Bash Built-In Commands

 Many built-in commands exist in Bash to perform different types of tasks. Bash has no built-in function like other programming languages. But the tasks of the built-in functions can be done easily using the different types of built-in commands of Bash. Bash has some special built-in commands and some inherited built-in commands which are inherited from the shell commands. The purposes of the most commonly used built-in commands of Bash and the uses of some built-in commands are described in this tutorial.

List of Some Useful Built-In Commands

 

Command	Purpose
echo	To write the string data in the output.
printf	To write the formatted string data in the output.
read	To take an input from the user.
pwd	To print the absolute path of the current working directory.
date	To print the current date and time of the system.
declare	To declare a variable with the data type.
set	To modify the behavior of the current shell.
unset	To remove any variable and function names.
ls	To print the list of all files and folders of the current location.
cat	To create or read a file.
rm	To remove one or more files.
mkdir	To create a new directory.
rmdir	To remove any directory.
dir	To print the directory stack of the shell.
cd	To change the current directory location.
let	To perform the arithmetic operations.
alias	To create a shortcut of any command.
unalias	To delete any previously created alias command.
return	To return the value from the function.
eval	To evaluate any expression as the command.
history	To print the history information.
export	To pass the shell variable or function from the parent to the child process.
help	To print the help information about built-in shell commands.
exit	To exit to the terminal from the shell with exit status code.

Examples of Some Bash Built-In Commands

The uses of some useful Bash built-in commands are shown in this part of the tutorial.

Example 1: “Ls” Command

Run the following command to print the list of the current files and folders:

$ ls

 
Run the following command to print the list of all files with the “bash” extension:

$ ls *.bash

 
Run the following command to print the list of all files and folders that starts with the “error”:

$ ls error*

 
Output:

The following output appears after executing the previous commands:

Example 2: “Cd” Command

Run the following command to change the current directory location to “/home/fahmida/code”:

$ cd /home/fahmida/code

 
Run the following command to check the content of the changed directory:

$ ls

 
Output:

The following output appears after executing the previous commands:

Example 3: “Cat” Command

Run the following command to create a new file named languages.txt and add some content to the file. Press Ctrl+D to go to the command prompt after adding the content.

$ cat > languages.txt

 
Run the following command to print the content of the file:

$ cat languages.txt

 
Run the following command to append the content to the file:

$ cat >> languages.txt

 
Run the following command to print the content of the file after appending:

$ cat languages.txt

 
Output:

The following output appears after executing the previous commands:

Example 4: “Echo” Command

Run the following command to print a simple string:

$ echo "Welcome to CMHTech"

 
Run the following command to print the string with a newline:

$ echo -e "Welcome\nTo\nCMHTECH"

 
Output:

The following output appears after executing the previous commands:

Example 5: “Read” Command

Run the following command to take the input in the $name variable:

$ read name

 
Run the following command to print the input value:

$ echo $name

 
Run the following command to take the input with the prompt message:

$ read -p 'Enter username:' un

 
Run the following command to print the input value:

$ echo $un

 
Output:

The following output appears after executing the previous commands:

Example 6: “Mkdir” Command

Run the following command to create a directory named “temp”:

$ mkdir temp

 
Run the following command to check whether the directory is created or not:

$ ls

 
Output:

The following output appears after executing the previous commands:

Example 7: “Date” Command

Run the following command to print the current date and time of the system:

$ date

 
Output:

The following output appears after executing the previous command:

Example 8: “Declare” Command

Run the following command to declare a variable named $number of integer type:

$ declare -i number

 
Run the following command to assign the string value to the $number variable that can take only the integer value:

$ number='abc'

 
Run the following command to print the current value of the $number variable which is 0 to assign the string value to the integer variable:

$ echo $number

 
Run the following command to assign the number value to the $number:

$ number=20

 
Run the following command to print the current value of the $number variable:

$ echo $number

 
Output:

The following output appears after executing the previous commands. The output shows that the number variable contains 0 when the string value is assigned to a variable to declare the variable by the “declare” command:

Example 9: “Eval” Command

Run the following command to assign a command to the variable:

$ cmd="echo"

 
Run the following command to assign a string value to a variable:

$ str="Hello World"

 
Run the following command to execute the “echo” command using the “eval” command:

$ eval $cmd $str

 
Output:

The following output appears after executing the previous commands:

Example 10: “History” Command

Run the following command to print the history information of the current terminal:

$ history

 
Output:

The following output appears after executing the previous commands. The output shows that four commands are executed after opening the terminal: