Capture -x debug commands into a file in Bash

Question

Using set -x in bash prints the shell-expanded commands to stderr. I would like to redirect these to a file or pipe. But not the whole output - only some commands. Something like:

set -x command.txt  ### <-- command.txt param is made up
echo $A $B
set +x

This would put the debug output to command. txt.

Can this be done?

Answer 1

With bash 4.1 or later:

#!/bin/bash

exec 5> command.txt
BASH_XTRACEFD="5"

echo -n "hello "

set -x
echo -n world
set +x

echo "!"

Output to stdout (FD 1):

hello world!

Output to command.txt (FD 5):

+ echo -n world
+ set +x