How to separate stdout & stderr from executed shell command

[jqwoods]

I would like to create a method which executes a given shell command
and returns an array containing three things: 1) stdout of the command
as a string, 2) stderr of the command as a string, and 3) the exit
status. I'm having trouble figuring out how to get stdout and stderr
as separate output.

Here's an illustration of the desired result I'm after:

irb> my_exec_command("ls existant_file")
=> ["existant_file\n", "", 0]

irb> my_exec_command("ls NON_existant_file")
=> ["", "ls: NON_existant_file: No such file or directory\n", 2]

Any pointers on how to implement my_exec_command?

 

[jqwoods]

I would like to create a method which executes a given shell command
and returns an array containing three things: 1) stdout of the command
as a string, 2) stderr of the command as a string, and 3) the exit
status. I'm having trouble figuring out how to get stdout and stderr
as separate output.

Here's an illustration of the desired result I'm after:

irb> my_exec_command("ls existant_file")
=> ["existant_file\n", "", 0]

irb> my_exec_command("ls NON_existant_file")
=> ["", "ls: NON_existant_file: No such file or directory\n", 2]

Any pointers on how to implement my_exec_command?

PS: Unless there's a simpler way, if someone could just point me to an
API that provides stdout and stderr of sub-processes as IO objects, I
could probably figure it out from there. Thanks!

 

[Rodrigo Bermejo]

irb> my_exec_command("ls NON_existant_file")
=> ["", "ls: NON_existant_file: No such file or directory\n", 2]

Any pointers on how to implement my_exec_command?

PS: Unless there's a simpler way, if someone could just point me to an
API that provides stdout and stderr of sub-processes as IO objects, I
could probably figure it out from there. Thanks!

Take a look at the IO class.
The Pickaxe book has some good examples.

-r.

 

[jqwoods]

On Mar 14, 11:40 am, (e-mail address removed) wrote:

irb> my_exec_command("ls NON_existant_file")
=> ["", "ls: NON_existant_file: No such file or directory\n", 2]
Any pointers on how to implement my_exec_command?

PS: Unless there's a simpler way, if someone could just point me to an
API that provides stdout and stderr of sub-processes as IO objects, I
could probably figure it out from there. Thanks!

Take a look at the IO class.
The Pickaxe book has some good examples.

-r.

The closest API I could find is IO:open. But as the doc for
IO:open says "...the subprocess's standard input and output will be
connected..." So far I haven't found any way to get stdout and stderr
of a sub-process *separately*.

Any other pointers?

 

[James Gray]

On Mar 14, 11:40 am, (e-mail address removed) wrote:

irb> my_exec_command("ls NON_existant_file")
=> ["", "ls: NON_existant_file: No such file or directory\n", 2]

Any pointers on how to implement my_exec_command?

PS: Unless there's a simpler way, if someone could just point me
to an
API that provides stdout and stderr of sub-processes as IO
objects, I
could probably figure it out from there. Thanks!

Take a look at the IO class.
The Pickaxe book has some good examples.

-r.

The closest API I could find is IO:open. But as the doc for
IO:open says "...the subprocess's standard input and output will be
connected..." So far I haven't found any way to get stdout and stderr
of a sub-process *separately*.

Any other pointers?

Yes, Ruby comes with a standard library for this, called open3. Here
are the docs for the method you want:

$ qri Open3#popen3
----------------------------------------------------------- Open3#popen3
popen3(*cmd) {|| ...}
------------------------------------------------------------------------
Open stdin, stdout, and stderr streams and start external
executable. Non-block form:

require 'open3'

[stdin, stdout, stderr] = Open3.popen3(cmd)

Block form:

require 'open3'

Open3.popen3(cmd) { |stdin, stdout, stderr| ... }

The parameter cmd is passed directly to Kernel#exec.

Hope that helps.

James Edward Gray II

 

[jqwoods]

unknown wrote:
On Mar 14, 11:40 am, (e-mail address removed) wrote:
irb> my_exec_command("ls NON_existant_file")
=> ["", "ls: NON_existant_file: No such file or directory\n", 2]
Any pointers on how to implement my_exec_command?
PS: Unless there's a simpler way, if someone could just point me
to an
API that provides stdout and stderr of sub-processes as IO
objects, I
could probably figure it out from there. Thanks!
Take a look at the IO class.
The Pickaxe book has some good examples.
-r.

The closest API I could find is IO:open. But as the doc for
IO:open says "...the subprocess's standard input and output will be
connected..." So far I haven't found any way to get stdout and stderr
of a sub-process *separately*.

Any other pointers?

Yes, Ruby comes with a standard library for this, called open3. Here
are the docs for the method you want:

$ qri Open3#popen3
----------------------------------------------------------- Open3#popen3
popen3(*cmd) {|| ...}
------------------------------------------------------------------------
Open stdin, stdout, and stderr streams and start external
executable. Non-block form:

require 'open3'

[stdin, stdout, stderr] = Open3.popen3(cmd)

Block form:

require 'open3'

Open3.popen3(cmd) { |stdin, stdout, stderr| ... }

The parameter cmd is passed directly to Kernel#exec.

Hope that helps.

James Edward Gray II

Exactly what I was looking for. Thanks!

 

[Arlen Cuss]

[Note: parts of this message were removed to make it a legal post.]

Hi,

require "open3"

stdin, stdout, stderr = Open3.popen3('nroff -man')

Try it!

Arlen