Unix / Linux - File Permission / Access Modes
File ownership is an important component of Unix that provides a secure method for storing files. Every file in Unix has the following attributes −
Owner permissions − The owner's permissions determine what actions the owner of the file can perform on the file.
Group permissions − The group's permissions determine what actions a user, who is a member of the group that a file belongs to, can perform on the file.
Other (world) permissions − The permissions for others indicate what action all other users can perform on the file.
The Permission Indicators
While using ls -l command, it displays various information related to file permission as follows −
$ls -l /home/amrood
-rwxr-xr-- 1 amrood users 1024 Nov 2 00:10 myfile
drwxr-xr--- 1 amrood users 1024 Nov 2 00:10 mydir
Here, the first column represents different access modes, ex - the permission associated with a file or a directory.
The permissions are broken into groups of threes, and each position in the group denotes a specific permission, in this order: read (r), write (w), execute (x) −
The first three characters (2-4) represent the permissions for the file's owner. For example, -rwxr-xr-- represents that the owner has read (r), write (w) and execute (x) permission.
The second group of three characters (5-7) consists of the permissions for the group to which the file belongs. For example, -rwxr-xr-- represents that the group has read (r) and execute (x) permission, but no write permission.
The last group of three characters (8-10) represents the permissions for everyone else. For example, -rwxr-xr-- represents that there is read (r) only permission.
File Access Modes
The permissions of a file are the first line of defense in the security of a Unix system. The basic building blocks of Unix permissions are the read, write, and execute permissions, which have been described below −
Read
Grants the capability to read, i.e., view the contents of the file.
Write
Grants the capability to modify, or remove the content of the file.
Execute
User with execute permissions can run a file as a program.
Directory Access Modes
Directory access modes are listed and organised in the same manner as any other file. There are a few differences that need to be mentioned −
Read
Access to a directory means that the user can read the contents. The user can look at the filenames inside the directory.
Write
Access means that the user can add or delete files from the directory.
Execute
Executing a directory doesn't really make sense, so think of this as a traverse permission.
A user must have execute access to the bin directory in order to execute the ls or the cd command.
Changing Permissions
To change the file or the directory permissions, you use the chmod (change mode) command. There are two ways to use chmod — the symbolic mode and the absolute mode.
Using chmod in Symbolic Mode
The easiest way for a beginner to modify file or directory permissions is to use the symbolic mode. With symbolic permissions you can add, delete, or specify the permission set you want by using the operators in the following table.
| Sr.No. | Chmod operator & Description |
|---|---|
| 1 | + Adds the designated permission(s) to a file or directory. |
| 2 | - Removes the designated permission(s) from a file or directory. |
| 3 | = Sets the designated permission(s). |
chmod command from the preceding table is run on the testfile, followed by ls –l, so you can see the permission changes −
$chmod o+wx testfile
$ls -l testfile
-rwxrwxrwx 1 amrood users 1024 Nov 2 00:10 testfile
$chmod u-x testfile
$ls -l testfile
-rw-rwxrwx 1 amrood users 1024 Nov 2 00:10 testfile
$chmod g = rx testfile
$ls -l testfile
-rw-r-xrwx 1 amrood users 1024 Nov 2 00:10 testfile
Here's how you can combine these commands on a single line −
$chmod o+wx,u-x,g = rx testfile
$ls -l testfile
-rw-r-xrwx 1 amrood users 1024 Nov 2 00:10 testfile
Using chmod with Absolute Permissions
The second way to modify permissions with the chmod command is to use a number to specify each set of permissions for the file.
Each permission is assigned a value, as the following table shows, and the total of each set of permissions provides a number for that set.
| Number | Octal Permission Representation | Ref |
|---|---|---|
| 0 | No permission | --- |
| 1 | Execute permission | --x |
| 2 | Write permission | -w- |
| 3 | Execute and write permission: 1 (execute) + 2 (write) = 3 | -wx |
| 4 | Read permission | r-- |
| 5 | Read and execute permission: 4 (read) + 1 (execute) = 5 | r-x |
| 6 | Read and write permission: 4 (read) + 2 (write) = 6 | rw- |
| 7 | All permissions: 4 (read) + 2 (write) + 1 (execute) = 7 | rwx |
