Linux file permissions control who can read, write, and execute the contents of a file or directory. Each permission is represented by an abbreviation (r, w, or x) and an octal value. The permissions are broken into groups of three, and each position in the group denotes a specific permission: First three characters (2–4): Represent the permissions for the file's owner Second group of three characters (5–7): Consist of the permissions for the group to which the file belongs Last group of three characters (8–10): Represent the permissions for everyone else  The three digits of a permission... Show more

Linux file permissions control who can read, write, and execute the contents of a file or directory. Each permission is represented by an abbreviation (r, w, or x) and an octal value. The permissions are broken into groups of three, and each position in the group denotes a specific permission:
First three characters (2–4): Represent the permissions for the file's owner
Second group of three characters (5–7): Consist of the permissions for the group to which the file belongs
Last group of three characters (8–10): Represent the permissions for everyone else 

The three digits of a permission represent the access rights of the user who owns the file, the group, and other users:
4: If read permission is granted
2: If write permission is granted
1: If execute permission is granted
755 permissions: Mean you can do anything with the file or directory 

You can use the chmod command to change file permissions. The format of this command is: chmod permissions list_of_files. You can use a + or - (plus or minus sign) to add or remove permissions for a file respectively. 
You can also use the ls command to check Linux file permissions. For example, ls -h changes the way file sizes are displayed, and ls -a displays hidden files. 

Related Test: Linux Basics Practice Test: Linux File Management

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