BASH_COMMAND does not expand correctly in subshells inside traps.

[Emanuele Torre]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -march=x86-64 -mtune=generic -O2 -pipe -fno-plt
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/bin'
-DSTANDARD_UTILS_PATH='/usr/bin' -DSYS_BASHRC='/etc/bash.bashrc'
-DSYS_BASH_LOGOUT='/etc/bash.bash_logout'
-DNON_INTERACTIVE_LOGIN_SHELLS
uname output: Linux t420 5.10.75-1-lts #1 SMP Wed, 20 Oct 2021
11:02:09 +0000 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 8
Release Status: release

Description:
  BASH_COMMAND does not expand to the expected value when used in a
  subshell inside a trap.

  I tried using the following DEBUG trap that i got from
  http://mywiki.wooledge.org/BashGuide/Practices in bash-5.1:

    trap '(read -p "[$BASH_SOURCE:$LINENO] $BASH_COMMAND?")' DEBUG

  And noticed that $BASH_COMMAND always expanded to:

    read -p "[...:...] $BASH_COMMAND?

  where the ...:... is the expanded value of $BASH_SOURCE:$LINENO.

  Then I tried to use the same trap in bash-4.4 and it worked as
  expected.

Repeat-By:

  $ cat test1
  trap 'echo "$BASH_COMMAND"' DEBUG
  echo hello
  $ cat test2
  trap '(echo "$BASH_COMMAND")' DEBUG
  echo hey
  bash

  $ ./bash test1 # bash-4.4
  echo hey
  hey
  $ ./bash test2 # bash-4.4
  echo hey
  hey
  $ bash test1 # bash-5.1
  echo hey
  hey
  $ bash test2 # bash-5.1
  echo "$BASH_COMMAND"
  hey

  $ ./bash --version | head -1
  GNU bash, version 4.4.0(3)-release (x86_64-unknown-linux-gnu)
  $ bash --version | head -1
  GNU bash, version 5.1.8(1)-release (x86_64-pc-linux-gnu)