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### What is Bash?

Bash is a "Unix shell": a command-line interface for interacting with the operating system. It is widely available, being the default shell on many GNU/Linux distributions and on Mac OS X; and ports exist for many other systems. It was created in the late 1980s by a programmer named Brian Fox, working for the Free Software Foundation. It was intended as a free-software alternative to the Bourne shell (in fact, its name is an acronym for "Bourne-again shell"), and it incorporates all features of that shell, as well as new features such as integer arithmetic and in-process regular expressions.
Example of Bash (shell) script
#!/bin/bash

if [[ -e source.txt ]] ; then
echo 'source.txt exists; copying to destination.txt.'
cp source.txt destination.txt
else
echo 'source.txt does not exist; exiting.'
exit 1 # terminate the script with a nonzero exit status (failure)
fi