Important File and Directory permissions in Unix

Are you looking for how permissions works in the Unix Operating system? Would you be interested in knowing what types of permissions are available in the Unix environment? The details about permissions in the Unix are explained in this article. This article also provides highlights on various characteristics about File and Directory permissions.

What are the types of file permissions?

The file permissions categories are as follows:

• Owner permissions − It determines what actions the owner of the file can perform on the file.
• Group permissions − It determines what actions a user, who is a member of the group to which a file belongs, can perform on the file.
• Other (world) permissions − It indicates what action all other users can perform on the file. 

How to display Permissions?

•  To display permissions on screen use ‘ls –l’ command -> read (r), write (w), execute (x)          e.g.

          ls –l /usr/tmp

    -rwxr-xr-- 1 testuser users 1017 Jan 2 00:10 myfile

    drwxr-xr-- 1 testuer users 1017 Jan 2 00:10 mydir

• Here, the first column represents different access modes, i.e., the permission associated with a file or a directory. The first character ‘-‘ stands for the file and the character ‘d’ stands for the directory.
• 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. 

Understanding File access modes

There are three types of file access modes: Read, Write and Execute. Mentioned below are the details about each mode:

• 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. 

Understanding Directory access mode

There are three types of directory access modes: Read, Write and Execute. Mentioned below are the details about each mode:

• 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. 

How to change permissions?

Use the chmod (change mode) command to change permissions.

There are two ways to use chmod:

1. The symbolic mode
2. The absolute mode 

Symbolic mode

With symbolic permissions we can add, delete, or specify the permission set we want by using the operators

 + : Adds the designated permission(s) to a file or directory

  - : Removes the designated permission(s) from a file or directory

  = : Sets the designated permission(s)

a) Change permission for other users

                  chmod o+wx test1file

b) Change permission for owner user

                  chmod u-x testfile

c) Change permission for group

                 chmod g=rx testfile

d) Change permission for users and groups

                 chmod o+wx,u-x,g=rx testfile

Absolute Mode

Use a number to specify each set of permissions for the file

Number	Description	Detail
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

Examples

a) chmod 755 testfile (all, read-write, read-write)

b) chmod 743 testfile (all, read,write-execute)

c) chmod 043 testfile (no permission, read, write-execute)

More details about the file and directory are explained with examples in the video below: