Linux - Changing user and group ownership

3.4.2.4. Changing user and group ownership

When a file is owned by the wrong user or group, the error can be repaired with the chown (change owner) and chgrp (change group) commands. Changing file ownership is a frequent system administrative task in environments where files need to be shared in a group. Both commands are very flexible, as you can find out by using the --help option.

The chown command can be applied to change both user and group ownership of a file, while chgrp only changes group ownership. Of course the system will check if the user issuing one of these commands has sufficient permissions on the file(s) she wants to change.

In order to only change the user ownership of a file, use this syntax:

chown newuser file

If you use a colon after the user name (see the Info pages), group ownership will be changed as well, to the primary group of the user issuing the command. On a Linux system, each user has his own group, so this form can be used to make files private:

jacky:~> id
uid=1304(jacky) gid=(1304) groups=1304(jacky),2034(pproject)

jacky:~> ls -l my_report
-rw-rw-r--  1 jacky   project       29387 Jan 15 09:34 my_report

jacky:~> chown jacky: my_report

jacky:~> chmod o-r my_report

jacky:~> ls -l my_report
-rw-rw----  1 jacky   jacky         29387 Jan 15 09:34 my_report

If jacky would like to share this file, without having to give everybody permission to write it, he can use the chgrp command:

jacky:~> ls -l report-20020115.xls
-rw-rw---- 1 jacky   jacky   45635 Jan 15 09:35 report-20020115.xls

jacky:~> chgrp project report-20020115.xls

jacky:~> chmod o= report-20020115.xls

jacky:~> ls -l report-20020115.xls
-rw-rw---- 1 jacky   project 45635 Jan 15 09:35 report-20020115.xls

This way, users in the group project will be able to work on this file. Users not in this group have no business with it at all.

Both chown and chgrp can be used to change ownership recursively, using the -R option. In that case, all underlying files and subdirectories of a given directory will belong to the given user and/or group.

Restrictions

On most systems, the use of the chown and chgrp commands is restricted for non-privileged users. If you are not the administrator of the system, you can not change user nor group ownerships for security reasons. If the usage of these commands would not be restricted, malicious users could assign ownership of files to other users and/or groups and change behavior of those users' environments and even cause damage to other users' files.