Cfxtrjtrk

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 "$$"

$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

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.

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 cmd= -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

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 at https://wiki.ubuntu.com/ChangingShells#Changing_your_login_shell_which_is_permanent