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I read that terminal is nothing but shell, and Unix provides different flavors of shells:

  • Bourne shell (sh)
  • C shell (csh)
  • TC shell (tcsh)
  • Korn shell (ksh)
  • Bourne Again shell (bash)

Questions:

  • When I open a terminal window, which shell is opened by default?
  • How do I check how many shells are installed?
  • How do I change the shell used from my account?
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604

You can type the following command in your terminal to see which shell you are using:

echo $0

The result will look something similar to the below if you are using the bash (Bourne Again Shell) terminal:

-bash
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    is 0 an environment variable? – Mi_Onim Feb 22 '16 at 17:21
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    @Mi_Onim $0 is the name of the running process. If you use it inside of a shell then it will return the name of the shell. If you use it inside of a script, it will be the name of the script. – kingmilo Jun 26 '16 at 4:49
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    NOTE: this won't work for interactive csh. % echo $0 No file for $0. , but it does work for tcsh – Sergiy Kolodyazhnyy Sep 18 '16 at 12:43
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    Doesn't work if when one is symlink. Like bash as sh – Anwar Dec 6 '16 at 17:43
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    @Anwar makes a good point, after you get the shell name from echo $0, ls -l `which <name>` to see if it is symlinked to another shell binary. – JivanAmara Feb 10 '17 at 23:38
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To find the shell you have on the default environment you can check the value of the SHELL environment variable:

echo $SHELL

To find the current shell instance, look for the process (shell) having the PID of the current shell instance.

To find the PID of the current instance of shell:

echo "$$"

Now to find the process having the PID:

ps -p <PID>

Putting it together:

ps -p "$$"
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    $SHELL is the default shell for the system (or user), which is usually (but not necessarily) the shell that is actually being used at any given moment. – kingmilo Feb 28 '15 at 4:13
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    @kingmilo: Right you are, answer modified. – heemayl Feb 28 '15 at 4:24
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    echo $SHELL gave me /bin/csh and ps -p $$ gave me 22673 pts/1 00:00:00 bash. Kingmilo explained (above) why they are not the same. – 18446744073709551615 Oct 30 '17 at 10:32
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    To grab ONLY the shell name for the current shell in a way portable between gnu & bsd ps versions, this works well: ps -cp "$$" -o command="" – James Tomasino Jan 7 '20 at 22:37
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    thank you @James Tomasino that worked the best! – Robert Houghton Feb 6 '20 at 1:26
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$SHELL gives you the default shell. $0 gives you the current shell.

For example: I have bash as my default shell, which I use for my Terminal App. But for my iTerm2 app, I use the command as the window opens: /bin/ksh.

So my $0 gives me /bin/ksh on iTerm2. $SHELL gives me /bin/bash on iTerm2. $0,$SHELL gives me /bin/bash on Terminal

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    This is a nice disambiguation of the other top-voted answers. Thank you. – Michael Hoffmann Feb 14 '18 at 18:58
  • For me, I have zsh installed and it's the current default, to change to bash, all I have to do type bash in the terminal. To switch back to zsh, type zsh – Yousuf Azad Feb 27 '19 at 11:11
  • You don't want to keep going back and forth that way because you stack up shell within shell within shell, and a fresh context within each. In general it's best to type ctrl-d or exit to return to the previous shell. – Phill Apley Apr 16 at 19:21
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The other answers tend to be using shell specific features, but we are trying to discover which shell we are using, so they assume the answer to the problem. For example none of the answers will work on fish.

sh -c 'ps -p $$ -o ppid=' | xargs ps -o comm= -p

Instead use the $$ syntax in an invocation of sh, but then we are looking for the PPID not the PID. Use the PPID to find the cmd.

sh -c 'ps -p $$ -o ppid=' | xargs -I'{}' readlink -f '/proc/{}/exe'

Thanks for improvement @muru

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    You ca use ppid=/cmd=to omit the headers (and so the tail -1s), and consider looking at /proc/.../exe to see what file is being run (since the cmd output can be manipulated by whatever ran the shell). – muru Apr 6 '18 at 4:07
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    Apparently this does not work in busybox, due to non posix compliance: ref: github.com/broadinstitute/cromwell/pull/… – Evan Benn Apr 30 '18 at 5:04
  • What does your first code block exemplify? – Andreas Feb 11 '20 at 22:29
  • this solution solves the problem of assuming the shell but introduces the problem of assuming the OS. The BSD version of readlink shipped with macOS (and presumably other BSD Distros) uses -f to specify "format" which is a printf style string that must start and end with % and has a variety of templating options. – masukomi May 21 '20 at 15:02
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To know which is the default shell for your user, you can run:

echo "$SHELL"

For example if you're using Bash you should get the following output:

/bin/bash

If you didn't change any configuration it should be Bash since Bash it's the default shell on Ubuntu.

