Before we start writing commands and programming with Bash, it's a good idea to find out what version of Bash you have installed. This is because newer versions of Bash have useful new features that might be discussed in this book, but that might not be available in your version of Bash.
You can check the version of your current running shell by printing the value of the BASH_VERSION variable:
bash$ declare -p BASH_VERSION declare -- BASH_VERSION="4.4.12(1)-release"
You can get the same information by invoking the bash program with its --version option, which provides some extra information about the program's version number and software license:
$ bash --version GNU bash, version 4.4.12(1)-release (x86_64-pc-linux-gnu) ...
