do it this way for other reasons and forgot you can do grep directly. Not a big deal so I will merge it.

[ilyam8]

It's a fallback, see the other thread.

Yes, I understood that.

My point is - why do we need this fallback? We had no fallback previously and it wasn't a problem.

[Dim-P]

I agree but it was suggested by both @thiagoftsm and @Ferroin so I just added it.

[thiagoftsm]

Installation was successful after changes and netdata is running as expected, LGTM!