chmod Command in Unix( File Permissions)

In Unix,files access is managed through the file permissions, attributes, and ownership. This ensures that only authorized users and processes can access files and directories.chmod command in Unix is used to change the access permissions of files and directories.

We will learn below topics in this article:

• • • File Security in Unix
    • Unix User class and classes of permissions for a file
    • Absolute mode to change file permissions
    • symbolic mode to change file permissions
    • Use of chmod command in Unix

File Security

To grant or restrict access to the file , Unix makes use of file access permission.

Unix comes with excellent security features, customization file permissions are one of them. Unix file permissions allow you to define who can read, write, and execute each file on your system.

Files can be secured through UNIX file permissions.
Unix assigns three types of owners to each file: User, Group, and Other.This access controls makes the Unix system extremely secure.

Unix users class 

In Unix, each file  is associated with an owner and a group and assigned with permission access rights for three different classes of users:

• • Owner: Anyone who creates a new file in a Unix system will automatically be granted User rights over that file.
  • Group: Group usually contains more than one users. Everyone who belongs to the same user group as User will be automatically the member of Group. Group is used to assign group permissions to a certain file for one or more users.
  • Others: Other means everyone else who can access the file. Unix treats them as the third kind of owners, and you can set separate permissions for them
  • All: it includes all users.

File ownership can be changed using chown command in Unix and chgrp command. We will further learn about it in this topic.

Classes of Permissions for a file

There are three file permissions that apply for each class. They are as follows:

1. 1. 1. Read (r) : The Read permission grants users the right to open or read a file. The user can only see the contents of the file but cannot modify it. When the Read permission belongs to a directory, the user can only list its contents but cannot modify or delete it.
         • • character ‘-‘ means the directory’s content cannot be shown.
           • character ‘r’ means the directory’s content can be shown
      2. Write (w) : Write allows users to edit the contents of a file. When the Write permission is set on a directory, users can add, rename, and remove all the files.
         • • character ‘-‘ means the directory’s content cannot be altered.
           • character ‘w’ means the directory’s content can be altered
      3. Execute (x) :The Execute permission means that a user can run the file as a program.
         • • character ‘-‘ means the directory cannot be changed to.
           • character ‘x’ means the directory can be navigated using cd command in Unix.

Two modes of setting file permissions 

Permission of a file can be changed using any of the two modes:

• • • Absolute Mode (Octal Integer) 
    • Symbolic Mode (String)

       1. Absolute Mode(Octal Integer)

• • Uses numbers for mentioning the permissions
  •  For example numerical equivalent for all permissions on a file are 777

SYNTAX:chmod [OPTIONS] NUMBER FileName

While using absolute mode you can set permissions for all three user classes (owner, group, and all others).
When 3 digits number is used, the first digit represents the permissions of the file’s owner, the second one the file’s group, and the last one all other users.

read, write and execute permissions has following value:

• • • r (read) = 4
    • w (write) = 2
    • x (execute) = 1
    • no permissions = 0

EXAMPLE:

Below example shows how permission is set for a fle. Here to give read, write and execute permission to the file’s owner, read and execute permissions to the file’s group and only read permissions to all other users, do the following:

Owner: rwx=4+2+1=7
Group: r-x=4+0+1=5
Others: r-x=4+0+0=4
Number 754, which represents the desired permissions.