Ownership with chown and chgrp

Permissions apply to an owner and a group — so to truly control access you also need to control who owns a file. That is the job of chown and chgrp.

Changing the owner with chown

chown alice notes.txt             # make alice the owner
chown alice:developers notes.txt  # owner alice, group developers

Changing the group with chgrp

chgrp developers notes.txt        # group becomes developers

chown user:group file can do the same thing, so chgrp is optional — use whichever you find clearer.

Changing whole trees

chown -R alice:developers /srv/www   # apply to a directory and everything inside

sudo chown root:root config.yml

Why does this matter?

Consider a web server. The web user needs to read your site files, but you don't want to hand out write permission to the world. The clean solution is a group:

sudo chown -R alice:www-data /var/www/mysite
sudo chmod 750 /var/www/mysite

Now alice owns everything, the www-data group can read and execute, and everyone else gets nothing.

Try it

1. ls -l /etc/hostname — who owns it?
2. chown $(whoami) /tmp/copy.txt after copying a system file — observe what changed
3. groups — which groups are you a member of?