Is it possible to silence the initial call in xtrace, when BASH_XTRACEFD is set?

Question

#!/usr/bin/env bash

exec {BASH_XTRACEFD}>./xtrace.log
declare -p BASH_XTRACEFD
set -x
{ : "how do you hide this in ./xtrace.log?"; } 2>/dev/null # fail
# { :; } "${BASH_XTRACEFD:-2}">/dev/null # syntax error

$ ./above_script.bash
declare -- BASH_XTRACEFD="10"

$ cat ./xtrace.log
+ : 'how do you hide this in ./xtrace.log?'

After a block, a literal, hardcoded file descriptor number works, but you cannot use a variable value.

{ : something; } 2>/dev/null works, it outputs nothing in xtrace, when BASH_XTRACEFD is unset, but:

{ : "How do you hide this from xtrace?"; } "${BASH_XTRACEFD:-2}">/dev/null produces a syntax error.

Is there any way, to silence the call, when BASH_XTRACEFD is set?

Answer 1

It apparently can be done, but you will need to know the file descriptor, or to plan ahead.

---

Knowing the file descriptor:

exec 5>xtrace.log
BASH_XTRACEFD=5

set -x

: before

( : test1 ; ) 5>/dev/null  
{ : test2 ; } 5>/dev/null  
: test3 5>/dev/null  

: after

For me, this shows only "before", "test3", and "after" in the log.

---

planning ahead:

exec {BASH_XTRACEFD}>xtrace.log
exec {null}>/dev/null

echo BASH_XTRACEFD=$BASH_XTRACEFD null=$null

set -x

: before 

BASH_XTRACEFD=$null : test1

: after

For me, this shows only "before" and "after" in the log.

Answer 2

This seems to work:

exec {trace}>> xtrace.log {null}<>/dev/null
shopt -s expand_aliases
alias 'hide{={ {' \
      '}hide=} <&$((BASH_XTRACEFD=null, 0)); ((BASH_XTRACEFD=trace, !$?)); }'
BASH_XTRACEFD=$trace
set -o xtrace

echo this shows in log

hide{ echo this does not; }hide

echo this also shows in log

hide{ echo this fails; (exit 42); }hide ||
  echo "It failed indeed but the exact exit status is lost: $?"

echo It works | hide{ sed 's/works/also & great/'; }hide
hide{ echo It works great; }hide | sed 's/works/also &/'

Then:

$ bash ./that-script
this shows in log
this does not
this also shows in log
this fails
It failed indeed but the exact exit status is lost: 1
It also works great
It also works great
$ cat xtrace.log
+ echo this shows in log
+ echo this also shows in log
+ echo 'It failed indeed but the exact exit status is lost: 1'
+ echo It works
+ sed 's/works/also &/'

The trick being to assign $null (the fd opened on /dev/null) to $BASH_XTRACEFD as part of what ends up a no-op <&0 dummy redirection using an arithmetic expansion.

You lose the exact value of the exit status of the hidden command though.

I'd still use a fixed fd above 2 instead of a dynamically assigned one and not bother with that kind of nonsense.

Answer 3

In bash redirections [n]>word the [n] must be a literal number, it cannot be a variable. The part after the redirection operator can be a variable. You can do this:

exec {BASH_XTRACEFD}>./xtrace.log
declare -p BASH_XTRACEFD
set -x
{ : "how do you hide this in ./xtrace.log?"; } 2>/dev/null
{ :; } {BASH_XTRACEFD}>/dev/null # a syntax error
{ : "how do you hide this in ./xtrace.log?"; } {BASH_XTRACEFD}>>./xtrace.log
foo=bar

Answer 4

This is my current best effort to answer my own question. Thank you to everyone else, who answered, it's all based on your ideas!

#!/usr/bin/env bash

: shown 1

{
    exec 2>&3 # restore stderr

    : hidden

    echo error 1>&2

    case $fd_xtrace in
        2) unset BASH_XTRACEFD ;;
        *) BASH_XTRACEFD=$fd_xtrace ;;
    esac
} \
    3>&2 \
    2>/dev/null \
    4>&2 \
    4>&$((fd_xtrace=${BASH_XTRACEFD:-2}, BASH_XTRACEFD=4))
# ^ syntax compatible with Bash 3.0 (tested; possibly compatible with earlier
#   versions as well), but it only starts working (in the sense of hiding
#   xtrace output) from Bash >= 4.1 on.

: shown 2

Output:

$ bash -x ./script.bash
+ : shown 1
error
+ : shown 2

$ { BASH_XTRACEFD=80 bash -x ./script.bash; } 80>&2
+ : shown 1
error
+ : shown 2

$ { BASH_XTRACEFD=80 bash -x ./script.bash 2>/dev/null; } 80>&2
+ : shown 1
+ : shown 2

$ docker run -it --rm -v .:/mnt:ro bash:4.1 bash -x /mnt/script.bash
+ : shown 1
error
+ : shown 2

$ docker run -it --rm -v .:/mnt:ro bash:4.1 bash -c '{ BASH_XTRACEFD=80 bash -x /mnt/script.bash; } 80>&2'
+ : shown 1
error
+ : shown 2

$ docker run -it --rm -v .:/mnt:ro bash:3.0 bash -x /mnt/script.bash
+ : shown 1
+ : hidden
+ echo error
error
+ case $fd_xtrace in
+ unset BASH_XTRACEFD
+ : shown 2