The Difference Between &> and 2>&1 in Bash: Which One to Use?

In the world of Bash scripting, understanding the nuances of command redirection is crucial. Two commonly used redirection operations are &> and 2>&1. Both of these operations redirect both standard output (stdout) and standard error (stderr) to the same location, but there are subtle differences between them that can impact their use. In this article, we’ll delve into the specifics of these two operations and discuss when and why you might choose one over the other.

Understanding Redirection in Bash

Before we dive into the specifics of &> and 2>&1, it’s important to understand the concept of redirection in Bash. Redirection allows you to control where the output of a command goes, whether that’s to a file, a device, or another command. It’s a powerful tool that can make your scripts more flexible and efficient.

There are three standard streams in Bash:

1. Standard Input (stdin): This is the stream where a program receives its input data. The default is the keyboard.
2. Standard Output (stdout): This is the stream where a program writes its output data. The default is the terminal.
3. Standard Error (stderr): This is the stream where a program writes its error messages. The default is also the terminal.

Each of these streams is associated with a file descriptor:

• stdin is file descriptor 0
• stdout is file descriptor 1
• stderr is file descriptor 2

Redirection operations allow you to manipulate these streams and file descriptors to control where data goes.

The &> Operator

The &> operator is a shorthand form that redirects both stdout and stderr to a single location. It’s equivalent to > file 2>&1, where > is the redirection operator, file is the target file, and 2>&1 redirects stderr to stdout.

Here’s an example of how you might use the &> operator:

command &> file

In this example, both the standard output and standard error of command are redirected to file.

The 2>&1 Operator

The 2>&1 operator is a bit more complex. It redirects file descriptor 2 (stderr) to file descriptor 1 (stdout). This means that both stdout and stderr will be sent to the same location.

Here’s an example of how you might use the 2>&1 operator:

command > file 2>&1

In this example, the standard output of command is redirected to file, and then the standard error is redirected to the same location as the standard output.

Which One to Use?

The &> and 2>&1 operators are semantically equivalent, meaning they achieve the same result. However, there are a few key differences to keep in mind:

• Compatibility: The 2>&1 form is the standard Bourne/POSIX shell syntax and is supported by most shells. The &> form is a bash extension and is not part of the standard. If you’re writing scripts that need to be compatible with different shells, you should use the 2>&1 form.
• Readability: The &> form is more concise and easier to read. If you’re certain your script will only be run in a bash environment, you might prefer this form for its simplicity.

Conclusion

In Bash scripting, both &> and 2>&1 are used to redirect both standard output and standard error to the same location. The choice between them depends on your shell compatibility requirements and personal preference. If you need your script to be compatible with a variety of shells, stick with 2>&1. If you’re only working in bash and prefer a more concise syntax, &> may be the better choice.

Remember, understanding these nuances and making the right choice can make your scripts more efficient and flexible. Happy scripting!