How can I redefine stdout/stderr in bash?

Question

Is it possible to redefine file descriptors (e.g. stderr) in bash?

I would like to send all output to a file by default while still being able to use the original stderr and stdout.

#!/bin/bash
echo "Error: foo bar" 1>2
REAL_STDERR=2
REAL_STDOUT=1
2=open("/tmp/stderr.log")
1=open("/tmp/stdout.log")
echo "This goes to stdout.log"
if ! curl doesntexist.yet; then
    echo "Error: Unable to reach host. See stderr.log for details" 1>REAL_STDERR
fi

Answer 1

The exec builtin does this when not given a name of a command to run.

exec 3>&1 4>&2 2>/tmp/stderr.log >/tmp/stdout.log

echo "This goes to stdout.log"
echo "This goes to stderr.log" >&2

echo "This goes directly to real stderr" >&4

Note that redirections are processed in the order they're given on the command line left-to-right. Thus, &1 and &2 are interpreted as-modified by any previous redirections on the same command.

See the relevant POSIX spec.

---

If you want to use variable names for your file descriptors (with the numbers automatically allocated), you'll need bash 4.1 or newer. There, you can do:

exec {real_stderr}>&2 {real_stdout}>&1 >stdout.log 2>stderr.log
echo "This goes stdout.log"
echo "This goes to stderr.log" >&2
echo "This goes to real stderr" >&$real_stderr