The chmod Command

Do you know how to rename a file you can't read? Better yet, do you know how other users can rename your files? Have you ever ftp'd a program from another host and been unable to run it?

The subject of file permissions, and how to manipulate them with the chmod command, is a good place to start learning about these situations.

First, let's create a file and examine its long listing. (In order to fit in the magazine, all the listings in this article are trimmed to fit.)

$ touch test_file
$ ls -l test_file
-rw-rw-r--   1 eric     users

Since I created this file, it makes sense that the third column shows my user name as the file's owner and that the fourth shows my group. (On some systems, the group name may be the same as the user name.) As you follow along in these examples, you will see your username in place of “eric”.

The leftmost column of the directory listing shows the file's mode. Mode is the term used to refer to a file's permissions. ls displays the file's type and mode together as a grouping of ten one-character fields:

The type field has several valid values. For the sake of this tutorial, we are only concerned with two: empty (-) for a regular file, and d for directories.

The other three columns cover the three classes of access that are stored for each file in a Unix-like file system. Linux (and Unix) evaluates access in terms of user ownership, group ownership and world (or other).

For each of these classes, rights are evaluated in terms of three operations: reading (r), writing (w) and executing (x). The permissions above specify “full” access for the owner, reading and writing for group, and only reading for world (an unusual combination used for demonstration). Those permissions specify that

If test_file were a very important document that we did not want anyone to be able to modify or delete, we would need to remove write access from group:

$ chmod g-w test_file
$ ls -l test_file
-rw-r--r--   1 eric     users

We see that the w for group is now replaced with a -, signifying that write permission is denied to members of the group users.

If test_file contained sensitive information that only members of the group users should be able to review:

$ chmod o-r test_file
$ ls -l test_file
-rw-r-----   1 eric     users

Now we see that the last triplet of the mode field, which specifies permissions for world, are all dashes. This means that other users who do not belong to the users group have no permissions to do anything with test_file whatsoever.

The command line usage for chmod mode looks like this:

chmod [options] new-mode filename

The new mode is specified in octal mode or symbolic mode. We'll cover symbolic mode first. In the first example we used g-w to remove write permission for group. As you might be able to guess, g stood for group, - for remove and w represented write permission.

$ chmod g+wx test_file
$ ls -l test_file
-rw-rwx---   1 eric     users

This operation added permission for group to write and execute.

Let's look at an example of these permissions in action.

$ chmod u-rwx test_file
$ ls -l test_file
----rwx---   1 eric     users
$ cat test_file
cat: test_file: Permission denied
$ cat .profile > test_file
bash: test_file: Permission denied

We are not able to display the file's contents because we do not have read access to our own file. When we specified u-rwx to chmod, we removed all access for the user (the file's owner). We were also denied permission when we attempted to add the contents of another file to it since we removed write access. (I should note that rm would still be able to delete this file, although it will normally request confirmation.)

$ chmod u+rwx test_file
$ ls -l test_file
-rwxrwx---   1 eric     users

When we specify u+rwx, all permissions are restored. Removing permissions from a file we own does not affect our ability to restore the permissions, because the mode is not stored in the file. It is stored in a structure called an inode entry. Only the owner of the file (and root) may modify this.