Add version numbers with the pessimistic constraint operator to all gems in your Gemfile
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Who is this for?

Anyone who works with a Gemfile, i.e. a project that uses bundler.

What does it do?

Pessimize adds version numbers with the pessimistic constraint operator (~>, a.k.a. "spermy" operator) to all gems in your Gemfile.


You should be using ~> x.x to limit the version numbers of your gems, otherwise bundle update could potentially break your application. Read the section on "why bundle update can be dangerous" for a more detailed description, or take a look at the rubygems explanation.

But why a gem?

I.e. why not just do it by hand?

When you start building an application that uses bundler, you aren't yet sure which versions of gems will work together, so you probably won't specify the precise versions. You would normally only do this when the application and it's dependencies are in a fairly stable state. However, if you have more than a few gems, it's very tedious to collect the version numbers for each gem and add the pessimistic constraint operator to each line in the Gemfile.

Pessimize works out the version of each gem from Gemfile.lock then generates a new Gemfile with the added version constraints. But don't worry: it backs up the existing Gemfile before running, so you can get the original back if it goes wrong. (If it does go wrong, please submit an issue.)


You don't need to add it to your Gemfile - it's best kept as a system-wide gem. All you need to do is install it from the command line:

$ gem install pessimize

This installs the command line tool pessimize.


Change to a directory that contains a Gemfile and execute:

$ pessimize

This backs up the existing Gemfile and creates a new one with everything neatly organised and versioned.

And that's it!


The executable has various options, which you can read with the --help argument:

Usage: pessimize [options]

Add the pessimistic constraint operator to all gems in your Gemfile, restricting the maximum update

Run this in a directory containing a Gemfile to apply the version constraint operator to all gems, at
their current version. By default, it will restrict updates to the minor version number, but this can be
changed to patch level updates.

  --version-constraint, -c <s>:   Version constraint ('minor' or 'patch') (default: minor)
     --backup, --no-backup, -b:   Backup existing Gemfile and Gemfile.lock (default: true)
                 --version, -v:   Print version and exit
                    --help, -h:   Show this message

By default, the versions will be constrained to the minor version number (e.g. "> 3.2"). If you supply the option --version-constraint patch then it will only allow updates on the patch version number (e.g. "> 3.2.5").

Also, by default, the Gemfile and Gemfile.lock are copied as a form of backup. To skip this backup (for instance, if you're confident that your Gemfile is committed into version control) then add the --no-backup option.

Known issues

Recent issues with certain Gemfile syntax edge cases have been fixed, so there are currently no known issues. If you do find pessimize doing something strange, or not correctly picking up information in your Gemfile, it would be greatly appreciated if you submit an issue.

Why bundle update can be dangerous

If you add gems to your Gemfile without specifying a version, bundler will attempt to get the latest stable version for that gem. When you first run bundle install, bundler will get and install the latest versions, then create a Gemfile.lock which specifies the versions used.

This is fine until someone runs bundle update. In this case, bundler will try to update each gem to the maximum possible version. If no constraints have been applied, that means that major versions can potentially be incremented. Gems have interdependencies with other gems, and if those gems haven't specified the version constraints then breakages could occur.

The pessimistic constraint operator will only allow the final number of the version string to increase. You can use this to only allow patch or minor level upgrades. This means that when you run bundle update, there's a limit on how far gems will update.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request