POpen4 provides the Rubyist a single API across platforms for executing a command in a child process with handles on stdout, stderr, stdin streams as well as access to the process ID and exit status. It does very little other than to provide an easy way to use either Ara Howard’s Open4 library or the win32-popen3 library by Park Heesob and Daniel Berger depending on your platform and without having to code around the slight differences in their APIs.

Consider this my first attempt at a response to Daniel’s request for a consensus on the API back in 2005.

It is my hope that this project will be shortly absorbed or replaced with a better solution. POpen4 offers very little in the way of features or flexibility, but it does work across platforms without extra coding.


Rather than adopting either Open4’s API or win32-open3’s (they differ in several respects, but most obviously in the order in which streams are passed to the block) I am proposing a third API for couple of reasons. First, Open4 passes the PID first and win32-open3 passes stdin neither of which seem to me to be the streams we are most likely to use when we don’t need all four. POpen4 passes stdout and stderr first so that when the others are not required we can omit them from the block. Second, I thought it best to break everybody’s code rather than to be a drop in replacement on one platform and be a surprise on another. No surprises–it’s a new API on either platform.


$ gem install --remote --include-dependencies POpen4

Presently POpen4 ships in four platform specific gems:


Users of Linux, Unix, BSD, etc., will want the unix package. Ara’s Open4 gem is a dependency. Users of the Ruby One-Click Installer (OCI) will want to select the gem appropriate their version of Ruby. The real issue here is compiler version. The wrong version will SEGFAULT due to some incompatibility somewhere in the VC++ compiler or libraries (which I really wish we could resolve, but oh well). Others using a Windows platform but not the OCI will want to compile from source using last of these gems. Be sure to have your environment set up for doing a build before running gem install POpen4.


Ara Howard, Park Heesob, Daniel Berger and others have done the real work. Many thanks to them for the many hours they have poured into sharing their work with the Ruby community at large.