Erlang Shell October 4, 2024 by Marcos Benevides

:ID: bcd04f60-0a4c-4213-90f7-139c0ddf5bef

Some features of the Erlang shell.

Basics

Compiling and Running “Hello World” in the Shell

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

  start() ->
      io:format("Hello world~n").

Running the hello.erl program in the erlang shell.

  $ erl
  Erlang/OTP 26 [erts-14.2.5] [source] [64-bit] [smp:24:24] [ds:24:24:10] [async-threads:1] [jit:ns]

  Eshell V14.2.5 (press Ctrl+G to abort, type help(). for help)
  1> c(hello).
  {ok,hello}
  2> hello:start().
  Hello world
  ok

Compiling Outside the Erlang Shell

  $ erlc hello.erl
  $ erl -noshell -s hello start -s init stop

erlc evokes the Erlang compiler from the command line. The compiler compiles the code in hello.erl and produces an object code file called hello.beam.

The erl -noshell command loads the module hello and evaluates the function hello:start(). After this, it evaluates the expression init:stop(), which terminates the Erlang session. (Armstrong 2013, 14)

Commands

Seeing Your Bound Variables

b() shows all bound variables in the current erlang shell.

    > b().
    N = 1
    Number = 5

Clearing Bound Variables in the Shell

  > f(N).
  > f().

Getting Help

The Erlang shell has a number of built-in commands. You can see them all with the shell command help(). (Armstrong 2013, 173).

Compiling and running Erlang programs

Modifying the Environment

You can find the value of the current load path by starting an Erlang shell and giving the command code:get_path(). The two most common functions that we use to manipulate the load path are as follows:

% Add a new directory to the start of the load path.
-spec code:add_patha(Dir) => true | {error, bad_directory}
% Add a new directory to the end of the load path.
-spec code:add_pathz(Dir) => true | {error, bad_directory}

Alternatively, you can start Erlang with a command like this:

$ erl -pa Dir1 -pa Dir2 ... -pz DirK1 -pz DirK2

Scripting

Often we want to be able to execute an arbitrary Erlang function from the OS command line. The -eval argument is very handy for quick scripting.

  erl \
      -eval 'io:format("Memory: ~p~n", [erlang:memory(total)]).' \
      -noshell -s init stop

Run As an Escript

  #!/usr/bin/env escript

  main([A]) ->
      I = list_to_integer(A),
      F = fac(I),
      io:format("factorial ~w = ~w~n", [I, F]).

  fac(0) ->
      1;
  fac(N) ->
      N * fac(N - 1).

Debugging

Reading Crash Dumps

If Erlang crashes, it leaves behind a file called erl_crash.dump. To start the analyzer, give the following command:

  crashdump_viewer:start()

References