Solving File Access Problems

When users cannot access files or directories that they previously could access, the permissions or ownership of the files or directories probably has changed.

Changing File Permissions

Table 71-1 shows the octal values for setting file and directory permissions. You use these numbers in sets of three to set permissions for owner, group, and other (in that order). For example, the value 644 sets read/write permissions for owner, and read-only permissions for group and other.

Changing File and Group Ownerships

Frequently, file and directory ownerships change because someone edited the files as superuser. When you create home directories for new users, be sure to make the user the owner of the dot (.) file in the home directory. When users do not own "." they cannot create files in their own home directory.

Access problems can also arise when the group ownership changes or when a group of which a user is a member is deleted from the /etc/group database.

Use the chown command to change file ownership.

# chown new-owner filename

Use the chgrp command to change group ownership.

# chgrp new-owner filename

How to Change File Permissions

1. List the file permissions.

   # ls -l filename

   -l	Displays the long listing, which includes current permissions for the file.
   filename	Is the specified file or directory.

2. Change the file permissions.

   # chmod nnn filename

   nnn	Are numbers representing the permissions you are assigning to the file owner, the group owner, and all others, in that order.
   filename	Is the specified file or directory.

   Permissions are changed using the numbers you specify.

   ---

   Note -

   You can change permissions on groups of files or on all files in a directory using meta characters such as (*) in place of file names or in combination with them.

   ---

3. Verify that the permissions have been changed by using the ls -l command.

   $ ls -l filename

   The long listing shows the current permissions for the file.

Example--Changing File Permissions

This example shows changing the permissions of a public directory from 744 (read/write/execute, read-only, and read-only) to 755 (read/write/execute, read/execute, and read/execute).

$ ls -ld public_dir
drwxr--r--  1 ignatz   staff    6023 Aug  5 12:06 public_dir
$ chmod 755 public_dir
$ ls -ld public_dir
drwxr-xr-x  1 ignatz   staff    6023 Aug  5 12:06 public_dir

This example show changing the permissions of an executable shell script from read/write to read/write/execute.

$ ls -l my_script
-rw------- 1 ignatz   staff    6023 Aug  5 12:06 my_script
$ chmod 700 my_script
$ ls -l my_script
-rwx------ 1 ignatz   staff    6023 Aug  5 12:06 my_script

How to Change File Ownership

You must own a file or directory (or have root permission) to be able to change its owner.

1. Become superuser.

2. List the file permissions.

   # ls -l filename

   -l	Displays the long listing, which includes the owner of the file, displayed in the third column.
   filename	Is the specified file or directory.

3. Change the file owner.

   # chown new-owner filename

   Ownership is assigned to the new owner you specify.

4. Verify the file ownership change.

   # ls -l filename

Example--Changing File Ownership

# ls -l quest
-rw-r--r--  1 fred   staff    6023 Aug  5 12:06 quest
# chown ignatz quest
# ls -l quest
-rw-r--r--  1 ignatz   staff    6023 Aug  5 12:06 quest

How to Change Group Ownership

1. List the file permissions.

   # ls -l filename

2. Change the group that owns the file or directory.

   $ chgrp GID filename

   The group ID for the file or directory you specify is changed.

3. Verify the file ownership change.

   # ls -l filename

Example--Changing Group Ownership

$ ls -l junk
-rw-r--r-- 1 kryten other 3138 Oct 31 14:49 junk
$ chgrp staff junk
$ ls -l junk
-rw-r--r-- 1 kryten staff 3138 Oct 31 14:49 junk

See Chapter 51, Securing Files (Tasks) for information about how to edit group accounts.