How to determine the current interactive shell that I'm in? (command-line)

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?

• 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.

  ---

• 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.

ps -p $$

should work anywhere that the solutions involving ps -ef and grep do (on any Unix variant which supports POSIX options for ps) and will not suffer from the false positives introduced by grepping for a sequence of digits which may appear elsewhere.

Try

ps -p $$ -oargs=

or

ps -p $$ -ocomm=

If you just want to ensure the user is invoking a script with Bash:

if [ -z "$BASH" ] ;then echo "Please run this script $0 with bash"; exit 1; fi

You can try:

ps | grep `echo $$` | awk '{ print $4 }'

Or:

echo $SHELL

$SHELL need not always show the current shell. It only reflects the default shell to be invoked.

To test the above, say bash is the default shell, try echo $SHELL, and then in the same terminal, get into some other shell (KornShell (ksh) for example) and try $SHELL. You will see the result as bash in both cases.

To get the name of the current shell, Use cat /proc/$$/cmdline. And the path to the shell executable by readlink /proc/$$/exe.

There are many ways to find out the shell and its corresponding version. Here are few which worked for me.

Straightforward

1. $> echo $0 (Gives you the program name. In my case the output was -bash.)
2. $> $SHELL (This takes you into the shell and in the prompt you get the shell name and version. In my case bash3.2$.)
3. $> echo $SHELL (This will give you executable path. In my case /bin/bash.)
4. $> $SHELL --version (This will give complete info about the shell software with license type)

Hackish approach

$> ******* (Type a set of random characters and in the output you will get the shell name. In my case -bash: chapter2-a-sample-isomorphic-app: command not found)

ps is the most reliable method. The SHELL environment variable is not guaranteed to be set and even if it is, it can be easily spoofed.

I have a simple trick to find the current shell. Just type a random string (which is not a command). It will fail and return a "not found" error, but at start of the line it will say which shell it is:

ksh: aaaaa: not found [No such file or directory]
bash: aaaaa: command not found

I have tried many different approaches and the best one for me is:

ps -p $$

It also works under Cygwin and cannot produce false positives as PID grepping. With some cleaning, it outputs just an executable name (under Cygwin with path):

ps -p $$ | tail -1 | awk '{print $NF}'

You can create a function so you don't have to memorize it:

# Print currently active shell
shell () {
  ps -p $$ | tail -1 | awk '{print $NF}'
}

...and then just execute shell.

It was tested under Debian and Cygwin.

The following will always give the actual shell used - it gets the name of the actual executable and not the shell name (i.e. ksh93 instead of ksh, etc.). For /bin/sh, it will show the actual shell used, i.e. dash.

ls -l /proc/$$/exe | sed 's%.*/%%'

I know that there are many who say the ls output should never be processed, but what is the probability you'll have a shell you are using that is named with special characters or placed in a directory named with special characters? If this is still the case, there are plenty of other examples of doing it differently.

As pointed out by Toby Speight, this would be a more proper and cleaner way of achieving the same:

basename $(readlink /proc/$$/exe)

My variant on printing the parent process:

ps -p $$ | awk '$1 == PP {print $4}' PP=$$

Don't run unnecessary applications when AWK can do it for you.

Provided that your /bin/sh supports the POSIX standard and your system has the lsof command installed - a possible alternative to lsof could in this case be http://glowworm.us/pid2path.html">pid2path</a></code> - you can also use (or adapt) the following script that prints full paths:

#!/bin/sh
# cat /usr/local/bin/cursh
set -eu
pid="$$"

set -- sh bash zsh ksh ash dash csh tcsh pdksh mksh fish psh rc scsh bournesh wish Wish login

unset echo env sed ps lsof awk getconf

# getconf _POSIX_VERSION  # reliable test for availability of POSIX system?
PATH="`PATH=/usr/bin:/bin:/usr/sbin:/sbin getconf PATH`"
[ $? -ne 0 ] && { echo "'getconf PATH' failed"; exit 1; }
export PATH

cmd="lsof"
env -i PATH="${PATH}" type "$cmd" 1>/dev/null 2>&1 || { echo "$cmd not found"; exit 1; }

awkstr="`echo "$@" | sed 's/\([^ ]\{1,\}\)/|\/\1/g; s/ /$/g' | sed 's/^|//; s/$/$/'`"

ppid="`env -i PATH="${PATH}" ps -p $pid -o ppid=`"
[ "${ppid}"X = ""X ] && { echo "no ppid found"; exit 1; }

lsofstr="`lsof -p $ppid`" || 
   { printf "%s\n" "lsof failed" "try: sudo lsof -p \`ps -p \$\$ -o ppid=\`"; exit 1; }

printf "%s\n" "${lsofstr}" | 
   LC_ALL=C awk -v var="${awkstr}" '$NF ~ var {print $NF}'

My solution:

ps -o command | grep -v -e "\<ps\>" -e grep -e tail | tail -1

This should be portable across different platforms and shells. It uses ps like other solutions, but it doesn't rely on sed or awk and filters out junk from piping and ps itself so that the shell should always be the last entry. This way we don't need to rely on non-portable PID variables or picking out the right lines and columns.

I've tested on Debian and macOS with Bash, Z shell (zsh), and fish (which doesn't work with most of these solutions without changing the expression specifically for fish, because it uses a different PID variable).

