In the shell, what does “ 2>&1 ” mean?

In a Unix shell, if I want to combine stderr and stdout into the stdout stream for further manipulation, I can append the following on the end of my command:

2>&1

So, if I want to use "head" on the output from g++, I can do something like this:

g++ lots_of_errors 2>&1 | head

so I can see only the first few errors.

I always have trouble remembering this, and I constantly have to go look it up, and it is mainly because I don't fully understand the syntax of this particular trick. Can someone break this up and explain character by character what "2>&1" means?

• Zero is stdin
• One is stdout
• Two is stderr

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

Here is one way to remember this construct (although it is not entirely accurate): at first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1". & indicates that what follows is a file descriptor and not a filename. So the construct becomes: 2>&1.

Symbole > mean redirection.

• > mean send to as a whole completed file, overwriting target if exist (see noclobber bash feature at #3 later).
• >> mean send in addition to would append to target if exist.

转自： http://stackoverflow.com/questions/818255/in-the-shell-what-does-21-mean