1. Become superuser or get an equivalent role using sudo command/su command
2. First, assign a new UID to user using the usermod command.
3. Second, assign a new GID to group using the groupmod command.
4. Finally, use the chown and chgrp commands to change old UID and GID respectively. You can automate this with the help of find command.
5. Verify that the group owner of the file has changed using the ls command

Change a USER and GROUP ID on Linux

It cannot be stressed enough how important it is to make a backup of your system before you do this. Make a backup. Let us say, our sample user name is foo

1. Foo’s old UID: 1005
2. Foo’s new UID: 2005
3. Our sample group name: foo
4. Foo’s old GID: 2000
5. Foo’s new GID: 3000

Let us see some examples and commands to change a user and group ID in Linux:

Linux command to change UID and GID

To assign a new UID to user called foo, enter:
# usermod -u 2005 foo
To assign a new GID to group called foo, enter:
# groupmod -g 3000 foo
Verify that you changed UID and GID for given users with the help of ls command:
# ls -l
Please note that all files which are located in the user’s home directory will have the file UID changed automatically as soon as you type above two command. However, files outside user’s home directory need to be changed manually. To manually change files with old GID and UID respectively, enter:
# find / -group 2000 -exec chgrp -h foo {} \;
# find / -user 1005 -exec chown -h foo {} \;

The -exec command executes chgrp or chmod command on each file. The -h option passed to the chgrp/chmod command affect each symbolic link instead of any referenced file. Use the following command to verify the same:
# ls -l /home/foo/
# id -u foo
# id -g foo
# grep foo /etc/passwd
# grep foo /etc/group
# find / -user foo -ls
# find / -group sales -ls

Conclusion

This page explained how to change group and user ownership of a file in Linux operating systems using the command-line utilities such as chgrp and others. See man page of chgrp for more info here.