# How to redirect a string to stdin within code?

**URL:** <https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545>\
**Category:** Ruby\
**Created:** [August 6, 2006, 12:00am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545 "2006-08-06T00:00:40Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 6, 2006, 12:00am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/1 "2006-08-06T00:00:40Z")

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Hello all, I am trying to create a unit test for a class that reads  
input  
from stdin (ie. the user enters his name when starting the class). I  
would  
like to write a unit test that provides the input via a string.  
something  
like:

input = “myname”  
$stdin = input  
start my class that I would like to test

Any help is greatly appreciated.

Christian

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<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 6, 2006, 12:51am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/2 "2006-08-06T00:51:43Z")

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On Aug 5, 2006, at 6:00 PM, Christian Seifert wrote:

> Christian

require ‘stringio’

input = StringIO.new(“myname”)

$stdin = input

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<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 6, 2006, 8:13am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/3 "2006-08-06T08:13:28Z")

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Logan C. wrote:

> > start my class that I would like to test  
> > $stdin = input  
> > But:

alex@pandora:~$ irb  
irb(main):001:0\> require ‘stringio’  
=\> true  
irb(main):002:0\> input = StringIO.new(“myname”)  
=\> #StringIO:0xb7a42864  
irb(main):003:0\> $stdin = input  
=\> #StringIO:0xb7a42864  
/usr/lib/ruby/1.8/irb/input-method.rb:97: [BUG] Segmentation fault  
ruby 1.8.4 (2005-12-24) [i486-linux]

Aborted

This is on Ubuntu Breezy. Any idea why that happens?

---

<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 6, 2006, 9:49am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/4 "2006-08-06T09:49:32Z")

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On 8/6/06, Alex Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > > input = “myname”  
> > > input = StringIO.new(“myname”)  
> > > =\> #StringIO:0xb7a42864  
> > > /usr/lib/ruby/1.8/irb/input-method.rb:97: [BUG] Segmentation fault  
> > > ruby 1.8.4 (2005-12-24) [i486-linux]
> 
> Aborted
> 
> This is on Ubuntu Breezy. Any idea why that happens?

Very interesting I was just trying it on my Dapper and I get similar  
but  
not identical behavior, see below.  
However this seems an IRB issue Logans code just works fine in pure ruby

510/11 \> irb  
irb(main):001:0\> require ‘stringio’  
=\> true  
irb(main):002:0\> input = StringIO.new(“myname”)  
=\> #StringIO:0xb7d2fe4c  
irb(main):003:0\> $stdin = input  
=\> #StringIO:0xb7d2fe4c  
irb(main):004:0\> irb(main):004:0\> NameError: undefined local variable or  
method `myname’ for main:Object  
from (irb):4  
from :0  
irb(main):004:0\> robert@:~/log/ruby/tests 09:43:16

and

510/12 \> uname -a  
Linux roma 2.6.15-19-386 #1 PREEMPT Mon Mar 20 16:46:02 UTC 2006 i686  
GNU/Linux  
robert@:~/log/ruby/tests 09:45:04  
511/13 \> ruby --version  
ruby 1.8.4 (2005-12-24) [i686-linux]  
robert@:~/log/ruby/tests 09:45:20  
512/14 \> irb --version  
irb 0.9.5(05/04/13)  
robert@:~/log/ruby/tes

Cheers  
Robert

–

> Alex

–  
Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui  
concerne l’univers, je n’en ai pas acquis la certitude absolue.

- Albert Einstein

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<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 7, 2006, 12:02am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/5 "2006-08-07T00:02:35Z")

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Alex Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

> > However this seems an IRB issue Logans code just works fine in pure ruby  
> > Yes… I can see why it’s not allowed in IRB (if you get rid of  
> > stdin, how the hell would you use the console? :-), I just don’t know  
> > why it segfaults.

Just a guess: the readline extensions directly accesses $stdin without  
checking its type.

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<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 7, 2006, 12:20am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/6 "2006-08-07T00:20:36Z")

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On Aug 6, 2006, at 2:59 PM, Christian N. wrote:

> Alex Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:
> 
> > > However this seems an IRB issue Logans code just works fine in  
> > > pure ruby  
> > > Yes… I can see why it’s not allowed in IRB (if you get rid of  
> > > stdin, how the hell would you use the console? :-), I just don’t know  
> > > why it segfaults.
> 
> Just a guess: the readline extensions directly accesses $stdin without  
> checking its type.

