Sometimes you want the opposite. For instance, you may need to send a command's standard output to the screen and grab the error messages (standard error) with backquotes. Or you might want to send a command's standard output to a file and the standard error down a pipe to an error-processing command. Here's how to do that in the Bourne shell. (The C shell can't do this, although tcsh can.)

$ command | ...
$ var=`command`

But there's no filename associated with the pipe or backquotes, so you can't use the 2> redirection. You need to rearrange the file descriptors without knowing the file (or whatever) that they're associated with. Here's how. You may find it useful to run this short Perl script, which simply prints "stdout" to standard output, and "stderr" to standard error:

#!/usr/bin/perl

print STDOUT "stdout\n";
print STDERR "stderr\n";

Let's start slowly. We will combine both standard output and standard error, sending them both as output, to be used as the input to a pipe or as the output of backquotes. The Bourne shell operator n>&m rearranges the files and file descriptors. It says, "Make file descriptor n point to the same file as file descriptor m." Let's use that operator on the previous example. We'll send standard error to the same place standard output is going:

$ command 2>&1 | ...
$ var=`command 2>&1`

In both those examples, 2>&1 means "send standard error (file descriptor 2) to the same place standard output (file descriptor 1) is going." Simple, eh?

You can use more than one n>&m operator. The shell reads them left-to-right before it executes the command.

$ command 2>&1 1>&2 | ...     wrong...

Sorry, Charlie. When the shell sees 2>&1 1>&2, the shell first does 2>&1. You've seen that before -- it makes file descriptor 2 (stderr) go the same place as file descriptor 1 (stdout). Then the shell does 1>&2. It makes stdout (1) go the same place as stderr (2)... but stderr is already going the same place as stdout, down the pipe.

This is one place the other file descriptors, 3 through 9 (and higher in bash), come in handy. They normally aren't used. You can use one of them as a "holding place," to remember where another file descriptor "pointed." For example, one way to read the operator 3>&2 is "make 3 point the same place as 2." After you use 3>&2 to grab the location of 2, you can make 2 point somewhere else. Then make 1 point where 2 used to (where 3 points now).

We'll take that step-by-step below. The command line you want is one of these:

$ command3>&2 2>&1 1>&3 | ...
$ var=`command 3>&2 2>&1 1>&3`

Figure 36-3. File descriptors before redirection

Figure 36-4. File descriptors after 3>&2 redirection

The figures use a grep command reading two files. afone is readable, and grep finds one matching line in it; the line is written to the standard output. bfoen is misspelled and so is not readable; grep writes an error message to the standard error. In each figure, you'll see the terminal output (if any) just after the variable-setting command with the backquotes. The text grabbed by the backquotes goes into the shell variable; the echo command shows that text.

Figure 36-5. File descriptors after 3>&2 2>&1 redirection

Figure 36-6. File descriptors after 3>&2 2>&1 1>&3 redirection