- Bash - Launches the Bourne-Again SHell, an sh-compatible shell that incorporates useful features from the Korn and C shells (ksh and csh). Syntax bash options Options -c string Commands are read from string.If there are arguments after the string, they are assigned to.
- This command works across all unix platforms, whether it’s Mac OS X, Linux, FreeBSD, or whatever, and it’s always reported back the same. For the vast majority of Mac OS X users, you will be using the Bash shell by default, which is both the standard in all recent versions of OS X and also one of the easier to use shells out there.
Free Mneptok Mac/OS X 10.0 Version 2.05 Full Specs. Download Now Secure Download. Recommended for shell newbies and vets alike, this is the UNIX shell of choice. Free Mneptok Mac/OS X 10.0 Version 2.05 Full Specs. Download Now Secure Download. Recommended for shell newbies and vets alike, this is the UNIX shell of choice. Shell scripts are normally written and run on Linux systems but they’re almost as at home on a Mac system as they are on a Linux system. You can write a Shell script on macOS, and you can run it easily, far more easily than you can on a Windows 10 system. In fact, on macOS, a shell script will likely run and accomplish what it was written to.
The Terminal app allows you to control your Mac using a command prompt. Why would you want to do that? Well, perhaps because you’re used to working on a command line in a Unix-based system and prefer to work that way. Terminal is a Mac command line interface. There are several advantages to using Terminal to accomplish some tasks — it’s usually quicker, for example. In order to use it, however, you’ll need to get to grips with its basic commands and functions. Once you’ve done that, you can dig deeper and learn more commands and use your Mac’s command prompt for more complex, as well as some fun, tasks.
Curated Mac apps that keep your Mac’s performance under control. Avoid Terminal commands, avoid trouble.
Download FreeHow to open Terminal on Mac
The Terminal app is in the Utilities folder in Applications. To open it, either open your Applications folder, then open Utilities and double-click on Terminal, or press Command - spacebar to launch Spotlight and type 'Terminal,' then double-click the search result.
You’ll see a small window with a white background open on your desktop. In the title bar are your username, the word 'bash' and the dimensions of the window in pixels. Bash stands for 'Bourne again shell'. There are a number of different shells that can run Unix commands, and on the Mac Bash is the one used by Terminal.
If you want to make the window bigger, click on the bottom right corner and drag it outwards. If you don’t like the black text on a white background, go to the Shell menu, choose New Window and select from the options in the list.
If Terminal feels complicated or you have issues with the set-up, let us tell you right away that there are alternatives. MacPilot allows to get access to over 1,200 macOS features without memorizing any commands. Basically, a third-party Terminal for Mac that acts like Finder.
For Mac monitoring features, try iStat Menus. The app collects data like CPU load, disk activity, network usage, and more — all of which accessible from your menu bar.
Basic Mac commands in Terminal
The quickest way to get to know Terminal and understand how it works is to start using it. But before we do that, it’s worth spending a little time getting to know how commands work. To run a command, you just type it at the cursor and hit Return to execute.
Every command is made up of three elements: the command itself, an argument which tells the command what resource it should operate on, and an option that modifies the output. So, for example, to move a file from one folder to another on your Mac, you’d use the move command 'mv' and then type the location of the file you want to move, including the file name and the location where you want to move it to.
Let’s try it.
Type cd ~/Documentsthen and press Return to navigate to your Home folder.
Type lsthen Return (you type Return after every command).
You should now see a list of all the files in your Documents folder — ls is the command for listing files.
To see a list of all the commands available in Terminal, hold down the Escape key and then press y when you see a question asking if you want to see all the possibilities. To see more commands, press Return.
Unix has its own built-in manual. So, to learn more about a command type man [name of command], where 'command' is the name of the command you want find out more about.
Terminal rules
There are a few things you need to bear in mind when you’re typing commands in Terminal, or any other command-line tool. Firstly, every character matters, including spaces. So when you’re copying a command you see here, make sure you include the spaces and that characters are in the correct case.
You can’t use a mouse or trackpad in Terminal, but you can navigate using the arrow keys. If you want to re-run a command, tap the up arrow key until you reach it, then press Return. To interrupt a command that’s already running, type Control-C.
Commands are always executed in the current location. So, if you don’t specify a location in the command, it will run wherever you last moved to or where the last command was run. Use the cdcommand, followed by a directory path, like in Step 1 above, to specify the folder where you want a command to run.
There is another way to specify a location: go to the Finder, navigate to the file or folder you want and drag it onto the Terminal window, with the cursor at the point where you would have typed the path.
Here’s another example. This time, we’ll create a new folder inside your Documents directory and call it 'TerminalTest.'
Open a Finder window and navigate to your Documents folder.
Type cd and drag the Documents folder onto the Terminal window.
Now, type mkdir 'TerminalTest'
Go back to the Finder, open Text Edit and create a new file called 'TerminalTestFile.rtf'. Now save it to the TerminalTest folder in your Documents folder.
In the Terminal window, type cd ~/Documents/TerminalTest then Return. Now type lsand you should see 'TerminalTestFile' listed.
To change the name of the file, type this, pressing Return after every step:
cd~/Documents/Terminal Test
mv TerminalTestFile TerminalTestFile2.rtf
That will change the name of the file to 'TerminalTestFile2'. You can, of course, use any name you like. The mv command means 'move' and you can also use it to move files from one directory to another. In that case, you’d keep the file names the same, but specify another directory before typing the the second instance of the name, like this:
mv ~/Documents/TerminalTest TerminalTestFile.rtf ~/Documents/TerminalTest2 TerminalTestFile.rtf
More advanced Terminal commands
Terminal can be used for all sorts of different tasks. Some of them can be performed in the Finder, but are quicker in Terminal. Others access deep-rooted parts of macOS that aren’t accessible from the Finder without specialist applications. Here are a few examples.
Copy files from one folder to another
In a Terminal window, type ditto [folder 1] [folder 1] where 'folder 1' is the folder that hosts the files and 'folder 2' is the folder you want to move them to.
To see the files being copied in the Terminal window, type -v after the command.
Download files from the internet
You’ll need the URL of the file you want to download in order to use Terminal for this.
cd ~/Downloads/
curl -O [URL of file you want to download]
If you want to download the file to a directory other than your Downloads folder, replace ~/Downloads/ with the path to that folder, or drag it onto the Terminal window after you type the cd command.
Change the default location for screenshots
If you don’t want macOS to save screenshots to your Desktop when you press Command-Shift-3, you can change the default location in Terminal
defaults write com.apple.screencapture location [path to folder where you want screenshots to be saved]
Hit Return
killall SystemUIServer
Hit Return
Change the default file type for screenshots
By default, macOS saves screenshots as .png files. To change that to .jpg, do this:
defaults write com.apple.screencapture type JPG
Press Return
killall SystemUIServer
Press Return
Delete all files in a folder
The command used to delete, or remove, files in Terminal is rm. So, for example, if you wanted to remove a file in your Documents folder named 'oldfile.rtf' you’d use cd ~/Documents to go to your Documents folder then to delete the file. As it stands, that will delete the file without further intervention from you. If you want to confirm the file to be deleted, use -i as in rm -i oldfile.rtf
To delete all the files and sub-folders in a directory named 'oldfolder', the command is rm -R oldfolder and to confirm each file should be deleted, rm -iR oldfolder
Just because you can use Terminal to delete files on your Mac, doesn’t mean you should. It’s a relatively blunt instrument, deleting only those files and folders you specify.
Another way to free up space
If your goal in removing files or folders is to free up space on your Mac, or to remove junk files that are causing your Mac to run slowly, it’s far better to use an app designed for the purpose. CleanMyMac X is one such app.
It will scan your Mac for files and recommend which ones you can delete safely, as well as telling you how much space you’ll save. And once you’ve decided which files to delete, you can get rid of them in a click. You can download CleanMyMac here.
As you can see, while Terminal may look scary and seem like it’s difficult to use, it really isn’t. The key is learning a few commands, such as those we’ve outlined above, and getting to know the syntax for those commands.
However, you should be careful when using Terminal, it’s a powerful tool that has deep access to your Mac’s system files. Check commands by googling them if you’re not sure what they do. And if you need to delete files to save space, use an app like CleanMyMac X to do it. It’s much safer!
These might also interest you:
Writing a shell script is like riding a bike. You fall off and scrape your knees a lot at first. With a bit more experience, you become comfortable riding them around town, but also quickly discover why most people drive cars for longer trips.
Shell scripting is generally considered to be a glue language, ideal for creating small pieces of code that connect other tools together. While shell scripts can be used for more complex tasks, they are usually not the best choice.
If you have ever successfully trued a bicycle wheel (or paid someone else to do so), that’s similar to learning the basics of shell scripting. If you don’t true your scripts, they wobble. Put another way, it is often easy to write a script, but it can be more challenging to write a script that consistently works well.
This chapter and the next two chapters introduce the basic concepts of shell scripting. The remaining chapters in this document provide additional breadth and depth. This document is not intended to be a complete reference on writing shell scripts, nor could it be. It does, however, provide a good starting point for beginners first learning this black art.
Shell Script Dialects
There are many different dialects of shell scripts, each with their own quirks, and some with their own syntax entirely. Because of these differences, the road to good shell scripting can be fraught with peril, leading to script failures, misbehavior, and even outright data loss.
To that end, the first lesson you must learn before writing a shell script is that there are two fundamentally different sets of shell script syntax: the Bourne shell syntax and the C shell syntax. The C shell syntax is more comfortable to many C programmers because the syntax is somewhat similar. However, the Bourne shell syntax is significantly more flexible and thus more widely used. For this reason, this document only covers the Bourne shell syntax.
The second hard lesson you will invariably learn is that each dialect of Bourne shell syntax differs slightly. This document includes only pure Bourne shell syntax and a few BASH-specific extensions. Where BASH-specific syntax is used, it is clearly noted.
The terminology and subtle syntactic differences can be confusing—even a bit overwhelming at times; had Dorothy in The Wizard of Oz been a programmer, you might have heard them exclaim, 'BASH and ZSH and CSH, Oh My!' Fortunately, once you get the basics, things generally fall into place as long as you avoid using shell-specific features. Stay on the narrow road and your code will be portable.
Some common shells are listed below, grouped by script syntax:
Bourne-compatible shells
shbashzshksh
C-shell-compatible shells
cshtcshbcsh(C shell to Bourne shell translator/emulator)
Many of these shells have more than one variation. Most of these variations are denoted by prefixing the name of an existing shell with additional letters that are short for whatever differentiates them from the original shell. For example:
The shell
pdkshis a variant ofksh. Being a public domain rewrite of AT&T'sksh, it stands for 'Public Domain Korn SHell.' (This is a bit of a misnomer, as a few bits are under a BSD-like open source license. However, the name remains.)The shell
tcshis an extension ofcsh. It stands for the TENEX C SHell, as some of its enhancements were inspired by the TENEX operating system.The shell
bashis an extension ofsh. It stands for the Bourne Again SHell. (Oddly enough, it is not a variation ofash, the Almquist SHell, though both are Bourne shell variants. This should not be confused with thedashshell—anash-derived shell used in some Linux distributions—whose name stands for the Debian Almquist SHell.)
And so on. In general, with the exception of csh and tcsh, it is usually safe to assume that any modern login shell is compatible with Bourne shell syntax.
Note: Because the C shell syntax is not well suited to scripting beyond a very basic level, this document does not cover C shell variants in depth. For more information, see About the C Shell.
About the C Shell
The C shell is popular among some users as a shell for interacting with the computer because it allows simple scripts to be written more easily. However, the C shell scripting language is limited in a number of ways, many of which are hard to work around. For this reason, use of the C shell scripting language for writing complex scripts is not recommended. For more information, read “CSH Programming Considered Harmful” at http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/. Although many of the language flaws it describes are fixed by some modern C shells, if you are writing a script that must work on multiple computers across different operating systems, you cannot always guarantee that the installed C shell will support those extensions.
However, the C shell scripting language has its uses, particularly for writing scripts that set up environment variables for interactive shell environments, execute a handful of commands in order, or perform other relatively lightweight chores. To support such uses, the C shell syntax is presented alongside the Bourne shell syntax within this 'basics” chapter where possible.
Outside of this chapter, this document does not generally cover the C shell syntax. If after reading this, you still want to write a more complex script using the C shell programming language, you can find more information in on the C shell in the manual page for csh.
Shell Variables and Printing
What follows is a very basic shell script that prints “Hello, world!” to the screen:
The first thing you should notice is that the script starts with ‘#!’. This is known as an interpreter line. If you don’t specify an interpreter line, the default is usually the Bourne shell (/bin/sh). However, it is best to specify this line anyway for consistency.
