bash: set -x logs to file

Based on this ServerFault answer Send bash -x output to logfile without interupting standard output, modern versions of bash include a BASH_XTRACEFD specifically for specifying an alternate file descriptor for the output of set -x

So for example you can do

#!/bin/bash

exec 19>logfile
BASH_XTRACEFD=19

set -x
command1
command2
...

to send the output of set -x to file logfile while preserving regular standard output and standard error streams for the following commands.

Note the use of fd 19 is arbitrary - it just needs to be an available descriptor (i.e. not 0, 1, 2 or another number that you have already allocated).

Steeldriver gave you one approach. Alternatively, you can simply redirect STDERR to a file:

script.sh 2> logfile

That, however, means that both the output created by the set -x option and any other error messages produced will go to the file. Steeldriver's solution will only redirect the set -x output which is probably what you want.

After more than a year I've found the right solution to have both the "normal" output (stdout + stderr - bash trace) on the screen and all together (stdout + stderr + bash trace) in a file (bash.log):

exec   > >(tee -ia bash.log)
exec  2> >(tee -ia bash.log >& 2)
exec 19> bash.log

export BASH_XTRACEFD="19"
set -x

command1
command2