Shell scripting in macOS – Part 1 | Arun Patwardhan's Blog

The scripts in the following articles are written in macOS Big Sur. You can use these concepts to create scripts on UNIX and Linux too.

This is the first part of a multipart series. You can find links to further articles at a table located at the bottom of this article. I will be updating this article with links to new articles as I publish them.

What is shell scripting?

Shell scripts are simply files that list out a series of commands in the sequence in which they are to be executed. By commands we typically mean other shell commands. But these could also be other executables, scripts, or commands from other languages.

Why do we need it?

There are several reasons. The most common reason being automation. For example, If there are steps that we perform on a repeated basis such as checking for the presence or absence of particular files we could easily automate this task with the help of a script. Or If we want to perform certain tasks at scale: such as creating a set of files and folders that should always be there within the user’s home folder.

Scripting also has the added benefit of consistency. By performing the tasks the same way we can ensure that our desired outcome is the same every time.

What is required for creating shell scripts?

Before we go ahead and look at how to create our own scripts there are a few things we need to keep ready at hand.

First we would need to know the commands we would have to execute to achieve our goal. This list is quite large and one would not necessarily know all the commands supported. But overtime your knowledge of these commands will grow. So do not worry!
Second, We need to pick a shell interpreter.
We also need to decide how we will be accessing the command line interface. This would most likely be via the Terminal application, but there are other ways too.
Finally we need to decide on the editor we will use to create our scripts. I will talk about this a little later.

Shell interpreters

The shell interpreter is as the name says the object that will interpret the commands and execute them. The default shell interpreter for macOS is zsh starting macOS Catalina. We can choose to use that or any other interpreter. While most commands we will be using will be common ones that are available across all interpreters be aware that some commands may be unique to certain interpreters only.

Commands

We will need to know some basic commands that will help us compose our scripts.There are several commands available in macOS. We will be learning about quite a few of those over the course of the next few articles. The table below lists some of the commands that we will be using.

Command	Description
cd	Change directory. This command changes the current working directory to the specified path. We use this command to navigate to another folder.
mv	This command moves the contents from the specified folder to another folder.
ls	Lists the contents of the folder.
rm	Remove the specified content.
cp	Copy the contents of a folder to another folder.
touch	Update the timestamp for a file or folder.
pwd	Print the complete path to the present working directory.
mkdir	Create a folder.
echo	Print the string out onto stdout.

Be aware that many commands will create/modify/delete items in the current folder if the absolute path is not specified in the command. This may result in unexpected or unintended behavior.

Editor

I will be using Xcode as the editor for our scripts. However, you can use any editor you wish. You will find the a list of editors at the bottom of the article.

_{Using Xcode as an editor for scripting may be a bit of an overkill. It is a very heavy application primarily designed for app development. If you are currently developing apps and are already using Xcode then you can go ahead and use it for scripting too. Otherwise it might be a good idea to go in for a different tool.}

Building our first shell script

In order to build our script. Let us take a simple scenario. Let us suppose that every user in our organisation must have the following folders:

Tools
Reports
Help

All these folders must be located in the home folder for each user. So let us take it step by step. We will perform these commands manually from the Terminal application.

The first command is the command to navigate to the home folder.

cd ~/

The ~/ represents the path to the current user’s home folder. The cd command is used to change the working directory to the newly specified path.

Now we will create the 3 folders.

mkdir Tools
mkdir Reports
mkdir Help

All the 3 commands are creating a new folder. Since we did not specify the complete path to the folder. These items are created in the working directory.

Now we will step into each folder and create an empty hidden file.

cd Tools
touch .ToolsFolderCreated
cd ..

Let us break down these commands one by one.

First we go into the Tools folder.

Then we use the touch command to update the timestamp of the.ToolsCreated file. Since the file doesn’t exist the touch command creates the file for us. Also as the file starts with the . character it is hidden by default. Creating a hidden file like this is a good way of leaving behind some flag indicating that the script ran successfully. Of course, in our example this can be determined simply by seeing the folders that are created. But in more elaborate situations they are a very useful way of laying down milestones for a script.

