Permissions for directories

The meaning of the permissions is different for directories than it is for "normal" files. For normal files: r=permission to read the contents of the file, w=permission to modify the contents of the file, and x=permission to execute the file.

For directories: r=permission to list the filenames in the directory, w=permission to create or delete files in the directory, and x=permission to access the directory. Otherwise, the permissions are set the same way for directories as they are for normal files.

Default file permissions with umask. When a new file is created, it is given default permissions. On my system, these are:

-rw-r--r--

This means that files created by a user can be read and written by this user; the group and the others can only read the file. Still, on my default RedHat system, users cannot read the files in the other users' home directories because the permissions on the home directories are:

drwx------

I can check the default file permissions given to my newly created files using:

umask -S

(The option "-S" stands for "symbolic" and tells umask to display the permissions in an easy-to-read form, instead of the default numeric mode.)

I can change the default file permissions for newly created files using a command like:

umask u=rw,g=,o=

which will give the owner the read and write permissions on newly created files (r+w), and no permission to the group and others.

Using numbers to set default permissions with umask is more tricky. The number shows the permissions that you take away for users (opposite to chmod). Thus:

umask 000

will give full permissions to everybody on newly created files. The next example gives read and write permissions to the owner, and zero permissions for everybody else (seems that's what one might want):

umask 177

To make the settings permanent for all users on the system, adjust the appropriate line(s) in the file /etc/profile.