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In my attempt to delete the admin user group I accidentally deleted the adm system group. I "added it back" by issuing addgroup --system adm. I have a feeling this is going to bite me back later. Is there anything else that has to be done? It was added with GID 126 and I couldn't find the default GIDs anywhere.

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    You definitely want to add it with the GID and obviously re-add any users to the group. Other than that I think you are OK. For me the adm GID is 4 on Debian Jessie. You are maybe better getting it from an installation disk for your release though. – Graeme Apr 13 '14 at 11:16
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    @Graeme Thanks! The GID was 4 alright, I listed all groups's GIDs and 4 was missing. adm had no users, while trying to remove admin and accidentally removing adm I used --only-empty. – Alex Apr 13 '14 at 12:00
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The adm group on Debian has a statically-allocated GID, which is 4. There are no system users in that group (and there shouldn't be any human users either).

So you don't need to do anything beyond adding the adm group back:

addgroup --gid 4 adm

You can also restore the group automatically (as well as undo any other changes that you made to the Debian standard system users and groups) by running the command update-passwd (update-passwd -n to see what changes would be done but not perform them).

You can find the description of system users in /usr/share/doc/base-passwd/users-and-groups.txt.gz. The adm group is the owner of some log files; that's all the use it gets in Debian. If you stayed without the adm group overnight, some crontabs that perform log file management (especially rotation) may have failed.

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