Linux Shell Scripting Cookbook - Third Edition

Checking for super user

The UID environment variable holds the User ID. Use this value to check whether the current script is being run as a root user or regular user. Consider this example:

If [ $UID -ne 0 ]; then 
  echo Non root user. Please run as root. 
else 
  echo Root user 
fi

Note that [ is actually a command and must be separated from the rest of the string with spaces. We can also write the preceding script as follows:

if test $UID -ne 0:1 
  then 
    echo Non root user. Please run as root 
  else 
    echo Root User 
fi

The UID value for the root user is 0.