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The Difference Between set, export, and env in Bash

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In the world of Bash scripting, there are several commands that allow you to set and manage environment variables: set, export, and env. While they may seem similar, they each serve different purposes and have unique characteristics. This article will delve into the differences between these three commands and provide examples of when and how to use each one.

Quick Answer

The set command is used to set shell options and local variables within a script or command. It does not automatically export variables to the environment. The export command is used to mark a variable for export to the environment of subsequently executed commands. It makes variables available to child processes or other scripts. The env command is used to run a command in a modified environment, allowing you to set environment variables for a specific command without affecting the current shell environment.

Understanding Environment Variables

Before we dive into the differences between set, export, and env, it’s important to understand what environment variables are. Environment variables are named values that can be used to configure and control the behavior of your system and the applications running on it. They are stored in the system’s environment and can be accessed by scripts and programs.

The set Command

The set command in Bash is used to set shell options and positional parameters. It can also be used to set local variables within a script or command.

set VARNAME="value"

In this example, VARNAME is the name of the variable and "value" is the value assigned to it. It’s important to note that variables set with set are local to the current shell. They are not automatically exported to the environment of subsequently executed commands. This makes set useful for setting variables that are only needed within the current script or command.

The export Command

The export command is used to mark a variable for export to the environment of subsequently executed commands. This means that any variables marked with export are available to child processes or other scripts that are called from the current script or command.

export VARNAME="value"

In this example, VARNAME is the name of the variable and "value" is the value assigned to it. Because VARNAME is exported, it will be available to any commands or scripts that are executed after this line.

It’s important to note that export is a Bash built-in command. This means that it is part of the shell itself and does not require a separate program to run. This makes export faster than env for setting environment variables.

The env Command

The env command is used to run a command in a modified environment. It can be used to set environment variables for a specific command without affecting the current shell environment.

env VARNAME="value" command

In this example, VARNAME is the name of the variable, "value" is the value assigned to it, and command is the command to be executed. The env command modifies the environment, sets the VARNAME variable, and then executes the command.

The env command is a separate program, not a shell built-in like export. This means that it has more overhead than export and is slower to execute. However, env has the advantage of being able to modify the environment for a single command without affecting the current shell environment.

Conclusion

While set, export, and env can all be used to set environment variables in Bash, they each have their own unique characteristics and uses. set is useful for setting local variables, export is used to make variables available to child processes, and env can modify the environment for a single command. Understanding the differences between these commands will help you write more effective and efficient Bash scripts.

What is the difference between `set` and `export`?

The set command is used to set local variables within a script or command, while the export command is used to mark a variable for export to the environment of subsequently executed commands. Variables set with set are local to the current shell, while variables marked with export are available to child processes or other scripts.

Can variables set with `set` be accessed by other scripts or commands?

No, variables set with set are local to the current shell and are not automatically exported to the environment of subsequently executed commands. If you want to make a variable available to other scripts or commands, you need to use the export command.

How is the `env` command different from `export`?

The env command is used to run a command in a modified environment and can be used to set environment variables for a specific command without affecting the current shell environment. On the other hand, the export command is used to mark a variable for export to the environment of subsequently executed commands. env is a separate program, while export is a built-in command in Bash.

Is there a difference in performance between `export` and `env`?

Yes, there is a difference in performance between export and env. Since export is a built-in command in Bash, it is faster than env, which is a separate program. However, the difference in performance might not be significant unless you are working with a large number of environment variables or frequently modifying the environment for multiple commands.

Can `env` be used to modify the current shell environment?

No, the env command only modifies the environment for a single command and does not affect the current shell environment. If you want to modify the current shell environment, you need to use the export command.

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