Access Rights

• Every user has a unique username and is member of 1+ groups.

• Every file and directory has a owner, a group and a set of permission flags.

• Flags specify permissions:

  • read, write and execute(rwx)

  • owner, group and others (ugo).

• Programs need to have execute (x) permission to run in shell

ls -lt

Permissions are controlled with the commands chmod, chown, chgrp

• Only owner can change permissions of a file

chmod

• chmod has 2 different syntaxes

Syntax 1

• assign (=), gives(+), or take away(-) permission

• who corresponds to --> u (user), g (group), o (other)

• permission corresponds to --> read (r), write (w), execute (x)

  • chmod who=permission

  • chmod who+permission

  • chmod who-permission

 ls -l catfile.txt
 chmod g=rwx catfile.txt
 ls -l catfile.txt

 ls -l catfile.txt
 chmod g-w catfile.txt
 ls -l catfile.txt

 ls -l catfile.txt
 chmod o+x catfile.txt
 ls -l catfile.txt

chmod n

• numeric shortcuts to change mode in a compact way.

• format: chmod n filename

  • n is a 3 digit number, values from 0 to 7

  • each digit is associated to user, group, other

• examples:

ls -l catfile.txt
chmod 644 catfile.txt
ls -l catfile.txt
chmod 700 catfile.txt
ls -l catfile.txt