During development the Erlang shell is often used for quickly testing functionality. Erlang's ability to reload modules while running makes this workflow even more efficient. To go a step further in removing manual intervention tools like sync and active have been created. These libs will listen for file modifications, recompile and reload the changed modules.

With rebar3 there is a plugin rebar3_auto which will start the shell, begin listening for modifications in the source directories of the project and recompile/reload when changes occur. Add to your global rebar3 configuration file to be able to use it on any of your projects ~/.config/rebar3/rebar.config:

{plugins, [rebar3_auto]}.

Running rebar3 auto will start the shell the same as running rebar3 shell but will be listening for file changes in your project's application source directories. When a file is change it will message the rebar3 agent to run compile and reload modules.

(master) $ rebar3 auto                       
===> Fetching rebar3_auto ({pkg,<<"rebar3_auto">>,<<"0.2.0">>})
===> Downloaded package, caching at /home/tristan/.cache/rebar3/hex/default/packages/rebar3_auto-0.2.0.tar
===> Fetching enotify ({pkg,<<"enotify">>,<<"0.1.0">>})
===> Version cached at ~/.cache/rebar3/hex/default/packages/enotify-0.1.0.tar is up to date, reusing it
===> Compiling enotify
===> Compiling rebar3_auto
Setting up watches.  Beware: since -r was given, this may take a while!  
Watches established.  
Erlang/OTP 18 [erts-7.1] [source] [64-bit] [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V7.1  (abort with ^G)  
1> rlx_util:test().  
** exception error: undefined function rlx_util:test/0

Oops, our function doesn't exist. Switch to your editor and add to rlx_util.erl a function test/0 and export it, when you save you'll see it is recompiled and now available in the shell:

Verifying dependencies...  
Compiling relx

2> rlx_util:test().  

In the shell all the features of the shell command, its configuration and the rebar3 agent are available, see the docs for more information.