If you just want to check that you are running (a particular version of) Bash, the best way to do so is to use the $BASH_VERSINFO array variable. As a (read-only) array variable it cannot be set in the environment, so you can be sure it is coming (if at all) from the current shell.

However, since Bash has a different behavior when invoked as sh, you do also need to check the $BASH environment variable ends with /bash.

In a script I wrote that uses function names with - (not underscore), and depends on associative arrays (added in Bash 4), I have the following sanity check (with helpful user error message):

case `eval 'echo $BASH@${BASH_VERSINFO[0]}' 2>/dev/null` in
    */bash@[456789])
        # Claims bash version 4+, check for func-names and associative arrays
        if ! eval "declare -A _ARRAY && func-name() { :; }" 2>/dev/null; then
            echo >&2 "bash $BASH_VERSION is not supported (not really bash?)"
            exit 1
        fi
        ;;
    */bash@[123])
        echo >&2 "bash $BASH_VERSION is not supported (version 4+ required)"
        exit 1
        ;;
    *)
        echo >&2 "This script requires BASH (version 4+) - not regular sh"
        echo >&2 "Re-run as \"bash $CMD\" for proper operation"
        exit 1
        ;;
esac

You could omit the somewhat paranoid functional check for features in the first case, and just assume that future Bash versions would be compatible.

echo $$ # Gives the Parent Process ID 
ps -ef | grep $$ | awk '{print $8}' # Use the PID to see what the process is.

From How do you know what your current shell is?.

None of the answers worked with fish shell (it doesn't have the variables $$ or $0).

This works for me (tested on sh, bash, fish, ksh, csh, true, tcsh, and zsh; openSUSE 13.2):

ps | tail -n 4 | sed -E '2,$d;s/.* (.*)/\1/'

This command outputs a string like bash. Here I'm only using ps, tail, and sed (without GNU extesions; try to add --posix to check it). They are all standard POSIX commands. I'm sure tail can be removed, but my sed fu is not strong enough to do this.

It seems to me, that this solution is not very portable as it doesn't work on OS X. :(

This is not a very clean solution, but it does what you want.

# MUST BE SOURCED..
getshell() {
	local shell="`ps -p $$ | tail -1 | awk '{print $4}'`"

	shells_array=(
	# It is important that the shells are listed in descending order of their name length.
		pdksh
		bash dash mksh
		zsh ksh
		sh
	)

	local suited=false
	for i in ${shells_array[*]}; do
		if ! [ -z `printf $shell | grep $i` ] && ! $suited; then
			shell=$i
			suited=true
		fi
	done

	echo $shell
}
getshell

Now you can use $(getshell) --version.

This works, though, only on KornShell-like shells (ksh).

Do the following to know whether your shell is using Dash/Bash.

ls –la /bin/sh:

• if the result is /bin/sh -> /bin/bash ==> Then your shell is using Bash.

• if the result is /bin/sh ->/bin/dash ==> Then your shell is using Dash.

If you want to change from Bash to Dash or vice-versa, use the below code:

ln -s /bin/bash /bin/sh (change shell to Bash)

Note: If the above command results in a error saying, /bin/sh already exists, remove the /bin/sh and try again.

I like Nahuel Fouilleul's solution particularly, but I had to run the following variant of it on Ubuntu 18.04 with the built-in Bash shell:

bash -c 'shellPID=$$; ps -ocomm= -q $shellPID'

Without the temporary variable shellPID, e.g. the following:

bash -c 'ps -ocomm= -q $$'

Would just output ps for me. Maybe you aren't all using non-interactive mode, and that makes a difference?

On Mac OS X (and FreeBSD):

ps -p $$ -axco command | sed -n '$p' 

Grepping PID from the output of "ps" is not needed, because you can read the respective command line for any PID from the /proc directory structure:

echo $(cat /proc/$$/cmdline)

However, that might not be any better than just simply:

echo $0

About running an actually different shell than the name indicates, one idea is to request the version from the shell using the name you got previously:

<some_shell> --version

sh seems to fail with exit code 2 while others give something useful (but I am not able to verify all since I don't have them):

$ sh --version
sh: 0: Illegal option --
echo $?
2

Why not get it with the $SHELL var, a simple sed could remove the path:

$ echo $SHELL | sed -E 's/^.*\/([aA-zZ]+$)/\1/g'
bash

Tested on MacOS, Ubuntu, CentOS

One way is:

ps -p $$ -o exe=

which is IMO better than using -o args or -o comm as suggested in another answer (these may use e.g. some symbolic link like when /bin/sh points to some specific shell as dash or bash).

The above returns the path of the executable, but beware that due to /usr-merge, one might need to check for multiple paths (e.g. /bin/bash and /usr/bin/bash)

Also note that the above is not fully POSIX-compatible (POSIX ps doesn't have exe).

Kindly use the below command:

ps -p $$ | tail -1 | awk '{print $4}'

And i came up with this

sed 's/.*SHELL=//; s/[[:upper:]].*//' /proc/$$/environ

You can use echo $SHELL|sed "s/\/bin\///g"

This one works well on Red Hat Linux (RHEL), macOS, BSD and some AIXes:

ps -T $$ | awk 'NR==2{print $NF}' 

alternatively, the following one should also work if pstree is available,

pstree | egrep $$ | awk 'NR==2{print $NF}'