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) Copyright (C) 2016 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software; you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
The most recent stable minor version of GNU Bash at the time of writing is version 4.4, with version 4.4.0 released in September 2016.
In this book, we will focus on the features of Bash that were available in GNU Bash version 4.0, which was released in 2011, and is very widely available as a minimum version.
If the version of Bash installed on your computer is older than 4.0, some of the scripts and features discussed in this book may not work correctly. You or your system administrator should use your operating system to upgrade your Bash shell to a newer version.