su vs. 'su -' The Mystery Revealed

2008-06-24

Inquiring minds want to know, what is this su - and why is it different than su? Well, if you are like me, you didn’t even know that su - existed. I ran into this problem when I was trying to add a new user to a new Linux machine. Remote root login was disabled(for obvious reasons:D) and I was logged in with my regular non-privileged user account. Well, normally the story goes like this...

1. su
2. Authenticate
3. useradd account
4. set password
5. log out of privileged account
6. Take coffee break from all the hard work and call it a day

No so today:( . When I attempted to run step three(useradd), I received a command not found error. "That's weird" I thought. How could a machine not have this basic command. I looked in the bin directory to see if it was some kind of path error I was having. Thinking that this was the case, I was shocked to see that none of the user commands(adduser, usermod, deluser) where there.

After a few minutes of scratching my head, I asked someone more versed in Linux than I what they thought. After walking through each of the steps I had taken, my mistake was easily spotted. I hadn’t added the - to the su command. Thinking that the explanation was weird, I checked with google for the su - command. Sure enough, the link to Wikipedia had this to say about it,

Optionally, you can use a hyphen with su to invoke a login shell and assume the target user’s complete user environment:

I am glad I had someone to point this out to me… It could have been a longer and very frustrating ordeal. It was also a lesson for me to refer more to the man pages(it had the answer there too). I hope this blog entry helps someone else who is confronted with this situation.