The next command takes us back a step outside the enclosing folder. In our case the Tools folder is inside the home folder. So we are going back to the home folder.

We will repeat the steps again for the Reports and Help folders.

cd Reports
touch .ReportsFolderCreated
cd ..

cd Help
touch .HelpFolderCreated
cd ..

Those are the commands we execute to get the desired result. You can switch to the graphical user interface to see if the items have been created. Note that the files created with the touch command will not be visible by default.

Now that we have seen how these commands work. Let us create a script.

Use any editor you like. I will start off with TextEdit. Create a new file. If you are using TextEdit then do not forget to convert the formatting to plain text. Format > Make Plain Text.
Give the file any name you want. I will call it folderCreator.zsh.
Save the file where ever you wish. I will save it on the Desktop folder for now.
On the first line we need to specify our interpreter. This indicates that the commands in our script need to be interpreted by the zsh interpreter.

#!/bin/zsh

One the next line we will type the command to go to the home folder.

#!/bin/zsh

cd ~/

Next we will type the command to create the 3 folders.

#! /bin/zsh

cd ~/

mkdir Tools
mkdir Reports
mkdir Help

Finally we will add the code to create the hidden files.

#! /bin/zsh

cd ~/

mkdir Tools
mkdir Reports
mkdir Help

cd Tools
touch .ToolsFolderCreated
cd ..

cd Reports
touch .ReportsFolderCreated
cd ..

cd Help
touch .HelpFolderCreated
cd ..

A nice addition to the script would be the echo command. This command would let the person who is running the script know about the different events taking place.

#! /bin/zsh

echo "Running script to create folders."

cd ~/

echo "Creating folders: Tools, Reports, Help"
mkdir Tools
mkdir Reports
mkdir Help

echo "Creating hidden file for Tools folder."
cd Tools
touch .ToolsFolderCreated
cd ..

echo "Creating hidden file for Reports folder."
cd Reports
touch .ReportsFolderCreated
cd ..

echo "Creating hidden file for Help folder."
cd Help
touch .HelpFolderCreated
cd ..

echo "Task completed. Have a nice day!"

Your completed script should look like:

#! /bin/zsh

echo "Running script to create folders."

cd ~/

echo "Creating folders: Tools, Reports, Help"
mkdir Tools
mkdir Reports
mkdir Help

echo "Creating hidden file for Tools folder."
cd Tools
touch .ToolsFolderCreated
cd ..

echo "Creating hidden file for Reports folder."
cd Reports
touch .ReportsFolderCreated
cd ..

echo "Creating hidden file for Help folder."
cd Help
touch .HelpFolderCreated
cd ..

echo "Task completed. Have a nice day!"

Save the script.

That’s it. You have just created your first script.

Running our first shell script

The next step would be to run our script. There are 2 ways of doing this. We will look at both the options.

Option 1

We can directly run the script using the zsh command.

zsh ~/Desktop/folderCreator.zsh

Note that we will need to provide the path to the script file.

This is a straightforward way. We simply tell the interpreter to execute the commands in our script.

Option 2

This option requires a few more steps.

First we need to change the permissions on the script. We need to make sure that all 3: Owner, Group, Everyone else have the read and execute permissions. Of course, you are free to change the permissions to whatever you want. But the execute capability is required. We will change the permissions from the command line.

chmod ugo+x ~/Desktop/folderCreator.zsh

There are other ways of writing this command too. But for now we are simply saying that we want to add the execute capability to the Owner, Group, Everyone else. If you look at the file in the GUI, you will see its icon has changed to the executable icon.

Next we will simply run the following command from the terminal application.

./Desktop/folderCreator.zsh

Now we can simply run the script by invoking it from the terminal application. Or we can trigger it from the graphical user interface by simply double clicking on the file.

There you go. You have successfully created and tested your own script. Try to play around with some of the terminal commands and create your own scripts.