I think this is probably fixed in newer versions of ruby:

% ruby -v -rstringio -e ‘log = StringIO.new(“blah”); $stdin = log;  
require “irb”; IRB.start( **FILE** )’  
ruby 1.8.3 (2005-09-28) [i386-freebsd4.11]  
/usr/local/lib/ruby/1.8/irb/input-method.rb:97: [BUG] Segmentation fault  
ruby 1.8.3 (2005-09-28) [i386-freebsd4.11]

Abort trap (core dumped)

versus:

% ruby -v -rstringio -e ‘log = StringIO.new(“blah”); $stdin = log;  
require “irb”; IRB.start( **FILE** )’  
ruby 1.8.4 (2006-03-04) [i686-darwin8.6.2]  
Read 100 saved history commands from /Users/ryan/.irb.hist.  
Saving 100 history lines to /Users/ryan/.irb.hist.  
/usr/local/lib/ruby/1.8/irb/input-method.rb:97:in `readline': wrong argument type StringIO (expected File) (TypeError) from /usr/local/lib/ruby/1.8/irb/input-method.rb:97:in `gets’  
from /usr/local/lib/ruby/1.8/irb.rb:132:in `eval_input' from /usr/local/lib/ruby/1.8/irb.rb:259:in `signal\_status’  
from /usr/local/lib/ruby/1.8/irb.rb:131:in `eval_input' from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:189:in `buf\_input’  
from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:104:in `getc' from /usr/local/lib/ruby/1.8/irb/slex.rb:206:in `match\_io’  
from /usr/local/lib/ruby/1.8/irb/slex.rb:76:in `match' from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:287:in `token’  
from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:263:in `lex' from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:234:in `each\_top\_level\_statement’  
from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:230:in  
`each_top_level_statement' from /usr/local/lib/ruby/1.8/irb/ruby-lex.rb:229:in `each\_top\_level\_statement’  
from /usr/local/lib/ruby/1.8/irb.rb:146:in `eval_input' from /usr/local/lib/ruby/1.8/irb.rb:70:in `start’  
from /usr/local/lib/ruby/1.8/irb.rb:69:in `start’  
from -e:1

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<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 6, 2006, 9:58am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/7 "2006-08-06T09:58:33Z")

</div>

Robert D. wrote:

> > Aborted
> > 
> > This is on Ubuntu Breezy. Any idea why that happens?
> 
> Very interesting I was just trying it on my Dapper and I get similar but  
> not identical behavior, see below.  
> However this seems an IRB issue Logans code just works fine in pure ruby  
> Yes… I can see why it’s not allowed in IRB (if you get rid of stdin,  
> how the hell would you use the console? :-), I just don’t know why it  
> segfaults.

> ```
> from (irb):4
> 
> ```
> 
> ruby 1.8.4 (2005-12-24) [i686-linux]  
> Interesting. In principle, we should get the same results - the only  
> difference is the target processor. Allegedly.

---

<div class="post-metadata">

**Author:** ![Christian\_Seifert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/edb3f5/32.png) [@Christian\_Seifert](https://www.ruby-forum.com/u/Christian_Seifert)\
**Post date:** [August 7, 2006, 3:41am UTC](https://www.ruby-forum.com/t/how-to-redirect-a-string-to-stdin-within-code/101545/8 "2006-08-07T03:41:45Z")

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Alex Y. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

> ruby 1.8.4 (2005-12-24) [i486-linux]
> 
> Aborted
> 
> This is on Ubuntu Breezy. Any idea why that happens?

Well, that particular line of the irb source is calling the method  
“readline” which is defined in readline.so - that is, in C. My guess  
is that it has some very specific assumptions about what type of  
object it’s reading from. When I try that, I get:

esau:~$ irb  
irb(main):001:0\> require ‘stringio’  
=\> true  
irb(main):002:0\> stream = StringIO.new(“p [1,2,3,4]”)  
=\> #StringIO:0xb7c9c734  
irb(main):003:0\> $stdin = stream  
=\> #StringIO:0xb7c9c734  
/usr/lib/ruby/1.8/irb/input-method.rb:97:in `readline': wrong argument type StringIO (expected File) (TypeError) from /usr/lib/ruby/1.8/irb/input-method.rb:97:in `gets’  
from /usr/lib/ruby/1.8/irb.rb:132:in `eval_input' from /usr/lib/ruby/1.8/irb.rb:259:in `signal\_status’  
from /usr/lib/ruby/1.8/irb.rb:131:in `eval\_input’  
(many more lines of stacktrace omitted)

This is on my Debian testing system, so I should be running the same  
binary as you are, in theory:

esau:~$ ruby -v  
ruby 1.8.4 (2005-12-24) [i486-linux]

Note that “irb --noreadline” won’t have this problem, but that has  
other issues (such as the fact that arrow keys don’t work any more):

esau:~$ irb --noreadline  
irb(main):001:0\> require ‘stringio’  
=\> true  
irb(main):002:0\> stream = StringIO.new(“p [1,2,3,4]”)  
=\> #StringIO:0xb7cdb7b8  
irb(main):003:0\> $stdin = stream  
=\> #StringIO:0xb7cdb7b8  
irb(main):004:0\> irb(main):004:0\> [1, 2, 3, 4]  
=\> nil  
irb(main):004:0\> esau:~$