[erlang-questions] os:cmd/1 with return code

Serge Aleynikov <>
Fri Oct 17 13:35:41 CEST 2008


You can take a look at this project:

http://code.google.com/p/erlexec/

It aims at solving this along with a handful of other problems.  It also 
allows to link Erlang Pids to spawned OS Pids, send POSIX signals.

Regards,

Serge

BJörn Lindqvist wrote:
> Hello good people,
> 
> I've been trying to implement and os:cmd/1 variant that also returns
> the status code of the executed command. But it proved to be
> exceedingly difficult.
> 
> First I tried something like os:cmd(Cmd ++ "; echo \n$?") and parsing
> out the exit code. But that doesn't work if the command string is
> broken in for example "ls ); echo \n$?" or "exit 99; echo \n$?".
> 
> Then I tried opening a port to "sh -c" like this:
> 
>     port_open({spawn, "sh -c '" ++ Cmd ++ "'"}, [exit_status]),
> 
> and then reading from the port and receiving the exit_status
> message. That works, but has escaping problems so it will fail if
> e.g. Cmd is "ls 'foo bar'".
> 
> Then I tried opening a port to "sh", sending the command to it and
> read the response from the port. Using this code:
> 
>     mycmd(Cmd)->
>         Opts = [stream, exit_status, stderr_to_stdout, eof],
>         Port = open_port({spawn, "sh"}, Opts),
>         port_command(Port, Cmd ++ "\nexit\n"),
>         get_data(Port, []).
> 
>     get_data(Port, Data) ->
>         receive
>             {Port, {data, Data1}} ->
>                 get_data(Port, Data ++ Data1);
>             {Port, {exit_status, N}} ->
>                 port_close(Port),
>                 {N, Data}
>         end.
> 
> This mostly works, except when it doesn't. Sometimes I don't get the
> {exit_status, N} message so the receive waits forever other times I
> get other messages that I don't want. What should I do? It shouldn't
> be that complicated to get this right. And why isn't this function
> already in the OTP library?
> 
> 




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