what does >& mean in a shell command?

Question

I actually have 2 questions:

1. What does >& mean in a shell script as in /usr/bin/x_app >& /tmp/blog.txt? Shouldn't a number follows & to make it a stdout/err thing?

2. As far as I know that > is overwriting and >> is appending, so 2>&1 should just replace stdout with stderr, but instead, it means putting stdout and stderr together. Why don't we use >>? I mean I know "appending" does not mean "putting them together" either but it's closer to it than "overwriting", right?

Answer 1

>& file is a csh operator (from the late 70s) to redirect both stdout and stderr to file. The Bourne shell (also from the late 70s) equivalent is > file 2>&1, or 2> file >&1, that is redirect fd 1 to file and then fd 2 to the same open file description or vice versa.

zsh and bash are two shells (from the late 80s, early 90s) that took features from both the Korn¹ shell (and by extension the Bourne shell) and csh, and support that >& csh operator in addition to the [i]>&j Bourne/Korn operator.

Those two conflicts though.

cmd >& "$var"

Will end up doing the Bourne/Korn redirection if $var contains a number and the csh one otherwise.

In practice, it's better to avoid that csh operator and use the Bourne syntax instead (cmd > "$var2" 2>&1) in those shells to avoid surprises.

Note that initially, bash didn't support csh's >& but had &> instead for the same feature. That also conflicted with Bourne syntax. In the Bourne shell, cmd &> file is cmd and > file run in parallel. zsh also added support for &> latter for compatibility with bash.

You'll find yet different syntax in different shells. In rc (late 80s) and derivatives (es, akanga), Bourne's 2>&1 is written >[2=1] (and it makes it easier to specify which fds are to be at each end of pipes: cmd1 |[3=4] cmd2 (connecting fd 3 of cmd1 to fd 4 of cmd2 via a pipe) which in Bourne syntax, you'd need to write { cmd1 3>&1 >&5 5>&- | 4<&0 <&6 6<&- cmd2; } 5>&1 6<&0).

The fish shell (mid 2000s) has different variations in I/O redirection syntax from the Bourne shell's.

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^{¹ The Korn shell (from the early 80s) was based on the Bourne shell but also borrowed features from csh like aliases, tilde expansion and brace expansion.}