Executable scripts

Making Deno scripts executable can come in handy when creating small tools or utilities for tasks like file manipulation, data processing or repetitive tasks that you might want to run from the command line. Executable scripts allow you to create ad-hoc solutions without setting up an entire project.

Creating an example script Jump to heading#

To make a script executable, start the script with a hashbang, (sometimes called a shebang). This is a sequence of characters (#!) that tells your operating system how to execute a script. It is followed by the path to the interpreter that should be used to run the script.

We'll make a small command-line greeter that reads its first argument from Deno.args and falls back to a default when none is supplied. This is the shape almost every Deno CLI tool starts with.

Create a file named hashbang.ts with the following content:

#!/usr/bin/env -S deno run
const name = Deno.args[0] ?? "world";

console.log(`Hello, ${name}!`);

The first line is the hashbang. It tells the system to run the file with /usr/bin/env, which then locates deno on your PATH and invokes deno run on the script. The -S flag splits deno run into separate arguments so env treats them correctly; without -S the whole string would be passed as a single argument name and env would fail to find a program called deno run. There are no --allow-* flags here because the script only reads from Deno.args, which is always available. Deno prompts for permissions only when the script tries to use a sandboxed API.

Deno.args is an array of the arguments that follow the script name on the command line. Deno.args[0] is the first one; ?? "world" substitutes "world" when no argument was supplied.

Execute the script Jump to heading#

Before running the file directly, give it execute permission with chmod (you only need to do this once per file):

chmod +x hashbang.ts

Now invoke it from the command line. With no arguments it uses the default; pass a name and it greets that name instead:

$ ./hashbang.ts
Hello, world!
$ ./hashbang.ts Ada
Hello, Ada!

Using hashbang in files with no extension Jump to heading#

For brevity, you may wish to omit the extension for your script's filename. In this case, supply one using the --ext flag in the script itself, then you can run the script with just the file name:

$ cat my_script
#!/usr/bin/env -S deno run --allow-env --ext=js
console.log("Hello!");
$ ./my_script
Hello!

🦕 Now you can directly execute Deno scripts from the command line! Remember to set the execute permission (chmod +x) for your script file, and you’re all set to build anything from simple utilities to complex tools. Check out the Deno examples for inspiration on what you can script.