Set command's exit status prior to invoking precmd functions.

- In zsh, each precmd function has access to "$?" as if it was executed
  immediately following the command. Simulate this by saving and setting the
  exit status prior to running each precmd function.

bash-preexec.sh

@@ -59,18 +59,29 @@ __bp_trim_whitespace() {
 #
 # It will invoke any functions defined in the precmd_functions array.
 __bp_precmd_invoke_cmd() {
+
+    # Should be available to each precmd function, should it want it.
+    local ret_value="$?"
+
     # For every function defined in our function array. Invoke it.
     local precmd_function
     for precmd_function in ${precmd_functions[@]}; do
 
         # Only execute this function if it actually exists.
         if [[ -n $(type -t $precmd_function) ]]; then
+            __bp_set_ret_value $ret_value
             $precmd_function
         fi
     done
     __bp_preexec_interactive_mode="on";
 }
 
+# Sets a return value in $?. We may want to get access to the $? variable in our
+# precmd functions. This is available for instance in zsh. We can simulate it in bash
+# by setting the value here.
+__bp_set_ret_value() {
+    return $1
+}
 
 __bp_in_prompt_command() {
 
@@ -176,7 +187,7 @@ __bp_preexec_and_precmd_install() {
     fi
 
     # Finally install our traps.
-    PROMPT_COMMAND="${existing_prompt_command} __bp_precmd_invoke_cmd";
+    PROMPT_COMMAND="__bp_precmd_invoke_cmd; ${existing_prompt_command}"
     trap '__bp_preexec_invoke_exec' DEBUG;
 
     # Add two functions to our arrays for convenience

test/bash-preexec.bats

@@ -40,6 +40,23 @@ test_preexec_echo() {
     [[ "$output" == "test echo" ]]
 }
 
+@test "precmd should set $? to be the previous exit code" {
+    echo_exit_code() {
+      echo "$?"
+      return 0
+    }
+    precmd_functions+=(echo_exit_code)
+
+    __bp_set_ret_value() {
+      return 251
+    }
+
+    run '__bp_precmd_invoke_cmd'
+    [[ $status == 0 ]]
+    [[ "$output" == "251" ]]
+}
+
+
 @test "preexec should execute a function with the last command in our history" {
     preexec_functions+=(test_preexec_echo)
     __bp_preexec_interactive_mode="on"