Difference between $0 and $SHELL?

Question

I am currently playing CTF in OverTheWire website . I was stuck on 32nd Level [ https://overthewire.org/wargames/bandit/bandit33.html ] to get password for next Level . Command to access Level 32 - ssh [email protected] -p 2220 and the password is 56a9bf19c63d650ce78e6ec0354ee45e .

SO when I login I get made up shell called UPPERCASE SHELL . Which takes commands and converts them to UPPERCASE throwing "command not found error " . When I run $SHELL , all I can see is a banner called "WELCOME TO UPPERCASE SHELL" . So I googled for walk-through after being stuck on for 3 hours . SO one of the guy enterd $0 which took him to Bourne Shell [ I guess ] . SO what is the difference between $0 and $SHELL .

By my understanding $SHELL is default shell [ UPPERCASE_SHELL ] which I verified after running cat /etc/paswd | grep bandit32 . SO what is $0 . If $0 is current shell then I should get the same banner[ UpperCase ] as output . How come I landed Bourne Shell .

Bottom line . what are these 2 shell variables how do thesework in this context ?

Answer 1

$SHELL is a funny variable. For most shells, it holds the name of your login shell (you see this in /etc/passwd). It doesn't automatically change if you launch a different shell interpreter.

For example, my login shell is fish. Let's see what different shells do with $SHELL:

$ set -e SHELL; bash -c 'echo $SHELL'
/usr/local/bin/fish
$ set -e SHELL; ksh -c 'echo $SHELL'
/bin/sh
$ set -e SHELL; zsh -c 'echo $SHELL'

$ set -e SHELL; dash -c 'echo $SHELL'

$ set -e SHELL; fish -c 'echo $SHELL'

set -e var is fish's way of unsetting a variable.

• bash sets SHELL to my login shell.
• ksh uses a default shell.
• zsh and dash and fish put no special meaning on SHELL.

So, $SHELL is simply informative at best. I wouldn't rely on it holding any magic value.

---

The value of SHELL is actually set by login, before the shell is invoked. From the (BSD) login manual on my Mac:

The login utility enters information into the environment (see environ(7)) specifying the user's home directory (HOME), command interpreter (SHELL), search path (PATH), terminal type (TERM) and user name (both LOGNAME and USER).

Answer 2

$SHELL should tell you the shell name - see glenn's answer for how vaguely that variable should be considered.

$0 is the name of the current script.

Make a shell script called test.sh like this:

#!/bin/bash
echo My shell is $SHELL
echo My script is called $0

and see what happens when you run it. Changing the shebang to #!/bin/zsh will not change the result. Again see glenn's answer.

As for your game: There anything might be happening. It might just be some emulated shell for the game or whatever simulated game element. No one can tell.