Match line from two lines

Get Bundler version from lock file.

Note: I no longer care about this specific case as this was to overcome an issue that was doesn’t matter anymore, but the detailed approach of using grep is still a good example to learn from.

This is an usual search - we can’t just search for a version alone, as we’ll match too many. We need to find the heading one line and version on next.

  • Gemfile.lock sample 
      # ...
  # ...
  BUNDLED WITH:
    1.17.2

Shell command:

$ VSN="$(grep -A 1 "BUNDLED WITH" Gemfile.lock | tail -n 1)"

Notes:

  • If the file does not exist, grep will show a warning on stderr. If you are existing on errors you can continue.
  • If file does not contain the line, grep will exit on a success code still and the value will be an empty string.
  • Use -A 1 as an offset so we get the line after it. And then tail to keep it one line - don’t know what happens if there’s more content after it.
  • Whitespace does not matter here but in other cases could be trimmed.

Using the version after matching it.

$ BUNDLE_VSN=gem install bundler \
-v "$(grep -A 1 '"BUNDLED WITH'" Gemfile.lock | tail -n 1)"
$ gem install bundler -v "${BUNDLE_VSN:-2.1.4}"

Copied from source.