How to update the bundler version in a Gemfile.lock

1. Install the latest bundler version:

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   gem install bundler
   Fetching bundler-2.3.5.gem
   Successfully installed bundler-2.3.5
   1 gem installed
   

2. Update the bundler version in Gemfile.lock:

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   bundle update --bundler  
   

3. Confirm it worked:

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   $ tail -n2 Gemfile.lock 
   BUNDLED WITH
     2.3.5
   

Notes:

• Bundler should automatically detect the latest installed version. If it does not, you can specify your preferred version like so:

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  bundle _2.1.2_ update --bundler
  

• Older Ruby and Rails can not use the latest bundler 2 version, so you need to stay on bundler 1.