Capturing the Output and Error Streams from a Unix Shell Command


You want to run an external program as in Recipe 20.8, but you also want to capture the standard error stream. Using popen only gives you access to the standard output.


Use the open3 library in the Ruby standard library. Its popen3 method takes a code block, to which it passes three IO streams: one each for standard input, output, and error.

Suppose you perform the Unix ls command to list a nonexistent directory. ls will rightly object to this and write an error message to its standard error stream. If you invoked ls with IO.popen or the %x{} construction, that error message is passed right along to the standard error stream of your Ruby process. You can capture it or suppress it:

	%x{ls no_such_directory}
	# ls: no_such_directory: No such file or directory

But if you use popen3, you can grab that error message and do whatever you want with it:

	require open3

	Open3.popen3(ls -l no_such_directory) { |stdin, stdout, stderr| }
	# => "ls: no_such_directory: No such file or directory


The same caveats in the previous recipe apply to the IO streams returned by popen3. If you e running a command that accepts data on standard input, and you read from stdout before closing stdin, your process will hang.

Unlike IO.popen, the popen3 method is only implemented on Unix systems. However, the win32-open3 package (part of the Win32Utils project) provides a popen3 implementation.

See Also

  • Recipe 20.8, "Driving an External Process with popen"
  • Like many other Windows libraries for Ruby, win32-open3 is available from



Date and Time



Files and Directories

Code Blocks and Iteration

Objects and Classes8

Modules and Namespaces

Reflection and Metaprogramming


Graphics and Other File Formats

Databases and Persistence

Internet Services

Web Development Ruby on Rails

Web Services and Distributed Programming

Testing, Debugging, Optimizing, and Documenting

Packaging and Distributing Software

Automating Tasks with Rake

Multitasking and Multithreading

User Interface

Extending Ruby with Other Languages

System Administration

Ruby Cookbook
Ruby Cookbook (Cookbooks (OReilly))
ISBN: 0596523696
EAN: 2147483647
Year: N/A
Pages: 399

Similar book on Amazon © 2008-2017.
If you may any questions please contact us: