Linux command shell commands

Shell Built-in Commands

In Linux, there are shell built-in commands which you are already using but never paid attention to. Learn more about them in this tutorial.

In Linux, there are commands and then there are shell built-ins. Linux purists refrain from calling them commands or shell built-in commands.

So, what’s so special about these shell built-ins and how are they different from other Linux commands? You’ll learn about them in this tutorial.

What is a shell built-in?

When you run a Linux command, it is run from a binary in a sub-shell by forking the existing process. However, a shell built-in is contained in the shell itself. It runs in the same shell without creating a new process. Thus, shell built-ins are faster. cd, pwd, exit, export, alias are some of the commonly used shell built-ins.

Want to know what other shell built-in commands are there, why, and how they are used? In this tutorial, I’m going to answer all such questions.

List of shell builtin commands

To list all the builtin bash commands, all you have to do is execute the help command in the terminal:

A long list of commands. Isn’t it?

Now, let’s have a look at why they are used in the first place:

Command	Description
alias	Used to create a shortcut or an alternative name for a command.
bg	Moves suspended or stopped jobs in the background.
bind	It is used to show or modify the keybindings.
break	Used within loops to terminate the loop and continue the execution outside the loop.
builtin	If you have built-in and external commands with the same name, using builtin , you can prioritize the internal command for the execution.
caller	Used in the script to get more information about that specific code block.
case	It allows you to create multiple conditions and their outcomes based on a specific block.
cd	Change directory.
command	Used to execute external commands by bypassing any alias or functions having the same name.`
compgen	It will generate completions for command options, files, directories, and other items based on the current context.
complete	Allows you to modify the command completions for specific commands.
compopt	It allows you to modify argument completion for a specific command.
continue	Used in loops to skip the execution of the remaining iteration and move to the next iteration of the given code,
coproc	Creates a co-process to establish communication between the script and the background process.
declare	Used to define variables and their properties.
dirs	It shows the list of the commands that you went through recently.
disown	By using it, you can detach currently running jobs from the shell, allowing you to continue their execution even after closing the shell.
echo	Used to print output.
enable	Used to enable or disable the built-in shell commands and functions.
eval	It treats the given arguments as a command and executes it in such it was written in as script or executed directly in the terminal.
exec	It is used to replace the current process with the new one.
exit	Used to terminate the current shell session or the script.
export	It allows you to create or modify the environment variables.
false	Used to return a non-zero exit status that indicates a false condition or failure.
fc	By using it, you can list, modify and re-execute previously used commands.
fg	Used to bring suspended, or the background processes, to the foreground.
for [condition]; do[execution];	A for loop is used in the shell script to execute repetitive tasks.
function name	Used to create a function that can be used multiple times in a shell session or the shell script.
getopts	Allows you to parse the command-line options and arguments inside a script.
hash	It helps the shell to remember the location of the executables.
help	Gives information about built-in commands in the shell.
history	Provides a list of previously executed commands.
if COMMANDS; then COMMANDS;	A condition statement that executed the command if the condition is true.
jobs	Displays the status of the background jobs running in the background.
kill	Sends a signal to kill to terminate the process.
let arg [arg . ]	Using the let command, you can perform the arithmetic operations.
local	It allows the user to define local variables within the specific function.
logout	As the name suggests, it logs you out of the current user session.
mapfile	Reads text from an input stream or a file and stores it in an array.
popd	It is used to replace the top directory from the directory stack with the current directory.
printf	Used to format and print the variable values or the text.
pushd	Allows you to change the directory and stores the previous directory to the directory stack, by which you can easily navigate between multiple directories with the least effort.
pwd	When executed, it shows the current working directory.
read	Reads input from a file or user and assigns it to a variable.
readarray	Used to read lines from the input file and turns it into an array variable.
readonly	Used to declare and define read-only variables.
return	It is used to exit a function and, optionally, you can also specify a return value.
select	By using it, you can create a simple menu or interactive selection prompt for a user.
set	Allows you to modify the shell settings and parameters.
shift	By using it, you can drop the first argument and lower the remaining ones a step lower in the list of positional parameters.
shopt	Provides various options to modify the behavior of the shell.
source	It is used to reload config files and execute the script in the current shell.
suspend	Used to suspend the current shell and returns the control to the parent shell.
test	Used within the shell script, allowing you to check conditions and perform various tests.
time	Used to measure the command or script execution time.
times	It is used to measure the CPU usage of a shell session or a shell script.
trap	It allows you to execute specific operations when received defined signals.
true	Returns a successful exit status.
type	Informs you about the type of the command.
typeset	Allows you to define and control the behavior of the variables.
ulimit	By using it, you can manage resources for shell sessions.
umask	Allows you to control permissions given to newly created files and directories.
unalias	Used to unset alias which was configured using the alias command.
unset	It is used to unset variables and functions which were defined previously.
until COMMANDS; do COMMANDS; done	It is a loop that will be iterated until the given conditions are met.
variables	Provides names and descriptions of some shell variables.
wait	Used to pause the execution until all the background tasks are complete.
while COMMANDS; do COMMANDS; done	It is a loop that will be iterated till the specified conditions are true.

Some Linux purists frown upon calling them shell built-in commands. They prefer not addressing them as commands. However, most Linux books call them built-in shell commands so I see no harm in using the ‘command’ word with shell built-ins.

No man pages and binaries for shell built-in commands

Regular or should I say external Linux commands come with a man page entry that gives information on how to use them.

However, shell built-in commands don’t have man pages. If you try using the man command, it will show an error like this:

[email protected]:~$ man cd No manual entry for cd

You’ll have to use the help command that you saw above:

help shell_built_in_command_name

And it will give you some hints on using the built-in command.

Similarly, you won’t see binaries for the shell built-ins with whereis and which commands.

Identify the shell built-in command

To identify which command is built-in, you can use the type command as shown:

For example, if I want to check whether the cd is a built-in command or not, I will be using the following:

Sometimes you’ll see a re-implementation of a shell built-in as an external command. echo is a common example. To identify, use type -a to see if there is any re-implementation of a built-in command.

But what if the command is not built-in? So what output can you expect from the type command?

Well, apart from the above output, it can give you four other types of output:

• alias : If the command is an alias.
• file : If the command is an executable file and is also configured with the PATH variable.
• function : If the command is function.
• No output : If the command is not of the above, then, it won’t show you anything.

More bash specials

Bash also has some special variables that you may want to know about next.

Since we are discussing ‘specials’, how about special file permissions?

I hope you liked this article on shell built-ins. Let me know if you have questions or suggestions.

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