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Tuesday, May 12, 2015

How to add and delete a user from a Linux group

Being in the right Linux group expedites many system administration tasks, and will save you time. For example, the Apache web server logs have the following file ownership and permissions.

# ls -al /var/log/apache2/*.log
-rw-r----- 1 root adm 882984 May 11 12:14 /var/log/apache2/access.log
-rw-r----- 1 root adm 418 May 11 01:55 /var/log/apache2/error.log
...

To read the Apache logs, you need root permissions. However, there is a shortcut that does not require you to run sudo. Note that adm - the admin group for Debian-based systems - is the group owner of the log files. So, if you become a member of adm, you don't need to sudo to read the log files.

To add peter to the adm group, execute any of the following commands:

  • $ sudo usermod -aG adm peter

  • $ sudo gpasswd -a peter adm

To verify that peter is now a member of the adm group, execute any of the following commands:

  • $ id -nG peter
    peter adm www-data

    You may be tempted, as I was, to not specify peter in the above command. Don't skip the parameter. Without the user parameter, you won't see the effect of the change in group membership - unless you log out and log back in. If you are running X, it means you have to log out of X, not just opening a new command shell window within the same X session.

  • $ groups peter
    peter : peter adm www-data

    Again, specify peter in the command. Otherwise, you must log out and then log back in before executing the command.

  • $ grep adm /etc/group
    adm:x:4:peter

If you have made a mistake, and now want to remove peter from the adm group, run any of the following commands:

  • $ sudo gpasswd -d peter adm
    Removing user peter from group adm

  • $ sudo deluser peter adm
    Removing user `peter' from group `adm' ...
    Done.

Besides the adm group, you should consider adding yourself to the www-data group. The Apache web server runs under the www-data user account on Debian systems. As a member of the www-data group, you can more easily modify web server files and directories.

An earlier post on group membership appears here.

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