The second thing you should notice is the echo command. The echo command is nearly universal in shell scripting as a means for printing something to the user’s screen. (Technically speaking, echo is generally a shell builtin, but it also exists as as standalone command, /bin/echo. You can read more about the difference between the builtin version and the standalone version in echo and Use Shell Builtins Wherever Possible.)
If you’d like, you can try this script by saving those lines in a text file (say “hello_world.sh”) in your home directory. Then, in Terminal, type:
Of course, this script isn’t particularly useful. It just prints the words “Hello, world!“ to your screen. To make this more interesting, the next script throws in a few variables.
Type or paste this script into the text editor of your choice (see Creating Text Files in Your Home Directory for help creating a text file) and save the file in your home directory in a file called test.sh.
Once you have saved the file in your home directory, type ‘chmod a+x test.sh’ in Terminal to make it executable. Finally, run it with ‘./test.sh leaders’. You should see “Hello, world leaders!” printed to your screen.
This script provides an example of a variable assignment. The variable $1 contains the first argument passed to the shell script. In this example, the script makes a copy and stores it into a variable called FIRST_ARGUMENT, then prints that variable.
You should immediately notice that variables may or may not begin with a dollar sign, depending on how you are using them. If you want to dereference a variable, you precede it with a dollar sign. The shell then inserts the contents of the variable at that point in the script. For all other uses, you do not precede it with a dollar sign.
Important: You generally do not want to prefix the variable on the left side of an assignment statement with a dollar sign. Because FIRST_ARGUMENT starts out empty, if you used a dollar sign, the first line:
FIRST_ARGUMENT were nonempty. (If you really want to assign a value to a variable whose name is in a different variable, use eval, as described in Using the eval Builtin for Data Structures, Arrays, and Indirection.)You should also notice that the argument to echo is surrounded by double quotation marks. This is explained further in the next section, Using Arguments And Variables That Contain Spaces.
C Shell Note: The syntax for assignment statements in the C shell is rather different. Instead of an assignment statement, the C shell uses the set and setenv builtins to set variables as shown below:
set and setenv is described in Exporting Shell Variables.Using Arguments And Variables That Contain Spaces
Take a second look at the script from the previous section:
Notice that the echo statement is followed by a string surrounded by quotation marks. Normally, the shell uses spaces to separate arguments to commands. Outside of quotation marks, the shell would treat “Hello,” and “world” as separate arguments to echo.
By surrounding the string with double quote marks, the shell treats the entire string as a single argument to echo even though it contains spaces.
To see how this works, save the script above as test.sh (if you haven’t already), then type the following commands:
The first line above prints “Hello, world leaders!” because the space after “leaders” ends the first argument ($1). Inside the script, the variable $1 contains “leaders”, $2 contains “and”, and $3 contains “citizens”.
The second line above prints “Hello, world leaders and citizens!” because the quotation marks on the command line cause everything within them to be grouped as a single argument.
Notice also that there are similar quotation marks on the right side of the assignment statement:
With most modern shells, these double quotation marks are not required for this particular assignment statement (because there are no literal spaces on the right side), but they are a good idea for maximum compatibility. See Historical String Parsing in Historical Footnotes and Arcana to learn why.
When assigning literal strings (rather than variables containing strings) to a variable, however, you must surround any spaces with quotation marks. For example, the following statement does not do what you might initially suspect:
If you type this statement, the Bourne shell gives you an error like this:
The reason for this seemingly odd error is that the assignment statement ends at the first space, so the next word after that statement is interpreted as a command to execute. See Overriding Environment Variables for Child Processes (Bourne Shell) for more details.
Instead, write this statement as:
Using quotation marks is particularly important when working with variables that contain filenames or paths. For example, type the following commands:
The above example creates a directory in /tmp called “My Folder”. (Don’t worry about deleting it because /tmp gets wiped every time you reboot.) It then attempts to list the files in that directory. The first time, it uses quotation marks. The second time, it does not. Notice that the shell misinterprets the command the second time as being an attempt to list the files in /tmp/My and the files in Folder.
Handling Quotation Marks in Strings
In modern Bourne shells, expansion of variables, occurs after the statement itself is fully parsed by the shell. (See Historical String Parsing in Historical Footnotes and Arcana for more information.) Thus, as long as the variable is enclosed in double quote marks, you do not get any execution errors even if the variable’s value contains double-quote marks.
