Changing Permissions

The Linux command chmod changes file permissions. You may want to change permissions for some popular directories in order to avoid logging in as root to install or configure software.

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NOTE:  Unless you have write permission for a file or directory, you can’t change the permissions. Of course, this means that you need to be logged in as root in order to change permissions.

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Permissions can be changed in numeric or symbolic form. Neither method is what could be called intuitive, so we’ll spend some time explaining each of them.

The Numeric Method

The numeric method uses numbers to track permissions. Like the permissions listings earlier in this section, the numeric method divides permissions into threes, albeit in a different manner.

The numeric method forces you to add three different sets of numbers in determining who has which permissions. The actual types of permissions (owner, group, world) haven’t changed—only the method of listing them.

You’ll use modes to track permissions, as seen in Table 4.3.

You must now translate these numbers into the numeric form by adding them together. For example, using the following directory listing:

     -rwx—x—x  1 kevinr  group1     854 Apr  2 19:12 test

we arrive at a numeric permission of 711:

A file or directory that’s totally open to the world would have a permission of 777; a file or directory inaccessible to anyone would have a permission of 000.

Changing the permissions entails combining the desired permissions with the chmod command. For example, to change the file permissions of the test command to make it totally accessible to all users, you’d use the following command line:

     gilbert:/$ chmod 777 test

To change the permissions so that only the owner of the file has the ability to totally access the file and at the same time permission is denied to every other user, you’d use the following command line:

     gilbert:/$ chmod 700 test

To change the permissions so that the owner of the file has the ability to totally access the file, but other users and the group have the ability to read and execute (but not change) the file, you’d use the following command line:

     gilbert:/$ chmod 744 test

The Symbolic Method

When using the numeric method, you don’t need to know the existing permissions of the file, which means that you need enter only the desired permissions. The other main method of setting permissions, called the symbolic method, requires that you know the existing permissions, as you’re setting new permissions relative to the existing permissions.

The symbolic method eschews numerals and uses letters instead. And it’s very precise in adding or subtracting permissions relative to existing permissions. For example, the following command line gives execute permissions to the world (all users):

     gilbert:/$ chmod o+x data

Here, o refers to “others” (in chmod parlance, the world), x refers to execute permission, and the plus sign (+) adds the execute permission to others. If a minus sign (-) were used, this command line would remove execute permission from others.

The symbolic method uses some quirky language, as you’ve already seen with the reference to others. The owner of the file is referred to as the user, and setting permissions for the owner means using u:

     gilbert:/$ chmod u+x data

Setting the permission for the group is a matter of using g:

     gilbert:/$ chmod g+x data

These statements, of course, would be meaningless if the users already had the ability to execute the file.

Table 4.4 lists the various symbols used with the chmod command.

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NOTE:  If you create your own shell scripts or use the Perl language, you’ll need to set permissions to make your scripts usable.

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