Linux Commands | File Management | chmod Change File/Directory Permissions

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Linux/UNIX Permissions

Linux/UNIX permissions are represented with 10 characters.

For example, if you check directory files with the ls -lrt command, you can see output like this.

% ls -lrt
total 8
-rwxrw-r--   1 kimkc  staff   37  5 13 08:40 README
drwxr-xr-x   8 kimkc  staff  256  5 13 08:40 java--tutorial

To understand the 10 characters, split them into “1 character / 3 characters / 3 characters / 3 characters”.

  • Directory or file indicator (1 character)
  • Owner permissions (characters 2-4)
  • Group permissions (characters 5-7)
  • Other permissions (characters 8-10)

The permission section to check is the part such as drwx at the beginning.

  • r: read permission
  • w: write permission
  • x: execute permission

Using the example again:

  • The first character d means directory, and - means file. In the example, one entry is a directory and one entry is a file.
  • The next three characters, owner permissions rwx, mean the owner has read, write, and execute permissions.
  • The next three characters, group permissions rw-, mean the group has read and write permissions only.
  • The last three characters, other permissions --r, mean others have read permission only.

chmod Command

The chmod command changes file permissions. chmod is short for “change mode”.

When r/w/x permissions are represented as numbers, they are 4/2/1. Permissions for each section are granted by simple addition.

  • 7/7/7 means owner permissions / group permissions / other permissions. The number 7 is 4+2+1.

For example, running chmod 777 README changes the permissions of the README file to -rwxrwxrwx (777).

Syntax

chmod [option (example: 744)] [file path/name to change]
  1. Permissions are represented as numbers: read (4), write (2), and execute (1).
  2. Read and write = 4+2=6, read and execute = 4+1=5, write and execute = 2+1=3, read, write, and execute = 4+2+1=7.
  3. Permissions are set in three sections: me / the group I belong to / other groups.
  4. If I can read, my group can write, and others can execute, the value is 421.