Networking / Beginners

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The chmod command

The chmod command lets you change the permissions for a Linux file. Before explaining the syntax of the chmod command, you need to look at the cryptic way Linux reports file permissions. Linux grants three different types of permissions - read, write, and execute - for three different scopes: owner, group, and everyone. That's a total of nine permissions.

When you use the ls command with the -l option, the permissions are shown as a ten-character string that begins with a hyphen if the entry is for a file or a d if the entry is for a directory. Then, the next nine letters are the nine permissions, in this order:

• Read, write, execute for the owner
• Read, write, execute for the group
• Read, write, execute for everyone

The letters r, w, or x appear if the permission has been granted. If the permission is denied, a hyphen appears.

For example, suppose the ls -l command lists these permissions:

-rw-r--r--

You interpret this permission string like this:

• The first hyphen indicates that this is a file, not a directory.
• The next three positions are rw-. Therefore, the owner has read and write permission on this file, but not execute permission.
• The next three positions are r--. That means the group owner has read permissions but not write or execute permission.
• The last three positions are also r--. That means that everyone else has read permission but not write or execute permission.

The full syntax of the chmod command is pretty complex. However, you can do most of what you need to do with this form:

chmod specification file

Here, specification is in the form u=rwx, g=rwx, or o=rwx to set the permissions for the user (owner), group, and others (everyone). You don't have to specify r, w, and x; you just list the permissions that you want to grant. For example, to grant read and write permission for the user to a file named rescue.plans, use this command:

$ chmod u=rw rescue.plans

You can also combine specifications, like this:

$ chmod u=rw,g=rw,o=r rescue.plans

To revoke all rights for the user, group, or others, don't type anything after the equal sign. For example, this command revokes all rights for others:

$ chmod o= rescue.plans