Nil

What Shell Am I Using?

I wanted to install the latest version of zsh on my MacBook Pro. This is usually the first thing I install when setting up a new machine. A brew install zsh and a chsh later and I'm good to go (well, technically I had to add the homebrew location of zsh to /etc/shells as well). Close out Terminal.app and reopen to make sure my changes took effect, and sure enough, there's my zsh prompt.

Yes, I know macOS Catalina ships with zsh as the default shell, but I want to make sure I have the latest... just because.

Curiosity strikes in and I wonder, even though the homebrew location of zsh is configured as my shell, am I actually using it?

which

~ % which zsh
/usr/local/bin/zsh

Hold on. I want the full location.

~ % ls -la $(which zsh)
lrwxr-xr-x  1 wally  admin  27 Dec 28 20:28 /usr/local/bin/zsh -> ../Cellar/zsh/5.7.1/bin/zsh

Yup, that looks about right.

But wait... that only told me the location of zsh the executable which is only correct because I added homebrew's /usr/local/bin location to my PATH. That is literally the first line of the man page for which.

WHICH(1)                  BSD General Commands Manual                 WHICH(1)

NAME
     which -- locate a program file in the user's path

That means that if I were to execute zsh from my current shell a new instance of zsh from /usr/local/bin/zsh would begin. This doesn't tell me the location of the shell I'm currently typing in which started after opening Terminal.app.

$SHELL

~ % echo $SHELL
/usr/local/bin/zsh

Eh, I don't know. That tells me that it's configured as my shell but does not necessarily tell me the location of the shell I'm currently typing in.

ps

Maybe I can find the currently running process of the instance of zsh I'm in right now.

~ % ps 
  PID TTY           TIME CMD
78525 ttys003    0:00.11 -zsh

Well that still doesn't give me the full path. And is that even the right process? How do I get the PID of the shell I'm currently typing in? Turns out there's a convenient shell variable $$.

~ % echo $$
78525

Hmm, that's good I suppose, but I still don't know if the process with PID 78525, the one I'm currently typing in, is the correct zsh.

lsof

Oh yeah, I probably should have used lsof a long time ago but knowing how to get the PID of the current shell makes things a bit easier.

~ % lsof -p $$
COMMAND   PID  USER   FD   TYPE DEVICE SIZE/OFF                NODE NAME
zsh     78525 wally  cwd    DIR    1,5     1120              362897 /Users/wally
zsh     78525 wally  txt    REG    1,5   614404             1052389 /usr/local/Cellar/zsh/5.7.1/bin/zsh

... trimmed output ...

Ahhh, finally! Not a single of instance of /bin/zsh in the output. I think I'm pretty happy with this result.

Are There Other Ways?

But now I'm even more curious. Are there other ways to find this same information? Turns out apropos process returned a few results but of the bunch the only interesting find was fuser.

fuser

~ % fuser /usr/local/Cellar/zsh/5.7.1/bin/zsh             
/usr/local/Cellar/zsh/5.7.1/bin/zsh: 78525

Neat. That PID matches $$.

~ % fuser /bin/zsh
/bin/zsh: 

And no results for the pre-installed macOS version of /bin/zsh.

macOS Activity Monitor

If you want a more point-and-click way of finding the same information then Activity Monitor has you covered. Make sure View > Applications in last 12 hours is selected. Expand Terminal.app and double click zsh. Clicking the Sample button on the bottom left gives you some detailed information about that running process.

Screen shot showing layered windows of macOS Activity Monitor and a process sample output of zsh

Fun fact: you can use the sample command for the same information.

Conclusion

Finding the shell you're actually using isn't all that simple at first. You can find the location of the executable of the shell. You can find all the processes currently running with that shell name. But gathering information about the shell you're typing all those commands in to find the shell you're using requires a little more poking around. The two easiest solutions, on macOS at least, are lsof and fuser. Activity Monitor also offers up a GUI for finding the same information but you need to how to dig into it.