The source command is a built-in shell command used to read and execute commands from a file inside the current shell session. The source command is commonly used to retain/change the environment variable in the current shell. In short, sourcing a script will run execute commands in the current shell.
The source command is useful for:
- Refreshing your current shell environment
- To execute a shell script in the context of the current environment
- To import a shell function in your script
- Read variables from a shell script
Syntax for source command
The syntax for this builtin shell command is human readable. It takes a file and if arguments are provided they serve as positional parameters for the script being passed.
source FILENAME [ARGUMENTS]
The . (dot) can also be used as an alternative for source command.
. FILENAME [ARGUMENTS]
How to use source command
Here I explain some practical examples where you can apply source command.
1. Refresh your current shell environment
As a user you can define an alias in your current shell environment. To define one for ls -l type:
alias ll = 'ls -l'
To use it type:
ll
Although the above list the files in the current directory in the long format, it works only for the current shell session. To make the changes permanently, open the ~/.bashrc file and add:
alias ll = 'ls -l'
To refresh the current shell environment type:
source ~/.bashrc
2. Execute a shell script in the context of the current shell environment
A shell script is not aware of the variables you define as a user in your current shell environment. The source command can be used to execute your shell script in the context of the current session.
To define a temporary variable type:
WEBSITE = example.com
To create a custom script type:
#!/bin/bash echo $WEBSITE
Save the file. To execute it in the context of the current shell session type:
source ./myscript.sh
The output is shown below.
example.com
3. Import a shell function
To define a custom shell script type:
!#/bin/bash foo() { echo "test" }
Save the above as script.sh.
To import the function of the above script in your current shell session, type:
source script.sh
To use the foo function type:
foo
The output is shown below.
test
4. Read variables from a shell script
To create a shell script with some variables, type:
#!/bin/bash a=1 b=2 c=3
To read the variables within another shell script type:
#!/bin/bash source abovescript.sh echo $a, $b, $c
The output should be:
1, 2, 3
5. Read and execute commands
Source command can read and execute commands from a file. Lets have a text file with a set of commands.
For example file commands.txt has the following content:
pwd
date
The output of source <filename>:
$ source firstexample.txt /home/developer Fri Feb 25 11:10:11:09 GMT 2021
6. Pass arguments to functions
This section describes how to pass the parameter to the function and same function we can re-use via source command.
functions.sh
!/usr /bin/bash var1=$1 var2=$2
execute.sh
!/usr/bin/bash source functions.sh 10 AA echo “var1 = $var1” echo “var2 = $var2”
Output.sh
var1 = 10 var2 = AA
Conclusion
Source command evaluvated script in the current shell whereas exec command runs in a new shell.
Through this article, you learned four practical examples of the source command. Although all of them are useful, the most important one for you as a user is the first one.
