2020-03-06
I promised that I would teach you the magical art of writing scripts for your computer to perform. And in the next few hundred words, I intend to fulfil that promise. But I must teach you not only how to play the part of a screenwriter writing scripts, but also of a producer actually making sure the script is acted out successfully. And while in Hollywood these different roles might be labelled 'screenwriter' and 'director', in the world of tech they are called #dev and #ops. Put them together and you get #devops.
First, let's start with bash. We already know how to engage
interactively with the REPL. Now let's write an actual script. And
following in the noble tradition of hackers everywhere, it's going to be
the apparently trivial command for the computer to print the words
"Hello World". In bash, all you need is a script that says:
echo Hello World
So you could open up your favourite text editor, type those words, and
save the file as hello.sh. Or you could just use the > operator to
redirect the output of the echo command to a file without bothering
with a text editor:
echo 'echo Hello World' > hello.sh
You might want to use a text editor to check that your file does say
echo Hello World. Now when you're ready, you can run the program
with bash hello.sh. And it should produce the global greeting
you've been awaiting.
Hello World
But I've lied to you. I've said that with bash hello.sh you would run
the program you've just written. But what's actually happened, is that
you've run bash, which has interpreted the instructed contained in the
hello.sh script and executed them. So what we really should do is
modify the permissions of the file to add execution rights:
chmod +x hello.sh
And now you'll be able to actually run it directly as a program in its own right:
./hello.sh
So now you can write programs in bash. Next time we'll learn some
other languages...