Start erlang with user defined variable (using rebar)? - erlang

Question

I'd like to be able to use the following macro in my modules:
-ifdef(debug).
My startup script looks something like the following:
#!/bin/sh
PWD="$(pwd)"
#NAME="$(basename $PWD)"
erl -pa "$PWD/ebin" deps/*/ebin -boot start_sasl \
-name foo#127.0.0.1 \
-debug 1 \
-s $NAME \
+K true \
+P 65536
What else would need to be added so that debug is defined in my module? I need this to be dynamic so I don't have to modify source code for deployment into production. Using different startup scripts per dev/qa/prod environments is fine, but modifying source code shouldn't be necessary.
With erlc this can be done with -Ddebug. I use rebar however, and am not sure how to do it with that. I've tried adding the following to my rebar.config:
{erl_opts, [{D, "debug"}]}.
This gives the following error:
{error,
{1,
erl_parse,
"bad term"}}

Answer 1

The define for the compiler in rebar.config should look like this:
{erl_opts, [{d, debug}]}.
Note: the syntax is exactly the same as the compiler module's syntax: http://www.erlang.org/doc/man/compile.html
Current version of rebar (rebar version: 2 date: 20111205_155958 vcs: git 54259c5) does support compiler defines, as well.
rebar -D <defines> compile
See rebar --help for more rebar options.

Answer 2

ifdef is a preprocessor macro, it gets evaluated and removed at compile time -- you would have to re-compile your module with something like erlc -Ddebug module.erl to change it. add the "-P" flag if you want to see the output from the preprocessor in module.P.
to get access to the "-debug 1" argument at runtime, you can use init:get_argument(debug).
# erl -debug 1
...
1> init:get_argument(debug).
{ok,[["1"]]}
2> init:get_argument(foo).
error