How to avoid starting multiple nested root shells?

Question

Consider the following Bash alias:

alias su='sudo -s'

The problem one might encounter is running it (that alias) while already root.

Demo:

$ su
[sudo] password for vlastimil:                
# su
# su
# 
exit
# 
exit
# 
exit
$ 

Is there a way to avoid the nested shell sessions?

Important note: I am using the same alias file for my user and root, not separate.

Answer 1

su and sudo -s are different commands with different APIs and semantic and behaviour, you shouldn't alias one to the other.

Here it seems you want a command that starts root's login shell as root via sudo but only when invoked from a non-privileged shell, so I would write a script called rootshell for instance as:

#! /bin/sh -

die() {
  printf >&2 '%s\n' "$@"
  exit 1
}

[ "$#" -eq 0 ] || die "Usage: $0

Starts a root shell, no argument accepted."

[ "$(id -u)" -eq 0 ] && die "You're already superuser!"

exec sudo -s

Or if you don't want to make a script, make it a shell function by wrapping that code as rootshell() (<that-code>) to add in your shell rc file (change $0 to the name of the function as not all shells put the function name in $0).

Answer 2

My solution follows:

alias su='[ $(id -u) -ne 0 ] && sudo -s'

---

• POSIX compatible, which means if you copy-paste it to dash or I suppose almost any other shell supporting aliases, you should get the same behavior.

• That is also why I avoided EUID and direct value comparison altogether.

---

Demo:

$ su
[sudo] password for vlastimil:                
# su
# su
# su
# 
exit
$ 

Considering I use sudo -s, I attach its comparison with sudo -s vs other variants.