Shebangs

dt is shebang-friendly. When you have something that gets too long for a one-liner, or you have something you want to save and not type out a lot, you can put that into a shebang file.

Note: It's been more than 20 years since I've been familiar with Windows. I think the equivalent in a Windows environment these days would be writing a PowerShell Script Module with a .psm1 extension. If we get Windows support and anyone knows a good pattern let us know!

An introduction

If you're familiar with Unix-like systems, you can skip this section.

The "Hash Bang" -- #! -- more often called "shebang" is a magic prefix for executable` files that tell operating systems like Linux, BSD, or OSX to use a specific interpreter to interpret the file.

More specifically, let's write a file named greet with contents like:

#!/usr/bin/env dt

"Hello world!" println

Mark it executable (chmod +x ./greet) and you should be ready to run it:

$ ./greet
Hello world!

If you've been following along in the past sections, you've now greeted the world at least 3 times. Hope the world has noticed you by now!

The #! says the rest of the line is something to execute as a process. /usr/bin/env dt helps locate a program called dt and execute it with the remaining arguments. dt in turn will get the filename as an argument and start to interpret the file contents.

If you had installed dt in a place like /home/me/.local/bin/dt you could also do something like:

#!/home/me/.local/bin/dt

"Hello world!" println

Shared scripts tend to use the /usr/bin/env lookup just in case the install location can't be known. I don't know about you, but I tend to share scripts, even if it's just with my future self! Who knows what that guy will do.

Anyway the point here is that you can put dt code in a file, and that can be an executable.

Shebang scripting with dt

Let's start our shebang examples by making a couple naive implementations of other common tools. (Of course: Do use the tools instead of these scripts!) We'll skip a lot of the niceties like usage messages and flag parsing, and just implement the core use case.

Let's create a head.dt N that can print the first N lines of its input. Put the following in a file called head.dt and mark it as executable.

#!/usr/bin/env dt

shebang-args first \n:

[readln println] n times

We use shebang-args to get the arguments passed to the shebang file. This is just the args of dt minus the first arg which is the script being interpreted. We bound the first argument to n and then did a read/print loop n times.

It can be used similar to the classic head -n N pattern:

$ seq 100 | head.dt 5
1
2
3
4
5

Now let's create a scream.dt that can be an equivalent of tr a-z A-Z.

#!/usr/bin/env dt

[readln upcase println] loop

$ echo -e "i\ncan't\nhear\nyou" | scream.dt
I
CAN'T
HEAR
YOU

Ok folks I get it, it's short, but we're here to demonstrate, not to optimize! The big thing to know is you can loop which is a "forever" kind of iteration, and when the pipe completes, dt will exit gracefully.

Note: Very short things also work as shell aliases. Here's a way to do the same thing in POSIX conventions: alias scream='dt stream [rl upcase pl] loop

Shebangs into REPLs

Another pattern that can be helpful is doing some pre-work like defining some commands or reading files and then dropping into a REPL with that state.

We won't go into a ton of detail here, but we'll demonstrate with a custom prompt and a single command definition.

#!/usr/bin/env dt

"MyShell 0.1" println

["/home/me/myshell.log" appendf]
"( s -- ) Save a string to the myshell log."
\log define

repl