Redirect output from erlang shell into a file

Question

Is there a way to redirect data printed by io:format() from erlang shell into a file ? I know that I can open a file (IoDevice) and write data directly into it, but it requires code change and I don't want to do now.

Answer 1

When a process calls io:format() and similar functions, the process sends io request messages to its group_leader process. So a simple hack is to open a file, and set it as the group_leader of the processes producing the output. Here is an example of redirecting the shell process's output to a file.

1> {ok, F} = file:open("z", [write]).
{ok,<0.36.0>}
2> group_leader(F, self()).
3> io:format("Where am I going to appear?~n").       
4>

This will only redirect the current shell process, so you will have to set the group_leader for all processes which you would like to redirect to the file.

The solution can be refined of course, for example by spawning a server process which proxies io request messages to rotated files, etc.

Answer 2

Just run it with erl -noinput -s module function -s init stop > file.

Here's an example.

Erlang code:

-module(test).    
-compile(export_all).

function() ->
   io:fwrite("Hello world!~n").

In shell:

$ erlc test.erl 
$ erl -noinput -s test function -s init stop > test.txt
$ cat test.txt
Hello world!

Answer 3

You can also use the IODevice argument to io:fwrite/3 and let it have the atom value standard_io when you don't want it directed to some file. Otherwise give it the file.

See the "Standard Input/Output" section of the io module's documentation.

Answer 4

You can recompile your code with a parse transform, transforms calls like

io:format("~p~n", "Hello world!")

into calls like

io:format(whereis(my_output_file), "~p~n", "Hello world!")

Plus you need to add this into your start-up code, and you are done:

{ok, F} = file:open("file", [write]),
register(my_output_file, F),