Puppet users - Aug 2013 - Replace file based on content

Hi,

I wanted to know if there is an easy way to replace a file in puppet (using 
the file directive) but only when a certain string exists.

My use case:
I have a package that installs a config file, I want to replace that file 
with puppet, but I in normal operation the application may also write to 
that file.
So a simple replace => true is not sufficient.

I tried with something like this:
define configfile ($source) {

                $grepcommand = ''/bin/grep --quiet "string to
check"''
                exec { "check_${name}":
                        command => "/bin/true",
                        onlyif => "${grepcommand} ${name}",
                }

                file { "$name":
                        ensure => "present",
                        replace => "true",
                        mode => "0644",
                        owner => ''user'',
                        group => ''user'',
                        source => $source,
                        require => Exec["check_${name}"]
                }

        }

        configfile {"/etc/software/config.xml":
                source => "puppet:///modules/software/config.xml"
        }

I can''t get that to work properly, the file is always getting replaced.

Regards,
Steven

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On Wednesday, August 14, 2013 8:23:47 AM UTC-5, redalert....@gmail.com 
wrote:
>
> Hi,
>
> I wanted to know if there is an easy way to replace a file in puppet 
> (using the file directive) but only when a certain string exists.
>
> My use case:
> I have a package that installs a config file, I want to replace that file 
> with puppet, but I in normal operation the application may also write to 
> that file.
> So a simple replace => true is not sufficient.
>
> I tried with something like this:
> define configfile ($source) {
>
>                 $grepcommand = ''/bin/grep --quiet "string to
check"''
>                 exec { "check_${name}":
>                         command => "/bin/true",
>                         onlyif => "${grepcommand} ${name}",
>                 }
>
>                 file { "$name":
>                         ensure => "present",
>                         replace => "true",
>                         mode => "0644",
>                         owner => ''user'',
>                         group => ''user'',
>                         source => $source,
>                         require => Exec["check_${name}"]
>                 }
>
>         }
>
>         configfile {"/etc/software/config.xml":
>                 source =>
"puppet:///modules/software/config.xml"
>         }
>
> I can''t get that to work properly, the file is always getting
replaced.
>
>
I think it would work if you rewrote your Exec like so:

exec { "check_${name}":
  command => ''grep -q <disqualifying-pattern>
${name}''
}

The point is that for the Exec to condition application of the File, its 
application must *fail* when you want to avoid applying the File.  When an 
Exec''s ''onlyif'' or ''unless''
parameter causes the ''command'' to be skipped,
that counts as the Exec succeeding.  It should be viewed as finding the 
Exec resource already in sync with the target node, so that the command 
does not need to be run.

Nevertheless, this is all the wrong approach.  Better alternatives include

   - create per-file-of-interest custom facts to communicate the grep 
   results to Puppet, and use them in Puppet conditional statements to control 
   whether the target File resources are declared at all.
   - if really all that matters is that the file contain a particular line, 
   then manage just that line.  PuppetLabs'' add-in "stdlib"
module provides a
   File_line resource serving exactly that purpose.
   - if the objective is to install a default file, but only if the target 
   does not already exist, then set replace => false on your File resource
   - flip around the File and Exec: sync the config to a temp file, and use 
   an Exec to conditionally update the true target with the temp version.  The 
   main advantage here is that Puppet will not report failed resources in the 
   course of normal operations.

John

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Hi John,

Thanks for having a look.
I already had the idea that there should be a better way, so I included the 
use case.
I used stdlib to solve my issue, I''m still relatively new to Puppet, so
I
didn''t know about this.

Below your suggested options again with a little comment. Thanks again for 
the solution.

custom-facts: I have no idea how to do that, but will look into it once I 
come across something else that might need that.
stdlib: it took a little figuring out and reading the documentation, but 
this does exactly what I need. In this case I need 2 File_line directives 
as there are 2 line to manage, but it definitely works.
file with replace = false: as I explained in my first post, this
doesn''t
work because the file is also included in the rpm package.
flip around the file and exec: seems a bit complicated for what I''m
trying
to achieve, haven''t tried this.

Best regards,
Steven

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