Unix chmod and chown

Following on from Basic UNIX file commands, here’s a bit there wasn’t time for on changing metadata on files.

These two commands change the files permissions and ownership. Permissions is information associated with a file that decides who can do what with it. This was once called the file mode, which is why the command is chmod (Change MODe). Files also have owners, both individuals and group of users, and the command to change this is chown (CHOWNer).

chown is easiest, so I’ll start there. To make a file belong to fred the command is:

chown fred myfile

To change the owning group to accounts:

chown :accounts myfile

And to change both at once:

chown fred:accounts myfile

Changing a files permissions is more tricky, and there are several ways of doing it, but this is probably the easiest to remember. You’ll recall that each file has three sets of permissions: Owner, Group and Other. The permissions themselves are read, write and execute (i.e. it’s an executable program).

chown can set or clear a load of permissions in one go, and the format is basically the type of permission, and ‘+’ or ‘-’ for set or clear, and the permissions themselves. What? It’s probably easier to explain with a load of examples.:

chmod u+w myfile

Allows the user of the file to write to it (u means user/owner)

chmod g+w myfile

Allows any user in the group the file belongs to write to it.

chmod o+r myfile

Allows any user who is not in the files group or the owner to read it. (o means “other”).

You can combine these options

chmod ug+rw

Allows the owner and the group to read and write the file.

chmod go-w

Prevents anyone but the user from being able to modify the file.

If you want to run a program you’ve just written called myprog.

chmod +x myprog

If you don’t specify anything before the +/- chmod assumes you mean yourself.

You might notice an ‘x’ permission on a directory – in this case it means the directory is searchable to whoever has the permission.