Shell redirection, and the "2>&1" is probably the most
common form. That simply means to send stderr to wherever stdout is
pointing. An easy way to demonstrate all this is to create a
direcory and put one file in it:
That gives us a known situation to work with. Sitting in that
ls foo t > y
Puts "t" in "y", but displays "ls: foo: No such file or
directory" on the screen. That's because ls writes what it doesn't
understand to stderr (file descriptor 2). If we want both things in
y, we just do:
ls foo t >y 2>&1
Or, if you wanted the error output in a file as you might with "cc", for
cc source.c 2>/tmp/errors
There are lots of ways to do this sort of thing. More
interesting is the case of a shell script where you want to capture
stderr but let any "ok" output go to the screen. To do that, take
advantage of other file descriptors:
x=`ls foo t 3>&2 2>&1 1>&3`
$x will have the error message but "t" will go to the screen.
How does that happen? Well, 3>&2 puts stderr into (normally
unused) file descriptor 3. Then 2>&1 puts 1 into 2. This
might be hard for you to follow, but hang in there a minute. Next,
1>&3 puts 3 (which is the original 2) into 1. Effectively,
this reverses stdout and stderr. Let's take a closer look:
When the shell starts , both stderr (1) and stdout (1) point to
our screen. However, the
x=`ls foo t `
Would leave stderr still at the screen, but stdout would
redirect to our "x" variable. We want the opposite of that: The
3>&2 makes 3 the same as 2, so now 3 is going to the screen.
2>&1 makes 2 go where 1 was, which is into our "x". Finally,
1>&3 puts 1 back to the screen by copying 3.
(See exec also)
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