# Redirecting stdin for 3. party program **URL:** \ **Category:** Ruby\ **Created:** [June 3, 2010, 5:46pm UTC](https://www.ruby-forum.com/t/redirecting-stdin-for-3-party-program/189593 "2010-06-03T17:46:50Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![tcsnide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tcsnide](https://www.ruby-forum.com/u/tcsnide)\ **Post date:** [June 3, 2010, 5:46pm UTC](https://www.ruby-forum.com/t/redirecting-stdin-for-3-party-program/189593/1 "2010-06-03T17:46:50Z")
OS: Debian Lenny Ruby version: 1.8.7.72-3lenny1 ==Code== stdin\_read,stdin\_write=IO.pipe pid=fork if !pid #$stdin.reopen("/home/admin/.bashrc") stdin\_write.close $stdin.reopen(stdin\_read) ``` a=gets puts "ruby child: got '#{a}'" system("echo bash; mawk '{print \"bash child: got \"$0;exit}'") exit! ``` end stdin\_read.close sleep 1 stdin\_write.puts “word1” sleep 1 stdin\_write.puts “word2” sleep 1 puts “parent: before wait” Process.wait(pid) puts “parent: after wait” ==/Code== =Expected result:= ruby child: got 'ruby word1 ’ bash bash child: got 'ruby word2 ’ parent: before wait parent: after wait =Actual result:= ruby child: got 'word1 ’ bash parent: before wait -\>after the line ‘parent: before wait’ has been printed, the program hangs. It gets even more interesting if I comment out all the pipe related stuff and instead redirect $stdin of the child to a file (the line '$stdin.reopen("/home/admin/.bashrc"): -I still only get output for ‘ruby child’, none for ‘bash child’ - however it finishes instead of hanging. Perhaps ruby buffers more stdin than the single ‘gets’ returns? Multiple ‘gets’ do work correctly. -If I comment out the ‘gets’, it works! So the subshell started by ‘system’ DOES get the redirected stdin… =Program that works= pid=fork if !pid $stdin.reopen("/home/admin/.bashrc") ``` exec("echo bash; mawk '{print \"bash child: got \"$0;exit}'") exit! ``` end puts “parent: before wait” Process.wait(pid) puts “parent: after wait” =Output= bash bash child: got # ~/.bashrc: executed by bash(1) for non-login shells. parent: before wait parent: after wait So: What is going wrong when redirecting it to a pipe? I tried playing around with Kernel.select and replacing system with exec, but neither changed the result in the least. I also looked through the implementation of popen4, but can’t spot anything done differently there. Maybe I’ve already looked at my own code for to long to spot something obvious though. So please: Can anyone provide me with a good hint? ---
**Author:** ![tcsnide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tcsnide](https://www.ruby-forum.com/u/tcsnide)\ **Post date:** [June 3, 2010, 7:18pm UTC](https://www.ruby-forum.com/t/redirecting-stdin-for-3-party-program/189593/2 "2010-06-03T19:18:55Z")
One small addendum: I’m not actually interested in reading in something via ‘gets’ before launching the external command, I only included it for demonstration purposes. What I am hoping to get to work is something like the second test case, just with stdin coming from a pipe instead of a file. ---
**Author:** ![tcsnide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tcsnide](https://www.ruby-forum.com/u/tcsnide)\ **Post date:** [June 3, 2010, 9:04pm UTC](https://www.ruby-forum.com/t/redirecting-stdin-for-3-party-program/189593/3 "2010-06-03T21:04:07Z")
On 6/3/10, Thomas S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > ``` > $stdin.reopen(stdin_read) > > ``` > > sleep 1 > ruby child: got 'ruby word1 > bash > parent: before wait > > -\>after the line ‘parent: before wait’ has been printed, the program > hangs. I’ve deliberately forgotten all the awk I ever knew, so I modified your program to use ruby where you had mawk, and it appears to work. I trust that I have correctly translated the intention of your original mawk script… Basically, I replaced the system line with: system(“echo bash; ruby -e ‘print "bash child: got "+readline;exit’”) And I see this output: ruby child: got 'word1 ’ bash bash child: got word2 parent: before wait parent: after wait With your original mawk line, I see the same behavior you do. I’d suspect a buffering problem, but inserting a flush after puts of word2 doesn’t change anything. However, when I close the handle stdin\_write after writing word2, it does work, even with mawk. Perhaps mawk is waiting for its stdin to close before it does anything? ---
**Author:** ![tcsnide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tcsnide](https://www.ruby-forum.com/u/tcsnide)\ **Post date:** [June 3, 2010, 9:17pm UTC](https://www.ruby-forum.com/t/redirecting-stdin-for-3-party-program/189593/4 "2010-06-03T21:17:14Z")
Thank you thank you thank you 🙂 Buffering by mawk simply wasn’t something that crossed my mind at all. With your ruby line it walks flawlessly. For anyone interested: mawk needs the Option “-W interactive” do process input as it arrives… And I dumbly and persistently searched for the problem on the ruby side… Thanks again - solved! ---
**Author:** ![tcsnide](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/eb8c5e/32.png) [@tcsnide](https://www.ruby-forum.com/u/tcsnide)\ **Post date:** [June 7, 2010, 10:12am UTC](https://www.ruby-forum.com/t/redirecting-stdin-for-3-party-program/189593/5 "2010-06-07T10:12:51Z")
2010/6/3 Thomas S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Thank you thank you thank you 🙂 > > Buffering by mawk simply wasn’t something that crossed my mind at all. > With your ruby line it walks flawlessly. > > For anyone interested: mawk needs the Option “-W interactive” do process > input as it arrives… > > And I dumbly and persistently searched for the problem on the ruby > side… Thomas, why do you use IO.pipe and fork instead of IO.popen or Open3.popen3? Kind regards robert