Use array PROMPT_COMMAND in Bash >= 5.1

Bash 5.1 started to support the array form of PROMPT_COMMAND, which
can be used to register multiple PROMPT_COMMAND commands more
robustly.  We perform this in a shell function to temporarily set IFS.

mcfly.bash

@@ -62,6 +62,25 @@ function mcfly_initialize {
  return "${exit_code}" # Restore the original exit code by returning it.
  }
 
+ function mcfly_add_prompt_command {
+ local command=$1 IFS=$' \t\n'
+ if ((BASH_VERSINFO[0] > 5 || BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] >= 1)); then
+ # Bash 5.1 supports array PROMPT_COMMAND, where we register our prompt
+ # command to a new element PROMPT_COMMAND[i] (with i >= 1) to avoid
+ # conflicts with other frameworks.
+ if [[ " ${PROMPT_COMMAND[*]-} " != *" $command "* ]]; then
+ PROMPT_COMMAND[0]=${PROMPT_COMMAND[0]:-}
+ PROMPT_COMMAND+=("$command")
+ fi
+ elif [[ -z ${PROMPT_COMMAND-} ]]; then
+ PROMPT_COMMAND="$command"
+ elif [[ $PROMPT_COMMAND != *"mcfly_prompt_command"* ]]; then
+ PROMPT_COMMAND="$command;${PROMPT_COMMAND#;}"
+ fi
+ }
+ # Set $PROMPT_COMMAND run mcfly_prompt_command, preserving any existing $PROMPT_COMMAND.
+ mcfly_add_prompt_command "mcfly_prompt_command"
+
  function mcfly_search_with_tiocsti {
  local LAST_EXIT_CODE=$?
  # shellcheck disable=SC2145
@@ -107,15 +126,6 @@ function mcfly_initialize {
  return "$LAST_EXIT_CODE"
  }
 
- # Set $PROMPT_COMMAND run mcfly_prompt_command, preserving any existing $PROMPT_COMMAND.
- if [ -z "${PROMPT_COMMAND-}" ]
- then
- PROMPT_COMMAND="mcfly_prompt_command"
- elif [[ ! "$PROMPT_COMMAND" =~ "mcfly_prompt_command" ]]
- then
- PROMPT_COMMAND="mcfly_prompt_command;${PROMPT_COMMAND#;}"
- fi
-
  # Take ownership of ctrl-r.
  if [[ ${BASH_VERSINFO[0]} -ge 4 ]]; then
  # shellcheck disable=SC2016