Access control

Multi-user system require a way to control what level of access each user has to a specific file or directory.

In Unix-like systems the access is governed through file permissions.

Viewing file permissions

Use the ls -l /path/to/file to view the permissions

ls -alh /bin/bash
# prints: -rwxr-xr-x 1 root root 1.2M Aug  1 22:56 /bin/bash

How are permissions structured

There are 3 types of access:

  • Read - view the contents of a file / directory
  • Write - modify the contents of a file / directory
  • Execute - execute a file (run as command) / open a directory

In Unix-like systems permissions are applied on 3 levels:

  • the user owning the file
  • the group owning the file
  • anyone else

When listing permissions we see that are 3 triplets:

spaces added for clarity

rwx r-x r--

In this example:

  • The user has full access
  • Members of the group can read and execute
  • Everyone else can only read

Permissions as octal numbers

Another way to represent the same information is using octal numbers:

rwx r-x r--

# same as

754

This is achieved by assigning a unique, complimentary number to each permission and summing them:

  • Read: 4
  • Write: 2
  • Execute: 1

So full access is 4+2+1 = 7

Modifying permissions

Use the chmod command to set the permissions for a file / directory:

# add the execute permissions for all levels to a file
chmod +x filename

# remove the execute permissions for all levels to a file
chmod -x filename

Use the octal representation to set the full permissions of a file / directory:

# only the owning user has full access to the file
chmod 700 filename

Useful Resources