If you're using a GNU/Linux system, and your system administrator hasn't changed your login shell, it's likely that Bash is starting up as soon as you log in with a TTY, in an xterm, or over SSH.

If you see a prompt that looks like one of these after you log in, it's a good sign you're in a bash session:

bash-4.4$
user@hostname:~$

If you want to check, you can enter this at the prompt:

bash$ declare -p BASH

If you get a response like this, with a path to your bash binary, you can be confident you are running bash:

declare -- BASH="/bin/bash"

If you get some other output, such as:

sh: declare: not found

Then you may be running some other kind of shell. You may be able to tell what it is by printing the value of the SHELL variable:

$ echo "$SHELL"

We'll use the bash$ prefix before commands throughout this book as a way to show commands you should enter at the Bash command line. We'll use just a $ prefix instead if the command should also work in other POSIX-compliant shells.

Even if it's not the shell that starts up when you log in, Bash may still be installed on your system, and you may still be able to change your login shell to it.

You might be able to start it by just typing bash:

$ bash

If you get output like command not found, you will probably need to install a Bash package specific to your system, or get your system administrator to do it for you. Consult your operating system's documentation to learn how to do this.

If you get a new prompt that looks like the Bash prompts in the previous section, you can then find the location of the bash program:

bash$ declare -p BASH
BASH="/usr/local/bin/bash"

Depending on the system, you might then be able to change Bash to your login shell to that path with the chsh tool:

$ chsh -s /usr/local/bin/bash

This might prompt you for your system password to allow you to make the change.

You may get an error message like this:

chsh: /usr/local/bin/bash is an invalid shell

In this case, the invalid shell part likely means that the path given needs to be added to the /etc/shells file, which specifies the programs the system and its administrator have allowed as login shells. You can inspect this list with cat:

$ cat /etc/shells

If you add your full path to bash on your system to that file, the chsh call should then succeed.