1. Overview

The source command can be used to read a file and treat its content as a set of commands to execute.

In this quick tutorial, we’ll explore how the Linux source command can help us in such situations to execute commands and refresh the environment variables.

2. Syntax

Let’s look at the basic syntax:

$ source FILENAME [arguments]

$ . FILENAME [arguments]

As we can see above, the dot or period character ‘. ‘ is a synonym for the source keyword. As such, there is no difference in output between the two.

To see the source command in action, let’s use it:

$ cat test.txt
printf "Today's date is: "
date
echo
cal -3

$ source test.txt
Today's date is: Sat May 23 07:26:50 IST 2020

                            2020
       April                  May                   June
Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa
          1  2  3  4                  1  2      1  2  3  4  5  6
 5  6  7  8  9 10 11   3  4  5  6  7  8  9   7  8  9 10 11 12 13
12 13 14 15 16 17 18  10 11 12 13 14 15 16  14 15 16 17 18 19 20
19 20 21 22 23 24 25  17 18 19 20 21 22 23  21 22 23 24 25 26 27
26 27 28 29 30        24 25 26 27 28 29 30  28 29 30
                      31

Here, we first created the file text.txt, which contains the commands to display the date and calendar on the console. After that, we ran the source command and passed our file name as an argument.

Finally, we were able to execute all these commands and see the output on the console.

3. Use Case

When we run a script using the source command, all the commands are executed in the same shell. As a result, any variable that has been assigned a value in the script will retain its value after the execution is complete.

This is the main reason it’s popularly used to source environment variables into the current terminal session.

4. Difference Between source and bash Commands

Sometimes, there’s confusion between the source and the bash commands because both can execute commands in a script. The key difference here is that the source command creates variables in the same shell, while the bash command creates a new shell and then executes all the script commands inside it.

Therefore, we can’t use the bash command to set the environment variables, as those variables are destroyed once the bash shell exits.

5. Conclusion

In this quick tutorial, we’ve seen how to use the source command and how it differs from the bash command.

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