Bash "declare -p" returns incomplete info

Question

Having the following script test1.sh I'm wondering what is the reason of not getting the full list of declared variables with declare -p. For BASH_COMMAND surprisingly I'm not seeing the value but only declare -- BASH_COMMAND. After the variable is used then declare -p returns it properly.

What is the reason behind? What am I missing? I couldn't find the meaning of the declare -- also. What can I do to see all the variables for debug purpose?

#!/bin/bash
set -o errexit -o pipefail -o noclobber -o nounset -o posix -o errtrace -o functrace -o hashall
# -o xtrace

function trapExit() {
  #here we get the INCORECT declare with BASH_COMMAND having no value
  declare -p | grep BASH_COMMAND
  declare -p | grep BASH_COMMAND
  value=$BASH_COMMAND
  #here we get the CORRECT declare with BASH_COMMAND having the value
  declare -p | grep BASH_COMMAND
  echo "valueOfCommand=$value"
}

function test2() {
  ${MISSING_PARAM?"Parameter is missing in function test2"}
}

trap trapExit exit

test2

: <<END_COMMENT

$ ./test1.sh
./test1.sh: line 15: MISSING_PARAM: Parameter is missing in function test2
declare -- BASH_COMMAND
declare -- BASH_COMMAND="\${MISSING_PARAM?\"Parameter is missing in function test2\"}"
valueOfCommand=${MISSING_PARAM?"Parameter is missing in function test2"}

END_COMMENT

Answer 1

BASH_COMMAND is a dynamic variable and its value is updated only when you reference to it.

1. Initially when Bash starts, BASH_COMMAND is only declared without being assigned with a value.
2. When you declare -p | grep BASH_COMMAND ^{(no matter how many times)} the var is still not referenced yet so it still has no value.
   • This can be verified by adding v=$BASH_COMMAND before invoking test2.
   • Vars like RANDOM are similar, you can try declare -p | grep -w RANDOM.
   • Rather than declare -p | grep BASH_COMMAND you can just declare -p BASH_COMMAND.
3. Then with value=$BASH_COMMAND it's being referenced and Bash updates it with the command which triggers the trap.
4. And then declare -p can also show its value.

---

-- as in declare -- has no special meaning. It just "signals the end of options and disables further option processing" though it's not really necessary for this case.