# What the heck (changing process uid question)

**URL:** <https://www.ruby-forum.com/t/what-the-heck-changing-process-uid-question/48655>\
**Category:** Ruby\
**Created:** [November 23, 2005, 1:25am UTC](https://www.ruby-forum.com/t/what-the-heck-changing-process-uid-question/48655 "2005-11-23T01:25:45Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![joevandyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c0e974/32.png) [@joevandyk](https://www.ruby-forum.com/u/joevandyk)\
**Post date:** [November 23, 2005, 1:25am UTC](https://www.ruby-forum.com/t/what-the-heck-changing-process-uid-question/48655/1 "2005-11-23T01:25:45Z")

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My problem: When I fork a new process and set the UID of that process  
and want to create a log file that I redirect output into, I’m having  
difficulties getting the log file to be owned by the correct person.

Below is a code snippet that demonstrates my problem.

require ‘ftools’

fail “run this as root please” if Process.uid != 0  
fail “usage: ruby #{ **FILE** } \<directory” if ARGV.size != 2

uid = ARGV.shift.to\_i  
dir = ARGV.shift

puts “uid is #{ Process.uid }”

puts “changing uid to #{ uid }”  
Process.uid = uid

puts “uid is now #{ Process.uid }”

File.makedirs dir

puts “Hopefully \<#{ dir }\> was created by uid \<#{ uid }\>”

raise “why the heck don’t I own this?” if !File.owned? dir

When I run this on my linux box, I get:  
crabbe@~$ id  
uid=7258(mz652c) gid=20(games) groups=20(games)

crabbe@~$ sudo ruby a.rb 7258 asdf  
uid is 0  
changing uid to 7258  
uid is now 7258  
Hopefully was created by uid \<7258\>  
a.rb:20: why the heck don’t I own this? (RuntimeError)

crabbe@~$ ls -ld asdf  
drwxr-xr-x 2 4294967294 4294967294 9 Nov 22 16:23 asdf

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<div class="post-metadata">

**Author:** ![joevandyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c0e974/32.png) [@joevandyk](https://www.ruby-forum.com/u/joevandyk)\
**Post date:** [November 23, 2005, 1:46am UTC](https://www.ruby-forum.com/t/what-the-heck-changing-process-uid-question/48655/2 "2005-11-23T01:46:00Z")

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On Wed, 23 Nov 2005, Joe Van D. wrote:

> My problem: When I fork a new process and set the UID of that process and  
> want to create a log file that I redirect output into, I’m having  
> difficulties getting the log file to be owned by the correct person.

afaik this isn’t possible from within a ruby, or any script. setuid  
programs  
are gaurded pretty heavily by the kernel. i worked on this for some  
time at  
one point and this was the only way it was possible at the time:

[http://codeforpeople.com/lib/ruby/setuidruby/](http://codeforpeople.com/lib/ruby/setuidruby/)

let me know if you figure out another method. the bottom line is that  
setuid  
program must be binary.

enjoy.

-a

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<div class="post-metadata">

**Author:** ![joevandyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c0e974/32.png) [@joevandyk](https://www.ruby-forum.com/u/joevandyk)\
**Post date:** [November 23, 2005, 1:58am UTC](https://www.ruby-forum.com/t/what-the-heck-changing-process-uid-question/48655/3 "2005-11-23T01:58:04Z")

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Joe Van D. wrote:

> My problem: When I fork a new process and set the UID of that process  
> and want to create a log file that I redirect output into, I’m having  
> difficulties getting the log file to be owned by the correct person.
> 
> Process.uid = uid

You want to change effective id, not real id:

Process.euid = uid

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<div class="post-metadata">

**Author:** ![joevandyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/c0e974/32.png) [@joevandyk](https://www.ruby-forum.com/u/joevandyk)\
**Post date:** [November 23, 2005, 2:10am UTC](https://www.ruby-forum.com/t/what-the-heck-changing-process-uid-question/48655/4 "2005-11-23T02:10:09Z")

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On 11/22/05, Bob S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Joe Van D. wrote:
> 
> > My problem: When I fork a new process and set the UID of that process  
> > and want to create a log file that I redirect output into, I’m having  
> > difficulties getting the log file to be owned by the correct person.
> > 
> > Process.uid = uid
> 
> You want to change effective id, not real id:
> 
> Process.euid = uid

Very nice, thank you.