Starting Erlang Programs

Starting an erlang program for someone new can be confusing. Erlang does not work like C or python or java. It really is unique in the way that it requires you to start your program.

Lets start off like I did for the first time, the books are nice and often give examples like.

1> c(something).
{ok, something}
2> something:do().
"hello"
3>

Well thats nice and dandy, now how do I just do something like I do with most programs, you know...

$ ./something
  "hello"

Lets see what options erl has.

$ erl --help
Erlang R14B02 (erts-5.8.3) [source] [64-bit] [smp:4:4] [rq:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.3  (abort with ^G)
1>

Ooops... where's help? What just happened? Turns out --help isn't used by erl.

Instead erl has built in man pages. Not unix man pages mind you.

$ erl -man erl

Here you can finally see a huge multitude of options. The one in particular we are interested in is '-s' or setting an init function. The '-s' option takes at least a module and optionally a function followed by optional arguments.

Another aspect of -s thats interesting to take note of is that you may have more than one -s argument given to erl!

As an example lets use a simple module.

-module(blah_me).
-export([start/0]).

start() ->
    io:format("started my program~n"),
    ok.

And then in your favorite shell (mine is zsh).

$ erlc blah_me.erl
$ erl -s blah_me

And viola, we can run erlang starting it with any module, function, and arguments we want. In fact you can run any number of functions this way.

Try for example.

$ erl -s blah_me -s blah_me

I often write small shell scripts to pass in lots of arguments and things to erl when developing a program. This saves a lot of time starting of many common things needed. At some point however its a good idea to make your program use OTP releases and all the benefits they provide.