File and Directory Permissions in UNIX / Linux

Source: UNIX / Linux: Beginners Guide to File and Directory Permissions

Unix file and directory permission is in the form of a 3×3 structure. i.e Three permissions (read, write and execute) available for three types of users (owner, groups and others).


Three file permissions:

• read: permitted to read the contents of file.
• write: permitted to write to the file.
• execute: permitted to execute the file as a program/script.

Three directory permissions:

• read: permitted to read the contents of directory (view files and sub-directories in that directory ).
• write: permitted to write in to the directory. ( create files and sub-directories in that directory )
• execute: permitted to enter into that directory.

Numeric values for the read, write and execute permissions:

• read 4
• write 2
• execute 1

To have combination of permissions, add required numbers. For example, for read and write permission, it is 4+2 = 6.

Change File and Directory Permissions Using Chmod Command
You can use either the octal representation or symbolic representation to change the permission of a file or directory.

Octal representation for permissions:

• First number is for user
• Second number is for group
• Third number is for others

For example, give read, write ( 4+2 = 6 ) to user and read ( 4 ) to group and others.

$ chmod 644 filename

For example, give read, execute ( 4 + 1 = 5 ) to user and read (4 ) to group, and nothing ( 0 ) to others.

$ chmod 540 filename

For example, give read, write ( 4 + 2 = 6 ) to user and nothing ( 0 ) to group, and read ( 4 ) to others.

$ chmod 604 filename