[CentOS-docs] becoming root

Sun Apr 6 01:11:31 UTC 2008
Ned Slider <nedslider at f2s.com>

Hi List,

I've just drafted a FAQ/mini-HOWTO on becoming root as this is a topic I 
see come up time and time again.

Perhaps someone with a reasonable understanding could check it for 
technical correctness, and if anyone would like to offer comments/feedback??

Any suggestions as to where might be an appropriate home for this on the 


(attached below)


*How to become root*

Many commands can only be run as the root user so to run these commands 
we need to become "root". To do this, we use the su command (substitute 

The su command takes the following format:

su - <user>

but most commonly we will use su to become the root user:

su - root

If no username is specified, then the root user is assumed, so the above 
is often shortened to:



su -

but the above are NOT the same thing.

Often a user will become root using just 'su', try to run a command (eg, 
ifconfig), and get a 'command not found' error:

bash: ifconfig: command not found

The reason is that regular system users and the root user have different 
PATHS (you can view a users PATH with 'echo $PATH'). When you type a 
Linux command, the shell with search the users PATH to try to locate the 
command to run. It starts searching each directory on the PATH until a 
match is found. Commands for regular users are mostly located in 
/usr/local/bin, /usr/bin, and /bin. However, root commands are mostly 
located in /usr/local/sbin, /usr/sbin, and /sbin and root's PATH 
reflects this difference.

When you become root by using 'su -', you also adopt root's PATH whereas 
using just 'su' retains the original users PATH, hence why becoming root 
using just 'su' and trying to run a command located in /usr/local/sbin, 
/usr/sbin, or /sbin results in a 'command not found' error.

So you either need to specify the full PATH to the command if you just 
used 'su' (eg, /sbin/ifconfig) or use the full 'su -'.