IO#popen and exit status

Ruby
1

I’m a bit stuck with this porblem:
I need to write data to a process and then wait for the exit code of it
to
see if it was successful or not.

Now I can IO.popen the process but there is no pclose instance method to
get the exitcode back. Additionally my workaround via Process.waitpid2
does
not work either. I think io.close does already all the cleanup.

e.g.

io = IO.popen(cmd, “w”)
io.write(msg)
io.close
pid, status = Process.waitpid2(0)

Raises a “Errno::ECHILD: No child processes” exception.

How do other handle this situation? Do I need to take the hard road via
pipe, fork, exec or did I miss something obvious?

2

On Tue, 21 Mar 2006 01:20:36 +0900, Claudio J.
[email protected] wrote:

[snip]

Now I can IO.popen the process but there is no pclose instance method to
get the exitcode back. Additionally my workaround via Process.waitpid2 does
not work either. I think io.close does already all the cleanup.

The $? variable will hold the exit status of the last child to
terminate.

andrew

3

On Tue, 21 Mar 2006, Claudio J. wrote:

io = IO.popen(cmd, “w”)
io.write(msg)
io.close
pid, status = Process.waitpid2(0)

Raises a “Errno::ECHILD: No child processes” exception.

How do other handle this situation? Do I need to take the hard road via
pipe, fork, exec or did I miss something obvious?

gem install open4

-a

4

Andrew J. wrote:

andrew

$? does not work with popen, or open3. I am not sure why one would want
to run a process, and monitor the output, yet not care what the exit
status is? One would think that this oversite will be fixed in future
releases.

~S

5

Actually I just solved this a few threads below. You can just run
Process.wait after you’ve done with the IO.popen and it should update
$? with the correct values.

6

Shea M. wrote:

The $? variable will hold the exit status of the last child to terminate.

andrew

$? does not work with popen, or open3. I am not sure why one would want
to run a process, and monitor the output, yet not care what the exit
status is? One would think that this oversite will be fixed in future
releases.

~S
ps - you can call Process.wait to update $?, but it is not reliable if
your program is multi threaded.

7

$? does not work with popen, or open3. I am not sure why one would want
to run a process, and monitor the output, yet not care what the exit
status is? One would think that this oversite will be fixed in future
releases.

~S

Before I get flamed, know Process.waitpid can be used…
~S

8

In article [email protected],
Claudio J. [email protected] writes:

I’m a bit stuck with this porblem:
I need to write data to a process and then wait for the exit code of it to
see if it was successful or not.

Now I can IO.popen the process but there is no pclose instance method to
get the exitcode back. Additionally my workaround via Process.waitpid2 does
not work either. I think io.close does already all the cleanup.

$? can be used after close.

I hope this documentation helps.

Index: io.c

RCS file: /src/ruby/io.c,v
retrieving revision 1.246.2.97
diff -u -p -r1.246.2.97 io.c
— io.c 14 Feb 2006 02:23:33 -0000 1.246.2.97
+++ io.c 28 Mar 2006 01:47:15 -0000
@@ -2149,6 +2149,9 @@ rb_io_close(io)

  • an IOError is raised if such an attempt is made. I/O
  • streams are automatically closed when they are claimed by the
  • garbage collector.
    • If ios is opened by IO.popen,
    • close sets $?.
      */

static VALUE
@@ -3062,7 +3065,9 @@ retry:
*

  • If a block is given, Ruby will run the command as a child connected
  • to Ruby with a pipe. Ruby’s end of the pipe will be passed as a
    • parameter to the block. In this case IO::popen returns
    • parameter to the block.
    • At the end of block, Ruby close the pipe and sets $?.
    • In this case IO::popen returns
    • the value of the block.
    • If a block is given with a cmd_string of ``-‘’,
      @@ -3078,6 +3083,7 @@ retry:
    • puts "Parent is #{Process.pid}"

    • IO.popen ("date") { |f| puts f.gets }

    • IO.popen("-") {|f| $stderr.puts "#{Process.pid} is here, f is

#{f}"}

    • p $?
    • produces:

@@ -3086,6 +3092,7 @@ retry:

  • Wed Apr  9 08:53:52 CDT 2003

  • 26169 is here, f is

  • 26166 is here, f is #<IO:0x401b3d44>

    • #<Process::Status: pid=26166,exited(0)>
    */

static VALUE

9

a = IO.popen(process)
a.close
and $? will be set immediately after the call to close succeeds.

Alternatively, if a block is given, at the end of block, Ruby closes the
pipe and sets $?.

See Method: IO.popen — Documentation for core (3.0.2)

“If a block is given, Ruby will run the command as a child connected to
Ruby with a pipe.
Ruby’s end of the pipe will be passed as a parameter to the block.
At the end of block, Ruby closes the pipe and sets $?.
In this case IO.popen returns the value of the block.”

10

not work either. I think io.close does already all the cleanup.

$? can be used after close.

I hope this documentation helps.

Just so there is no confusion for others that run into this since google
brought me straight here, you can do:

a = IO.popen(process)
a.close
and $? will be set immediately after the call to close succeeds.

Process.waitpid is not necessary.

11

Nice chain of topic resurrection…