Handle unbound PROMPT_COMMAND

Expressly handle if $PROMPT_COMMAND is previously undefined.

Author: gaelicWizard

bash-preexec.sh

@@ -159,7 +159,7 @@ __bp_set_ret_value() {
 __bp_in_prompt_command() {
 
     local prompt_command_array
-    IFS=$'\n;' read -rd '' -a prompt_command_array <<< "$PROMPT_COMMAND"
+    IFS=$'\n;' read -rd '' -a prompt_command_array <<< "${PROMPT_COMMAND:-}"
 
     local trimmed_arg
     __bp_trim_whitespace trimmed_arg "${1:-}"
@@ -297,7 +297,8 @@ __bp_install() {
 
     local existing_prompt_command
     # Remove setting our trap install string and sanitize the existing prompt command string
-    existing_prompt_command="${PROMPT_COMMAND//$__bp_install_string[;$'\n']}" # Edge case of appending to PROMPT_COMMAND
+    existing_prompt_command="${PROMPT_COMMAND:-}"
+    existing_prompt_command="${existing_prompt_command//$__bp_install_string[;$'\n']}" # Edge case of appending to PROMPT_COMMAND
     existing_prompt_command="${existing_prompt_command//$__bp_install_string}"
     __bp_sanitize_string existing_prompt_command "$existing_prompt_command"
 
@@ -328,7 +329,7 @@ __bp_install_after_session_init() {
     __bp_require_not_readonly PROMPT_COMMAND HISTCONTROL HISTTIMEFORMAT || return
 
     local sanitized_prompt_command
-    __bp_sanitize_string sanitized_prompt_command "$PROMPT_COMMAND"
+    __bp_sanitize_string sanitized_prompt_command "${PROMPT_COMMAND:-}"
     if [[ -n "$sanitized_prompt_command" ]]; then
         PROMPT_COMMAND=${sanitized_prompt_command}$'\n'
     fi;