What shell does sudo use

Question

My apologies as this has to be a question others have asked but it seems google proof. I'm trying to figure out what shell is being invoked as I have met an inconsistency. My script doesn't work if I sudo , but does work if I sudo bash . Yet when I sudo echo $0 , it says bash.

cpu=$(cat /proc/cpuinfo | grep Revision | cut -d' ' -f 2-);
if [[ "a22082" = $cpu || "a02082" = $cpu ]]; then
    echo 'do stuff';
fi

If I specify the command interpreter with #!/bin/sh it still fails but being more specific #!/bin/bash it does.

I (now) know that [[ is bash specific but despite the response it doesn't seem like sudo's default shell is bash.

Answer 1

Short answer: There is no default interpreter. Your script needs a shebang line #!/bin/bash at the top.

Long answer: sudo does not run a shell. It runs the command you pass it directly, no intermediate shell. If you want a shell you need to explicitly pass it one.

 sudo echo $0 

This command is misleading due to $0 being expanded by your shell before sudo is called . It's printing your current $0 , not the value of $0 inside the sudo 'd environment.

Let's compare some commands:

•  sudo ls 

  This executes the /bin/ls executable directly. There's no shell involved. The following commands will show when the distinction comes into play.
  
  

•  sudo ls /root/* 

  The current shell expands the glob. If you can't access /root/ then the glob isn't expanded and you get ls: cannot access /root/*: No such file or directory .
  
  

•  sudo 'ls /root/*' 

  This attempts to execute a program called ls /root/* . Literally, it's named * in the root sub-directory of ls ( ls with a trailing space). It fails with sudo: ls /root/*: command not found .
  
  

•  sudo ls '/root/*' 

  This runs /bin/ls and passes it the literal string /root/* . There's no glob expansion. It fails with ls: cannot access /root/*: No such file or directory .
  
  

•  sudo bash -c 'ls /root/*' 

  This executes /bin/bash and has it interpolate the ls /root/* command-line, expanding the glob. This one, finally, works, because we have an explicit shell to expand the glob, and we're doing the expansion within sudo instead of outside of it.

Answer 2

To complement John Kugelman's helpful answer by addressing the more general question implied by the post's title:

^{On a side note: It's best to always start your scripts with a shebang line to explicitly control what shell it is run by.}

sudo doesn't run a shell by default , but can be made to run one with either of the following options:

• -s (non-login shell)
• -i (login shell)

As for what shell is used: man states for -s (applies analogously to -i ; emphasis mine):

Run the shell specified by the SHELL environment variable if it is set or the shell specified by the invoking user's password database entry. If a command is specified, it is passed to the shell for execution via the shell's -c option. If no command is specified, an interactive shell is executed.

In other words: sudo uses the current user's default shell.