How to Make a Variable Available to Child Processes in Bash

In Bash, when you create a variable, it is local to the current shell session by default. If you want to make a variable available to child processes (subshells or scripts executed from the current shell), you need to export the variable. This guide will explain how to do this, along with examples and best practices.

1. Understanding Local vs. Exported Variables

When you define a variable in Bash, it is local to the current shell session unless explicitly exported. An exported variable can be accessed by any child processes spawned from that shell.

Example of a Local Variable

MY_VAR="Hello, World!"

In this example:

  • The variable MY_VAR is created but is not available to child processes.

Example of Accessing a Local Variable in a Child Process

bash -c 'echo $MY_VAR'

In this example:

  • The command attempts to access MY_VAR in a new Bash shell.
  • The output will be empty because MY_VAR is not exported.

2. Exporting a Variable

To make a variable available to child processes, you need to use the export command.

Example of Exporting a Variable

export MY_VAR="Hello, World!"

In this example:

  • The variable MY_VAR is exported, making it available to any child processes.

Example of Accessing an Exported Variable in a Child Process

bash -c 'echo $MY_VAR'

In this example:

  • The command will now output: Hello, World! because MY_VAR has been exported.

3. Exporting Multiple Variables

You can export multiple variables in a single command by using the export command followed by the variable assignments.

Example of Exporting Multiple Variables

export VAR1="Value1" VAR2="Value2"

In this example:

  • Both VAR1 and VAR2 are exported and will be available to child processes.

4. Exporting Variables in a Script

If you have a script that needs to use exported variables, you can export them within the script itself.

Example of Exporting Variables in a Script

#!/bin/bash
export MY_VAR="Hello from the script!"
bash -c 'echo $MY_VAR'

In this example:

  • The script exports MY_VAR and then runs a child Bash process that accesses it.
  • The output will be: Hello from the script!.

5. Unsetting Exported Variables

If you need to remove an exported variable, you can use the unset command.

Example of Unsetting an Exported Variable

unset MY_VAR

In this example:

  • The command removes < code>MY_VAR from the environment, making it unavailable to any child processes.

6. Conclusion

Exporting variables in Bash is essential for sharing data between the parent shell and its child processes. By using the export command, you can ensure that your variables are accessible wherever needed, enhancing the functionality of your scripts and command-line operations. Remember to unset variables when they are no longer needed to avoid potential conflicts in your environment.