Changing Permissions (chmod)

Use the chmod command to change permissions for a file or directory. You must be the owner of a file or directory, or have root access, to change its permissions. The general form of the chmod command is:

chmod permissions name

In this example, permissions indicates the permissions to be changed and name is the name of the affected file or directory.

You can specify the permissions in several ways. Here is one of the forms that is easy to use:

1. Use one or more letters to indicate the type of users.

   • u (for the user)

   • g (for group)

   • o (for others)

   • a (for all three of the previous categories.))

2. Indicate whether the permissions are to be added (+) or removed (-).

3. Use one or more letters to indicate the permissions.

   • r (for read)

   • w (for write)

   • x (for execute)

In the following example, write permission is added to the directory carrots for users who belong to the same group (thus, permissions is g+w and name is carrots).

$ cd veggies2
$ ls -l
drwxr-xr-x   2 user2    users        512 Nov  1 09:11 carrots
$ chmod g+w carrots
$ ls -l
drwxrwxr-x   2 user2    users        512 Nov  1 09:11 carrots
$

The chmod g+w carrots command in the previous example gives the group write permission on the file carrots. The hyphen (-) in the set of permissions for group is changed to a w.

To make this same directory unreadable and unexecutable by other users outside your group type the following commands.

$ ls -l
drwxrwxr-x   2 user2    users        512 Nov  1 09:11 carrots
$ chmod o-rx carrots
$ ls -l
drwxrwx---   2 user2    users        512 Nov  1 09:11 carrots
$

Now, the r (for read) and the x (for execute) in the set of permissions for other users are both changed to hyphens (-).

When you create a new file, the system automatically assigns the following permissions.

-rw-r--r--

When you create a new directory, the system automatically assigns the following permissions.

drwxr-xr-x

For example, to make a new file turnip executable by its owner (user2), type the following command.

$ ls -l turnip
-rw-r--r--   1 user2    users        124 Nov  1 09:14 turnip
$ chmod u+x turnip
$ ls -l turnip
-rwxr--r--   1 user2    users        124 Nov  1 09:14 turnip
$

If you want to change permissions for all categories of users, use the -a option of the ls command. To make a new file garlic executable by everyone, type the following command.

$ ls -l garlic
-rw-r--r--   1 user2    users        704 Nov  1 09:16 garlic
$ chmod a+x garlic
$ ls -l garlic
-rwxr-xr-x   1 user2    users        704 Nov  1 09:16 garlic
$

The x in the output of the ls -l command indicates garlic is executable by everyone.

You can also use the * wildcard character to change permissions for groups of files and directories. For example, to change the permissions for all the files in the current directory veggies so that the files can be written by you alone, type the following command.

$ pwd
/home/user2/veggies
$ ls -l
-rwxrwxrwx   1 user2    users       5618 Nov  1 09:18 beets
-rwxrwxrwx   1 user2    users       1777 Nov  1 09:18 corn
-rwxrwxrwx   1 user2    users       3424 Nov  1 09:18 garlic
-rwxrwxrwx   1 user2    users      65536 Nov  1 09:18 onions
$ chmod go-w *
$ ls -l
total 152
-rwxr-xr-x   1 user2    users       5618 Nov  1 09:18 beets
-rwxr-xr-x   1 user2    users       1777 Nov  1 09:18 corn
-rwxr-xr-x   1 user2    users       3424 Nov  1 09:18 garlic
-rwxr-xr-x   1 user2    users      65536 Nov  1 09:18 onions
$

Perform this chmod operation on the current directory only.