Printf command stops working when DEBUG trap is active?

Question

When I set a DEBUG trap in Bash like this:

set -o functrace
trap 'echo "# $BASH_COMMAND" >&2' DEBUG

Suddenly, this function stopped working:

getBase() {

  local base="$1"

  base=$(basename "${base%%\?*}")
  : "${base//+/ }"; printf -v base '%b' "${_//%/\\x}"
  base=$(echo "$base" | sed -e 's/[^A-Za-z0-9._-]/_/g')

  echo "$base"

  return 0
}

with the error

line 6: printf: missing hex digit for \x

and outputs ".__printf_-v_base____________x__x__" to the variable.

I am trying to understand why this is. As the DEBUG trap outputs everything to STDERR, so how can it influence the printf -v function? And how can I solve it?

Pleases edit your question to show a minimal reproducible example, not just a standalone function and a couple of additional lines. Make sure to run it through shellcheck.net first. — Ed Morton
– Ed Morton, Commented Oct 2 at 12:18
also, fyi, just set set -x, it does like exactly the same as your trap command — KamilCuk
– KamilCuk, Commented Oct 2 at 12:20
@KamilCuk With set -x this bug does not occur, so it can't be exactly the same. — Maestro
– Maestro, Commented Oct 2 at 15:29
@EdMorton I created a generic reproducable example, but then I noticed that the question already received an answer. So if I edit my question now with the generic example, the accepted answer does not make any sense anymore. — Maestro
– Maestro, Commented Oct 2 at 15:30

oguz ismail · Accepted Answer · 2025-10-02 13:35:47Z

2

You use the special variable _ (underscore) for moving the result of ${base//+/ } across commands but the DEBUG trap changes its value to something else. Define getBase like this instead and it'll work:

getBase() {
    local base
    base=${1%%\?*}
    base=${base%"${base##*[!/]}"}
    base=${base##*/}
    base=${base//+/ }
    base=${base//%/\\x}
    printf -v base '%b' "$base"
    base=${base//[!A-Za-z0-9._-]/_}
    printf '%s\n' "$base"
}

edited Oct 2 at 13:35

answered Oct 2 at 13:30

oguz ismail

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1 Comment

F. Hauri - Give Up GitHub Oct 3 at 13:09

Well done! ( I like base=${base%"${base##*[!/]}"} !), but as //[!A-Za-z0-9._-]/_, the step base=${base//+/ } is useless!! (both space ⠀ and plus sign + will be converted into underscore _ ;-)

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