# Guess what this will do

**URL:** <https://www.ruby-forum.com/t/guess-what-this-will-do/107874>\
**Category:** Ruby\
**Created:** [June 27, 2007, 1:28am UTC](https://www.ruby-forum.com/t/guess-what-this-will-do/107874 "2007-06-27T01:28:49Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![amin1426](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/4da419/32.png) [@amin1426](https://www.ruby-forum.com/u/amin1426)\
**Post date:** [June 27, 2007, 1:28am UTC](https://www.ruby-forum.com/t/guess-what-this-will-do/107874/1 "2007-06-27T01:28:49Z")

</div>

Without cheating (trying to run it) what will this pair of files do  
under Linux…

Here is a little script we want to be interpreted by the “flashbang”  
interpreter…  
===/home/johnc/foo/hashbang===========================================  
#!/home/johnc/foo/flashbang

puts ARGV[0]

======================================================================

So we expect  
./hashbang “Hello World”  
to be formally equivalent to running…

/home/johnc/foo/flashbang ./hashbang “Hello World”

Now assume…

===/home/johnc/foo/flashbang==========================================  
#!/bin/sh

echo flashbanger  
echo $0  
echo “$@”

# /usr/bin/ruby “$@”

Let’s try…  
/home/johnc/foo/flashbang -e ‘puts “Hello World”’

On my system the result is…  
flashbanger  
/home/johnc/foo/flashbang  
puts “Hello World”  
Hello World

No surprise…

Now what do you think happens if you say…?

./hashbang “Hello World”

Now what do you think happens if you say…?

/home/johnc/foo/flashbang ./hashbang “Hello World”

Scroll down for the answers…  
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./hashbang “Hello World”  
./hashbang: line 4: puts: command not found

Surprise Number 1 : The binfmt look up code looks up flashbang and  
finds it’s also a #! freaks out and just uses sh as the interpreter…

To check that lets try…  
==hashcat=============================================================  
#!/home/johnc/foo/flashcat

puts ARGV[0]

# echo “Look, we must be running $SHELL”

and  
==flashcat============================================================  
#!/bin/cat

echo flashbanger  
echo $0  
echo “$@”

# /usr/bin/ruby “$@”

## Then…

## ./hashcat “Hello World” ./hashcat: line 4: puts: command not found Look, we must be running /bin/bash

and…  
/home/johnc/foo/flashcat ./hashbang “Hello World”

Comes out as …  
/bin/cat /home/johnc/foo/flashcat ./hashcat “Hello World”

## Which results, no surprise as…

#!/bin/cat

echo flashbanger  
echo $0  
echo “$@”

/usr/bin/ruby “$@”  
#!/home/johnc/foo/flashbang

puts ARGV[0]

## /bin/cat: Hello World: No such file or directory

## Now for Surprise #2

## /home/johnc/foo/flashbang ./hashbang “Hello World” | head -20 flashbanger /home/johnc/foo/flashbang ./hashbang Hello World flashbanger /home/johnc/foo/flashbang ./hashbang Hello World flashbanger /home/johnc/foo/flashbang ./hashbang Hello World flashbanger /home/johnc/foo/flashbang ./hashbang Hello World flashbanger /home/johnc/foo/flashbang ./hashbang Hello World flashbanger /home/johnc/foo/flashbang ./hashbang Hello World flashbanger /home/johnc/foo/flashbang

WHAT THE HELL!? Why?

Well, deep within ruby/ruby.c is a chunk of code that looks for a #!  
on the first line. If it contains the word “ruby” anywhere, it ignores  
it. If it doesn’t it says, Oh dear. This one wasn’t actually for me. I  
better be gnice and exec that interpreter and feed it this script…

All in all, you can tell I have a had rather exotic and frustrating  
day or three.

John C. Phone : (64)(3) 358 6639  
Tait Electronics Fax : (64)(3) 359 4632  
PO Box 1645 Christchurch Email : removed\_email\_address@domain.invalid  
New Zealand

---

<div class="post-metadata">

**Author:** ![amin1426](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/4da419/32.png) [@amin1426](https://www.ruby-forum.com/u/amin1426)\
**Post date:** [June 27, 2007, 7:55pm UTC](https://www.ruby-forum.com/t/guess-what-this-will-do/107874/2 "2007-06-27T19:55:36Z")

</div>

On Wed, Jun 27, 2007 at 08:28:07AM +0900, John C. wrote:

> Without cheating (trying to run it) what will this pair of files do under Linux…

[lots of stuff snipped]

> WHAT THE HELL!? Why?
> 
> Well, deep within ruby/ruby.c is a chunk of code that looks for a #!  
> on the first line. If it contains the word “ruby” anywhere, it ignores  
> it. If it doesn’t it says, Oh dear. This one wasn’t actually for me. I  
> better be gnice and exec that interpreter and feed it this script…

That’s a Perlism. man perlrun says:  
If the #! line does not contain the word “perl”, the program named  
after the #! is executed instead of the Perl interpreter. This is  
slightly bizarre, but it helps people on machines that don’t do #!,  
because they can tell a program that their SHELL is /usr/bin/perl,  
and Perl will then dispatch the program to the correct interpreter  
for them.

I’m a bit surprised that ruby mimicks this behavior! Does it even  
_build_ on systems that don’t do ``#!’’?