Question

How can I determine the current shell I am working on?

Would the output of the ps command alone be sufficient?

How can this be done in different flavors of Unix?

Answer 1

• There are three approaches to finding the name of the current shell’s executable:

  Please note that all three approaches can be fooled if the executable of the shell is /bin/sh, but it’s really a renamed bash, for example (which frequently happens).

  Thus your second question of whether ps output will do is answered with "not always".

  1. echo $0 – will print the program name… which in the case of the shell is the actual shell.

  2. ps -ef | grep $$ | grep -v grep – this will look for the current process ID in the list of running processes. Since the current process is the shell, it will be included.

     This is not 100% reliable, as you might have other processes whose ps listing includes the same number as shell’s process ID, especially if that ID is a small number (for example, if the shell’s PID is "5", you may find processes called "java5" or "perl5" in the same grep output!). This is the second problem with the "ps" approach, on top of not being able to rely on the shell name.

  3. echo $SHELL – The path to the current shell is stored as the SHELL variable for any shell. The caveat for this one is that if you launch a shell explicitly as a subprocess (for example, it’s not your login shell), you will get your login shell’s value instead. If that’s a possibility, use the ps or $0 approach.

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• If, however, the executable doesn’t match your actual shell (e.g. /bin/sh is actually bash or ksh), you need heuristics. Here are some environmental variables specific to various shells:

• $version is set on tcsh

• $BASH is set on bash

• $shell (lowercase) is set to actual shell name in csh or tcsh

• $ZSH_NAME is set on zsh

• ksh has $PS3 and $PS4 set, whereas the normal Bourne shell (sh) only has $PS1 and $PS2 set. This generally seems like the hardest to distinguish – the only difference in the entire set of environment variables between sh and ksh we have installed on Solaris boxen is $ERRNO, $FCEDIT, $LINENO, $PPID, $PS3, $PS4, $RANDOM, $SECONDS, and $TMOUT.

UPDATE: Someone brought up "ash" (Almquist Shell) in comments. There seem to be 2001 variants of it including dash; so in the interest of not blowing up the answer unnecessarily, here’s a very useful page listing a ton of various flavours of ash and their differences from each other and often from stanard Bourne sh: https://www.in-ulm.de/~mascheck/various/ash/