What is the difference between `su` and `su -` in CentOS?

Question

I tried to switch to 'root' in CentOS by running su. It does change to 'root', but it doesn't seem to be a real 'root' user.

For example, by default, ifconfig command in /sbin is not found in the path for a normal user, but for 'root', the command can be found. When I run su, I still get bash: ifconfig: command not found output when I try to run ifconfig.

However, when I run su -, ifconfig just works.

What is the difference between su and su - in CentOS?

Solution 1

Adding the - option affects your environment behavior. For all practical purposes, the environment is completely reset. In general, you likely want to use su - instead of plain su. From the man page:

 -l      Simulate a full login.  The environment is discarded except for HOME, SHELL, PATH, TERM, and USER.  HOME and SHELL are modified as above.
         USER is set to the target login.  PATH is set to ``/bin:/usr/bin''.  TERM is imported from your current environment.  The invoked shell is
         the target login's, and su will change directory to the target login's home directory.

 -       (no letter) The same as -l.

Solution 2

su - invokes a login shell, which among other things, ensures that root's .bashrc and other shell startup scripts are run, just like as if you'd logged in directly as root via console or SSH. root's profile usually sets your path to include /sbin which is where ifconfig generally lives.