A shell script contains a sequence of commands for a UNIX-based operating system. It's called a shell script because it combines into a "script" in a single file a sequence of commands that would otherwise have to be presented to the system from a keyboard one at a time.Thus , a very useful tool in automating tasks in Linux OSes. All scripts can be run without the need of compiling it, so the user will have a way of prototyping commands seamlessly.
The bash (Bourne-Again Shell) is the default shell in most of the Linux distributions and OS X. It was developed by Brian Fox and was released in 1989.
Every Linux script starts with a shebang (#!) line. The bang line specifies the full path /bin/bash of the command interpreter that will be used to run the script.