Comparing package versions

Package version numbers are odd things. They look like decimal numbers, but they're not: a version number is often in the form of 2.6.4, for example. If you need to compare one version number with another, you can't do a straightforward string comparison: 2.6.4 would be interpreted as greater than 2.6.12. And a numeric comparison won't work because they're not valid numbers.

Puppet's versioncmp function comes to the rescue. If you pass two things that look like version numbers, it will compare them and return a value indicating which is greater:

versioncmp( A, B )

returns:

• 0 if A and B are equal
• Greater than 1 if A is higher than B
• Less than 0 if A is less than B

How to do it…

Here's an example using the versioncmp function:

1. Modify your site.pp file as follows:

   node 'cookbook' {
     $app_version = '1.2.2'
     $min_version = '1.2.10'
           
     if versioncmp($app_version, $min_version) >= 0 {
       notify { 'Version OK': }
     } else {
       notify { 'Upgrade needed': }
     }
   }

2. Run Puppet:

   [root@cookbook ~]# puppet agent -t
   Info: Caching catalog for cookbook.example.com
   Notice: Upgrade needed
   

3. Now change the value of $app_version:

   $app_version = '1.2.14'

4. Run Puppet again:

   [root@cookbook ~]# puppet agent -t
   Info: Caching catalog for cookbook.example.com
   Notice: Version OK
   

How it works…

We've specified that the minimum acceptable version ($min_version) is 1.2.10. So, in the first example, we want to compare it with $app_version of 1.2.2. A simple alphabetic comparison of these two strings (in Ruby, for example) would give the wrong result, but versioncmp correctly determines that 1.2.2 is less than 1.2.10 and alerts us that we need to upgrade.

In the second example, $app_version is now 1.2.14, which versioncmp correctly recognizes as greater than $min_version and so we get the message Version OK.