Added 'do' support for unix systems.

do.bat

@@ -1,6 +1,4 @@
 @echo off
-set errorlevel=0
-
 :: Bypass "Terminate Batch Job" prompt.
 if "%~1"=="-FIXED_CTRL_C" (
    :: Remove the -FIXED_CTRL_C parameter

do.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# Run Task
+DO_NAME=do
+DO_MANIFEST_PATH=tools/do-tasks/Cargo.toml
+cargo run --manifest_path $DO_MANIFEST_PATH --release --quiet -- $*

tools/do-tasks/src/util.rs

@@ -65,6 +65,9 @@ pub fn cmd_external(cmd: &str, default_args: &[&str], user_args: &[&str]) {
 
     #[cfg(windows)]
     {
+        // We use ping to cause a slight delay that gives time for the current
+        // executable to close because the subsequent command is expected to affect
+        // the current executable file.
         Command::new("cmd")
             .args(&["/C", "ping", "localhost", "-n", "3", ">", "nul", "&"])
             .arg(cmd)
@@ -72,9 +75,14 @@ pub fn cmd_external(cmd: &str, default_args: &[&str], user_args: &[&str]) {
             .spawn()
             .ok();
     }
+
     #[cfg(not(windows))]
     {
-        todo!("cmd_external only implemented in windows")
+        // We assume that if not on Windows we are in a Unix based system.
+        //
+        // We don't need a delay in Unix because it naturally permits repointing
+        // or removing a file name without affecting the current running file.
+        self::cmd(cmd, default_args, user_args);
     }
 }