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.

Wait, I'm assuming that `sh` and `bash` aren't the same shell. Why would you expect it to work when you're specifying the wrong shell? — Hasturkun, Jun 14 '16 at 16:42
I always found it personally to me to put `#!/bin/sh` for the shebang and found it better practice to put the exact shell to use. — jgr208, Jun 14 '16 at 16:43
@Hasturkun `sh` says use the default system shell, `bash` says to use the bash shell — jgr208, Jun 14 '16 at 16:43
@EdHeal, I just tried to do this on Mac OS (the question's about Linux, but still), and bash seems to execute `$(SHELL)` _before `sudo`_ as I first got `-bash: SHELL: command not found` and only then - a prompt to input a password. So I guess that bash is running `$(SHELL)` without `sudo`. — ForceBru, Jun 14 '16 at 16:44
@jgr208: `sh` is a Bourne Shell (compatible) shell, `bash` is the Bourne Again Shell, which has it's own features. See http://stackoverflow.com/a/5725402 . My point is that if you're using `bash` features, use `#!/bin/bash` — Hasturkun, Jun 14 '16 at 16:45
just put shebang like `#!/bin/bash` or `#!/usr/bin/env bash` at the beginning of your script and everything will work. — sotona, Jun 14 '16 at 17:10

John Kugelman · Accepted Answer · 2016-06-14T18:03:55.347

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.

@JiangYD `sudo` processes variable assignments preceding the command. — John Kugelman, Feb 16 '22 at 15:39
do you mean that the sudo program parse `a=e` string then invoke the `printenv` program with the parsed enviroments? — Jiang YD, Feb 17 '22 at 02:04

mklement0 · Answer 2 · 2023-04-20T18:19:36.277

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)
- uses the shell specified via the $SHELL environment variable, if defined, otherwise the shell defined in the user's password database entry (/etc/passwords)
-i (login shell, always uses the shell from the user's )
- uses the shell defined in the user's password database entry

In other words: sudo uses the current user's default shell, unless, for -s only, overridden via $SHELL.

See man sudo for details.

What shell does sudo use

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