This series is an excerpt from the first chapter of my upcoming book “Scripting macOS” which will teach you to use and create shell scripts on macOS.
- Part 1: First Script
- Part 2: The Script File
- Part 3: The Code
- Part 4: Running the Script
- Part 5: Lists of Commands
- Part 6: Turning it off and on again
- Part 7: Download and Install Firefox
I will publish one part every week over the summer. Enjoy!
Running the Script
Now that we have the code for a functional (even though minimal) script, you can run it from the command line with:
(Verify that the current directory is your script project folder.) You probably have wondered why you need the
./ before the script name. When you type the script name without the
./ it will fail:
zsh: command not found: hello.sh
What does the
./ mean and why do we need it?
Current Working Directory
In the shell, the
. character represents the current working directory or the directory that you ‘
cd-ed’ to. Usually, the working directory is shown in the window title bar of Terminal and in the shell prompt (the status text shown before your cursor in the Terminal window). You can also have the system show your current working directory with the
When you type
./hello.sh the shell will substitute the current working directory for the
. character to get
Your path will of course look different.
This means that when I use the
./hello.sh form, the shell knows exactly which file I want to execute and where to look for it. There is no ambiguity.
Now we know what the
./ means, but why is it necessary?
A command you enter in the shell is just a string of text to begin with. The shell has to parse this text into pieces to determine what needs to be done.
We will use the
chmod command from earlier as an example:
> chmod +x hello.sh
The text you entered is ‘
chmod +x hello.sh’. The shell will split this text into pieces on the spaces (or tab characters). This will yield three elements:
+x,’ and ‘
The shell is only really interested in the first element. The shell will try to interpret the first element of your entry as a command.
Note: Many programming languages start counting at zero. This first element is also called ‘argument zero’ or
The remaining elements are arguments, which the shell will pass on to the command. Arguments are optional. Not all commands require or even have arguments.
Note: This is a simplified description of the parsing process in shells. The reality is quite a bit more complex. But this description is ‘close enough’ for most situations. We will explore some of the nuances later.
If you are interested, you can get all the details in the shell’s documentation:
- Zsh Documentation: Shell Grammar
bashman page, search for ‘Command Execution’
When there is a ‘
/‘ character anywhere in the first element, the shell will interpret the first element as a full or relative path to an executable file and attempt to run that.
This is the behavior we are using when we type
./hello.sh. Since that contains a ‘
/‘ the shell will resolve the path and execute our script.
We are intentionally using the seemingly redundant
./ prefix to tell the shell to run the script from the current working directory.
When the first element does not contain a ‘
/,’ the shell will check if it is one of the following:
- a shell function
- a shell built-in command or reserved word
- an external command
Shell functions are (mainly) customized shell behavior declared by the user. They look and work like commands.
Shell built-in commands cover tasks that are either inherent to the shell or can be performed much faster within the shell than as an external command. These are commands that affect the internal state of the current shell process like
history, or commands that are simpler and faster to implement as built-ins like
You rarely need to worry about whether a command is a built-in.
Many built-in commands will have an executable external command file as well. This serves as a ‘fallback’ for the rare situation that the shell built-in is not available.
You can use the (aptly, but confusingly, named)
command built-in to determine if a command is built-in or external:
> command -V cd
cd is a shell builtin
> command -V sw_vers
sw_vers is /usr/bin/sw_vers
When the command entered is neither a function, nor a built-in, and does not contain a ‘
/,’ then the shell will go search for an executable file with that name.
PATH environment variable determines the locations in the file system and the order in which to search them.
PATH on macOS in an interactive shell is:
PATH variable contains a colon-separated list of directories. When you have third-party software installed, or customized your shell configuration, your shell may have additional directories. The default
PATH splits into the following five directories:
Note: Variable names in the shell are case-sensitive. The variable names
my_varrepresent different variable and values.
Zsh, however, has the concept of ‘connected variables.’ In zsh,
pathare connected variables. The upper-case
PATHcontains the colon-separated list of directory paths, while the lower-case path is an array of paths. The actual list of directories will be the same and changing one variable will change the other. Zsh has this concept to maintain compatibility with other shells with the
PATH, while also allowing you to use array operators on the path.
I will use the colon-separated
PATH, because it is more compatible with other shells.
When a user enters a command like this:
This is neither a function or alias, nor a built-in command. It also does not contain a ‘
The shell will check for the presence of an executable file with the name
system_profiler in all the directories given in the
PATH. It will start with
/bin, until it finally finds a matching file in
Then the shell will attempt to execute
When no matching files can be found in any of the directories listed in the
PATH, the shell will present a ‘command not found’ error.
zsh: command not found: cantFindMe
If you are curious which file the shell will use for a given command you can use the
command or the
> command -V system_profiler
system_profiler is /usr/sbin/system_profiler
> which system_profiler
Once the shell finds a matching file, it will stop searching the remaining paths. If there were a second matching executable in
/sbin, the shell would never find and execute it. The order of the directory paths given in the
PATH variable determines the precedence.
We can test this by placing an executable with a file name matching an existing command in
/usr/local/bin comes first in the default interactive
PATH, the shell should prefer our executable over the default command.
> sudo ditto hello.sh /usr/local/bin/system_profiler
You need administrator privileges to modify the contents of
ditto command preserves all the file’s metadata (like privileges and extended attributes), which makes it preferable to
cp for this task.
Then open a new Terminal window. You need to do this because every shell instance will cache or ‘remember’ the lookup for a command to speed up the process later. The shell instance in your current terminal window will remember the last lookup for the
system_profiler command. A new Terminal window will start a new ‘fresh’ shell instance, forcing a new lookup:
> which system_profiler
Obviously, overriding a system provided command this way can break your workflows and scripts. To be safe, remove our script from
/usr/local/bin right away:
> sudo rm /usr/local/bin/system_profiler
There are some situations where overriding a system-provided command is desirable, though. For example, you could install the latest version of bash 5 as
/usr/local/bin/bash. And then, when you invoke bash from your interactive terminal, you will launch that version, instead of the outdated version that comes with macOS.
Note: You cannot simply overwrite
/bin/bashwith the newer version on macOS. The
/sbinfolders are protected by System Integrity Protection and the read-only system volume.
When bash is invoked with the absolute path
/bin/bash, the path will need to be updated to use the newer version, though. This includes the shebangs in scripts, and the UserShell attribute in a user’s account record for their default shell.
Extending your tool set
As you get more confident and experienced with scripting, you will assemble a set of scripts that you will use regularly. When you use a script often, it would be nice if the shell recognized them as commands, without having to type the path to them.
To achieve that you can add the directory containing the scripts to the
PATH variable. I put my frequently used tools in
~/bin. The name is chosen to be somewhat consistent with the four standard locations for tools, but really does not matter. You can append your tool directory to the
> export PATH=$PATH:~/bin
This reads as: replace the value of the environment variable
PATH with its current value and append
:~/bin. You can verify the new value with
> echo $PATH
We added our custom directory to the end of the
PATH variable, so you cannot accidentally override system tools.
To prevent other local users from changing your command files, you should set the directory’s privileges, so that only you can access:
> chmod 700 ~/bin
Now you can copy our
hello.sh script to that folder. We will rename it to just
hello, so it looks more like a command:
> ditto hello.sh ~/bin/hello
Then you can run your script just by typing
Most of the scripts you build will not need to be as accessible as this.
Since you do not want to manually change the
PATH every time you create a new Terminal window, you should add this line to your shell configuration file:
PATH to find and possibly override commands can be a useful and powerful tool. However, when scripting, you have to always remember, that the interactive Terminal environment may not be the environment that your script will run in ‘production.’
This is especially important for scripts run by
- LaunchDaemons or LaunchAgents
- applications other than Terminal (such as Xcode)
- installer packages
- management systems
When run from any of these environments, the environment the script runs in will be different from your interactive shell environment. Most likely the
PATH in any of these environments will be set to the minimal four system folders:
This should work well for most commands and tools. But when you are working with third party software, especially newer versions, then you need to pay very close attention.
When you build scripts for these environments, I recommend setting the
PATH at the beginning of your script explicitly to the value you need:
When your scripts require tools, include their locations in the
PATH. This way, the
PATH environment is declared explicitly at the beginning of the script and there can be no confusion.
For the scripts in this series, the default
PATH will be sufficient, so we will not need to worry about this.
Next: Lists of Commands