In Bash, both export and declare are used to manage variables, but they serve different purposes and have distinct functionalities. Understanding the differences between these two commands is essential for effective shell scripting and variable management.

1. Purpose of export

The export command is used to set environment variables that are available to child processes. When you export a variable, it becomes part of the environment, allowing any subprocesses to access it.

Example of Using export

export MY_VAR=`Hello, World!`

In this example:

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

Accessing an Exported Variable in a Child Process

bash -c 'echo $MY_VAR'

In this example:

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

2. Purpose of declare

The declare command is used to create variables with specific attributes. It allows you to define the type of variable (e.g., integer, array) and set options such as readonly or export status. However, variables declared with declare are not automatically exported to child processes unless explicitly done so.

Example of Using declare

declare -i MY_INT=10

In this example:

• The variable MY_INT is declared as an integer using the -i option.
• This means that any arithmetic operations performed on MY_INT will treat it as an integer.

Accessing a Declared Variable in a Child Process

bash -c 'echo $MY_INT'

In this example:

• The command will output nothing because MY_INT is not exported and is not available in the child process.

3. Key Differences

Here are the key differences between export and declare:

• Functionality: export makes a variable available to child processes, while declare is used to define variable attributes and types.
• Exporting: Variables created with declare are not exported by default; you must use export separately to make them available to child processes.
• Variable Types: declare allows you to specify variable types (e.g., integer, array), while export does not.

4. Combining declare and export

You can use both commands together to declare a variable and export it in one line:

declare MY_VAR=`Hello, World!` && export MY_VAR

In this example:

• The variable MY_VAR is declared and then exported, making it available to child processes.

5. Conclusion

In summary, export and declare serve different purposes in Bash scripting. Use export when you need to make a variable available to child processes, and use declare when you want to define variable attributes or types. Understanding these differences will help you manage your variables more effectively in your scripts.