Passing Environment Variables to Bash Scripts Called from Functions

In the world of Bash scripting, one of the most common tasks is to pass variables from one script to another or from a function to a script. This article will provide a detailed guide on how to pass environment variables to Bash scripts called from functions.

Understanding Environment Variables

Before we dive into the main topic, let’s first understand what environment variables are. Environment variables are dynamic-named values that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP environment variable to discover a suitable location to store temporary files, or the HOME variable to find the directory structure owned by the user running the process.

The export Command

The export command in Unix is a built-in command which is used to set the environment variable for the current session. This command sets the environment variable on the left to the value on the right. To make the variable available to child processes (like our script called from a function), we use the export command.

Passing Environment Variables to Bash Scripts

Let’s consider a scenario where we have a bash script scr.sh that uses the REMOTE_SERVER environment variable:

#!/bin/bash
echo "Params = $@"
echo "REMOTE_SERVER=${REMOTE_SERVER}"

In the script above, $@ is a special variable in bash that holds all the parameters passed to the script. ${REMOTE_SERVER} is the syntax to access the value of the REMOTE_SERVER variable.

Now, we want to call this script from a function f defined in another script scr2.sh, and we want to pass the REMOTE_SERVER variable to it:

#!/bin/bash
f() {
 export REMOTE_SERVER=s001
 scr.sh "${@}"
}

In the function f, we first export the REMOTE_SERVER variable and set its value to s001. The export command makes the REMOTE_SERVER variable available to the scr.sh script when it is called on the next line. The "${@}" passes all the parameters received by the function to the scr.sh script.

To make the f function available, we source the scr2.sh script:

$ source scr2.sh

The source command reads and executes commands from the file specified as its argument in the current shell environment.

We can now call the f function with the desired parameters:

$ f par1 par2

This will output:

Params = par1 par2
REMOTE_SERVER=s001

To check that the REMOTE_SERVER variable is not set outside of the function, we can run:

$ echo "REMOTE_SERVER=${REMOTE_SERVER}"

This will output:

REMOTE_SERVER=

This shows that the REMOTE_SERVER variable is only available to the scr.sh script and does not affect the parent shell.

Conclusion

In this article, we covered how to pass environment variables to Bash scripts called from functions. We used the export command to make the variable available to child processes and we saw that this does not affect the parent shell. This technique is useful in a variety of scripting scenarios where you need to pass data between scripts or between functions and scripts.

Remember, the export command is a powerful tool in your scripting toolbox, but use it with care. It’s important to understand its scope and impact on your scripts and overall environment.