However, if you are using double quote marks within a literal string, you must quote that string properly. For example:
C Shell Note: The C shell handling of backslashes within double-quoted strings is different. In the C shell, the previous example should be changed to:
to achieve the desired effect. This difference is described further in Parsing, Variable Expansion, and Quoting.This quoting technique also applies to literal strings within commands entered on the command line. For example, using the script from earlier in Shell Variables and Printing, the command:
prints the phrase “Hello, world “leaders”!”
The details of quotes as they apply to variable expansion are explained in Parsing, Variable Expansion, and Quoting. (Variable safety with shells that predate this behavior is generally impractical. Fortunately, the modern behavior has been the norm since the mid-1990s.)
Shell scripts also allow the use of single quote marks. Variables between single quotes are not replaced by their contents. Be sure to use double quotes unless you are intentionally trying to display the actual name of the variable. You can also use single quotes as a way to avoid the shell interpreting the contents of the string in any way. These differences are described further in Parsing, Variable Expansion, and Quoting.
Exporting Shell Variables
One key feature of shell scripts is that variables are typically limited in their scope to the currently running script. The scoping of variables is described in more detail in Subroutines, Scoping, and Sourcing. For now, though, it suffices to say that variables generally do not get passed on to scripts or tools that they execute.
Normally, this is what you want. Most variables in a shell script do not have any meaning to the tools that they execute, and thus represent clutter and the potential for variable namespace collisions if they are exported. Occasionally, however, you will find it necessary to make a variable's value available to an outside tool. To do this, you must export the variable. These exported variables are commonly known as environment variables because they affect the execution of every script or tool that runs but are not part of those scripts or tools themselves.
A classic example of an environment variable that is significant to scripts and tools is the PATH variable. This variable specifies a list of locations that the shell searches when executing programs by name (without specifying a complete path). For example, when you type ls on the command line, the shell searches in the locations specified in PATH (in the order specified) until it finds an executable called ls (or runs out of locations, whichever comes first).
The details of exporting shell variables differ considerably between the Bourne shell and the C shell. Thus, the following sections explain these details in a shell-specific fashion.
Using the export Builtin (Bourne Shell)
Generally speaking, the first time you assign a value to an environment variable such as the PATH variable, the Bourne shell creates a new, local copy of this shell variable that is specific to your script. Any tool executed from your script is passed the original value of PATH inherited from whatever script, tool, or shell that launched it.
With the BASH shell, however, any variable inherited from the environment is automatically exported by the shell. Thus, in some versions of OS X, if you modify inherited environment variables (such as PATH) in a script, your local changes will be seen automatically by any tool or script that your script executes. Thus, in these versions of OS X, you do not have to explicitly use the export statement when modifying the PATH variable.
Because different Bourne shell variants handle these external environment variables differently (even among different versions of OS X), this creates two minor portability problems:
A script written without the
exportstatement may work on some versions of OS X, but will fail on others. You can solve this portability problem by using theexportbuiltin, as described in this section.A shell script that changes variables such as
PATHwill alter the behavior of any script that it executes, which may or may not be desirable. You can solve this problem by overriding thePATHenvironment variable when you execute each individual tool, as described in Overriding Environment Variables for Child Processes (Bourne Shell).
To guarantee that your modifications to a shell variable are passed to any script or tool that your shell script calls, you must use the export builtin. You do not have to use this command every time you change the value; the variable remains exported until the shell script exits.
For example:
Either of these statements has the same effect—specifically, they export the local notion of the PATH environment variable to any command that your script executes from now on. There is a small catch, however. You cannot later undo this export to restore the original global declaration. Thus, if you need to retain the original value, you must store it somewhere yourself.
In the following example, the script stores the original value of the PATH environment variable, exports an altered version, executes a command, and restores the old version.
Bash Shell For Mac Keyboard
If you need to find out whether an environment variable (whether inherited by your script or explicitly set with the export directive) was set to empty or was never set in the first place, you can use the printenv command to obtain a complete list of defined variables and use grep to see if it is in the list. (You should note that although printenv is a csh builtin, it is also a standalone command in /usr/bin.)
For example:
The resulting variable will contain 1 if the variable is defined in the environment or 0 if it is not.
Overriding Environment Variables for Child Processes (Bourne Shell)
Because the BASH Bourne shell variant automatically exports all variables inherited from its environment, any changes you make to preexisting environment variables such as PATH are automatically inherited by any tool or script that your script executes. (This is not true for other Bourne shell variants; see Using the export Builtin (Bourne Shell) for further explanation.)
