Create and Use Bash Scripts: A complete guide
Bash scripts? What are they and how can we use them?
This tutorial is about just that, and is here to help you get the most out of this great feature. Bash is available out of the box on Mac and UNIX systems, so if you’re using either of the two you’re ready to go!
Bash: “Bourne again shell”. There are many shells that can run UNIX commands, and the one used on the Mac Terminal is Bash
Command line: An interface for interacting with the operating system
Shebang: A String at the beginning of a script to indicate execution
Shell: A user interface for accessing an operating system’s services
Terminal: A command line interface to control the UNIX-based operating system that underlies the Mac
VIM: This is a contraction of Vi and IMproved. That is, it is a clone (with additions) of the vi text editor for Unix. In other words, it is a command-line interface for creating and manipulating files
A little help…
This tutorial is not an exhaustive guide to the bash terminal, and is rather a starter guide to scripting with some basic bash syntax. This guide does use VIM but you are certainly welcome to use other text editors.
Where each of the following commands are prefixed by,
$, which indicates that this should be typed into the terminal. To make this clear I’ve written it as a quote followed by the
$ symbol so you will see the following (followed by a command to type) within this guide:
Creating your bash script
Creating the bash script
We are going to create our first bash script (called, wonderfully myfirstscript). There’s an animated gif to help with this if you choose to scroll to the end of this section.
$ vim myfirstscript
Add the shebang line
From within VIM we need to tell the command line to run the code using bash. VIM requires that you edit the current line, i to insert will do just that, then type away!
Add the echo line
From VIM we add an echo line, which will (in the end) write “Hello,World” to the console.
Save the script
From within VIM we can (if we were editing a quick nudge of the escape key will allow us to do this) use the save command
This will return us to the console. Awesome.
Make the script executable
From terminal run the following to make the script executable
$ chmod +x myfirstscript
Running the script
After all of that, this isn’t too difficult
The video so you can see what to do? That’s right here:
Make the script Globally available
Running the script from this particular folder is great, but we really want to create utilities that can be run from anywhere on your device.
This section of the guide will help you with just that.
- Copy the script to
/usr/local/binby running the following command (
sudomeaning you are running this as the super user):
$ sudo cp ./ffmpegscript /usr/local/bin
you may well be asked for you password. This is no problem — don’t panic as this is a side-effect of using sudo.
2. Set the environment variable
- open the bash_profile
$ vim .bash_profile
- Add the path
Here is my
3. save the result using
You can now run the file from anywhere. In the case of this script
As we have seen a little earlier in this guide, we can use
echo to print to the terminal. There aren’t any painful quotes here — we can just write (including any spaces) straight to the screen!
echo hello big scary world
The syntax for
echo is as follows:
echo [option(s)] [string(s)]
But how can we see what the options are? We have a choice — the manual through
man or the short documentation through
help echo . Which would I recommend? At this point the shorter guide is probably enough
$ help echo
Using these parameters does bring back the quotations, simply because we need to use the escape characters
$ echo -e "\a Hello, World"
in the documentation \a gives an alert. For me, this gives a nice sound from the Mac and a bell symbol at the top of the Terminal screen (if I use iTerm).
Speaking of which, it looks like the following:
Multiple options? See the following:
$ echo -e “\a\n Hello, World”
Which puts our message on a new line. Escaping quotation marks will work…
You can use the output of a shell execution in a script. This basically uses the output of a shell as a variable (more on that in a minute):
$ echo "Hello, $(whoami)"
Arrays can be used to store data link in any language. This doesn’t even need to be done in a script, as it can be run directly on the command like:
Variables are declared with just the variable name, but are called with $.
I’ve set up a new script called myvariablesscript, and made it executable with
chmod +x myvariablesscript.
within the script I’ve added the following code
Which means we need to provide a name, which it will say hello to whichever variable is passed.
expected output: Hello to Me
This is fine. But if you don’t provide a name? It will just output Hello to. This isn’t great.
So we can provide a default message if no name given:
Now if you don’t provide a name the following error is given:
./myvariablesscript: line 3: 1: Error: no name given
Can we do even better? Yes, yes we can.
We can provide a default variable:
so if no parameter is provided we see Hello to ME
If you want something a little more interactive, you might want to use user input. This is done using the read keyword. The following is an alternative script to the one above that writes out a wonderful greeting message that is personalised for you.
Comparison and if statements
Again using examples of greetings, we can demonstrate the implementation of an if statement. Now this is tricky, as bash treats strings and number differently; however for this simple example equality == will do.
which, if your name is given as
You will return
Hello, you! and if not returns
Good Morning followed by your name.
Here are the Comparisons for Number and Strings
The operator for numbers works the same way as that for strings, that is should be between two operands that is
[20 -eq 20] resolves to true whereas
[21 -eq 20] resolves to false.
Here we can list the files in the current directory.
To demonstrate the use of functions with parameters I’ll write a simple function to (in practice) replace the echo function
If we wanted more parameters we would refer to them sequentially, that is $2, $3 and so on.
Ok, this is only debugging of a sort.
At the top of each file instead of using
we can use
#!/usr/bin/env bash -x
to get some rather juicy information!
This is the start, not the end of bash scripting. This should give you some of the (albeit basic) tools to be able to create works of beauty on your machine — all from the command line.
I hope this helps!
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