Ubuntu – the differences between &> and 2>&1

bashcommand lineredirect

There are two forms of redirection the standard output and standard error into standard output. But which one is better? and why the &> is considered the perfect?

I can't find what is the differences so that many tutorials and even bash manual state that &> is better!

So Why shall I use &> and not 2>&1

Mainly using bash shell

EDIT: Thanks for commentors

Only >& works in csh or tcsh

In ksh only 2>&1 works.

dash use >file 2>&1 redirection only

Then which one to use to ensure my script is compatible with other systems whatever the used shells are!

Best Answer

Bash's man page mentions there's two ways to redirect stderr and stdout: &> file and >& file. Now, notice that it says both stderr and stdout.

In case of this >file 2>&1 we are doing redirection of stdout (1) to file, but then also telling stderr(2) to be redirected to the same place as stdout ! So the purpose may be the same, but the idea slightly different. In other words "John, go to school; Suzzie go where John goes".

What about preference ? &> is a bash thing. So if you're porting a script, that won't do it. But if you're 100% certain your script will be only working on system with bash - then there's no preference

Here's an example with dash , the Debian Amquist Shell which is Ubuntu's default.

$ grep "YOLO" * &> /dev/null
$ grep: Desktop: Is a directory
grep: Documents: Is a directory
grep: Downloads: Is a directory
grep: Music: Is a directory
grep: Pictures: Is a directory
grep: Public: Is a directory
grep: Templates: Is a directory
grep: Videos: Is a directory
grep: YOLO: Is a directory
grep: bin: Is a directory

As you can see, stderr is not being redirected

To address your edits in the question, you can use if statement to check $SHELL variable and change redirects accordingly

But for most cases > file 2>&1 should work

In more technical terms, the form [integer]>&word is called Duplicating Output File Descriptor, and is a feature specified by POSIX Shell Command Language standard, which is supported by most POSIX-compliant and Brourne-like shells.

See also What does & mean exactly in output redirection?

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