While automatic export is usually convenient, you may sometimes wish to change a preexisting environment variable without modifying the environment of any script or tool that your script executes. For example, if your script executes a number of tools in /usr/local/bin, it may be convenient to change the value of PATH to include /usr/local/bin. However, you may not want child processes to also look in /usr/local/bin.
This problem is easily solved by overriding the environment variable PATH on a per-execution basis. Consider the following script:
This script prints the value of the variable MYVAR. Normally, this variable is empty, so this script just prints a blank line. Save the script as printmyvar.sh, then type the following commands:
Notice that the assignment statement MYVAR=7 applies only to the command that follows it. The value of MYVAR is altered in the environment of the command ./printmyvar.sh, so the script prints the number 7. However, the original (empty) value is restored after executing that command, so the echo statement afterwards prints an empty string for the value of MYVAR.
Thus, to modify the PATH variable locally but execute a command with the original PATH value, you can write a script like this:
Using the setenv Builtin (C shell)
In the C shell, variables are exported if you set them with setenv, but not if you set them with set. Thus, if you want your shell variable modifications to be seen by any tool or script that you call, you should use the setenv builtin. This builtin is the C shell equivalent to issuing an assignment statement with the export builtin in the Bourne shell.
If you want your shell variables to only be available to your script, you should use the set builtin (described in Shell Variables and Printing). The set builtin is equivalent to a simple assignment statement in the Bourne shell.
Notice that the local variable version requires an equals sign (=), but the exported environment version does not (and produces an error if you put one in).
To remove variables in the C shell, you can use the unsetenv or unset builtin. For example:
This will generate an error message. In the C shell, it is not possible to print the value of an undefined variable, so if you think you may need to print the value later, you should set it to an empty string rather than using unset or unsetenv.
If you need to test an environment variable (not a shell-local variable) that may or may not be part of your environment (a variable set by whatever process called your script), you can use the printenv builtin. This prints the value of a variable if set, but prints nothing if the variable is not set, and thus behaves just like the variable behaves in the Bourne shell.
For example:
This prints X is ' if the variable is either empty or undefined. Otherwise, it prints the value of the variable between the quotation marks.
If you need to find out if a variable is simply empty or is actually not set, you can also use printenv to obtain a complete list of defined variables and use grep to see if it is in the list. For example:
The resulting variable will contain 1 if the variable is defined in the environment or 0 if it is not.
Overriding Environment Variables for Child Processes (C Shell)
Unlike the Bourne shell, the C shell does not provide a built-in syntax for overriding environment variables when executing external commands. However, it is possible to simulate this either by using the env command.
The best and simplest way to do this is with the env command. For example:
As an alternative, you can use the set builtin to make a temporary copy of any variable you need to override, change the value, execute the command, and restore the value from the temporary copy.
You should notice, however, that whether you use the env command or manually make a copy, the PATH variable is altered prior to searching for the command. Because the PATH variable controls where the shell looks for programs to execute, you must therefore explicitly provide a complete path to the ls command or it will not be found (unless you have a copy in /usr/local/bin, of course). The PATH environment variable is explained in Special Shell Variables.
As a workaround, you can determine the path of the executable using the which command prior to altering the PATH environment variable.
Or, using env:
The use of the backtick (`) operator in this fashion is described in Inline Execution.
Security Note:If your purpose for overriding an environment variable is to prevent disclosure of sensitive information to a potentially untrusted process, you should be aware that if you use setenv for the copy, the called process has access to that temporary copy just as it had access to the original variable. To avoid this, be sure to create the temporary copy using the set builtin instead of setenv.
Deleting Shell Variables
For the most part, in Bourne shell scripts, when you need to get rid of a variable, setting it to an empty string is sufficient. However, in long-running scripts that might encounter memory pressure, it can be marginally useful to delete the variable entirely. To do this, use the unset builtin.
Bash Shell For Mac Osx
For example:
The unset builtin can also be used to delete environment variables.
C Shell Note: The C shell unset builtin is identical except that it cannot be used to delete environment variables. Use unsetenv instead, as shown in Overriding Environment Variables for Child Processes (C Shell).
Also, in C shell, if you try to use a deleted variable, it is considered an error. (In Bourne shell, an unset variable is treated like an empty string.)
Bash Shell For Mac Mojave
Bash Shell For Mac
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