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  • /bin/sh is the Shell Command Language and not the Bourne Shell, please edit your answer. – kingmilo Feb 28 '15 at 4:07
  • @kingmilo There's no /bin/sh reference in my answer – kos Feb 28 '15 at 4:12
  • @kol there was before edit. Please also note $SHELL is the default shell for the system (or user), which is usually (but not necessarily) the shell that is actually being used at any given moment. – kingmilo Feb 28 '15 at 4:14
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    @kingmilo Reading the answer again i think i understand what is your concern, probably i didn't express myself well. I clarified the meaning of "currently" in my answer – kos Feb 28 '15 at 4:49
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    @frederickjh fish is a bit of an odd one out here. The $$ variable is actually defined by POSIX and will work on the vast majority of shells. Fish has decided not to follow the standard here so I think it's fair to ignore it. I can confirm that $$ works as expected in sh, dash, bash, zsh, ksh, ash, tcsh and csh. In fact, off the top of my head I can't think of any other shell except fish where it doesn't work. – terdon Aug 18 '16 at 9:02
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The original post asked three questions. The answers given do cover the first question, "When I open a terminal window, which shell is opened by default?" They also answer a question which was NOT asked, namely "How can I find out which shell is currently running in the terminal?" However, as far as I can see nobody has answered either the second or third questions originally asked, namely "How do I check how many shells are installed?" and "How do I change the shell used from my account?"

  • To answer "How do I check how many shells are installed?" the following command will list all the available shells:

    cat /etc/shells
    

    For example, on a default installation of Ubuntu 18.10 this gives:

    # /etc/shells: valid login shells
    /bin/sh
    /bin/dash 
    /bin/bash
    /bin/rbash
    

    However, by default sh is a symbolic link to dash, while rbash links to bash with the option -r ("restricted bash") so there are actually only two shells, not four as the above list suggests. The following command will show you whether any of the listed shells are in fact symbolic links, and if so where they link to: ls -l /bin

  • Now for the question "How do I change the shell used from my account?" Assuming this means "How do I permanently change the default shell that a terminal will use", there is an answer here.

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To address your third question, "How do I change the shell used from my account?", the answer is to use chsh.

There are two modes:

  • interactive, and;
  • non-interactive.

From Changing Shells - Changing your login shell which is permanent, and paraphrasing it slightly:

You will use a program called chsh. There is a interactive method and non-interactive method. Type the following into your terminal:

INTERACTIVE METHOD

$ chsh

This results in a brief dialog in which the user is prompted first for their password and then for the full path of the desired new shell.

Caution should be exercised when changing one's default shell because it is possible to make an error that only the root user (i.e., system administrator) can repair (although it should be easy for a skilled user to repair it on a home system). In particular, it is important to first test the shell temporarily in the current session and then to make certain that a valid shell name is being entered when making the permanent change.

NON-INTERACTIVE METHOD

I will use csh as again an example.

$ chsh -s /bin/csh

The -s sets it for you without having to go into the editor to do it.

Once this is executed successfully, then echo $SHELL will still say that you are in the same shell as before. However, you need to log out and back in for the change to take effect. Now do echo $SHELL. You should see it shows the new shell.

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In one of the servers I connect to, the login shell is /bin/sh which is a symlink to /bin/bash

Most answers here will give sh, which would make the OP consider it's Bourne shell and not GNU bash, except this one that gives /bin/bash

Another option that works for this case is:

$ echo $SHELL
/bin/sh

$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 May 31 16:15 /bin/sh -> bash

$ /bin/sh --help
GNU bash, version 4.2.10(1)
Usage:  /bin/sh [GNU long option] [option] ...
        /bin/sh [GNU long option] [option] script-file ...

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