The IO#expect in the expect ruby module is very useful for automating interaction with interactive shell scripts. However, it has a few shortcomings:

• Its return value is not as useful as it could be
• Logging can only be achieved by setting a global variable
• Using regexps for matching becomes slow with large amounts of data
• The default expect timeout is a very large number (this doesn't actually affect the functionality negatively, it just feels awkward)

To adress this, the CExpect (Christer's Expect) gem offers a different interface:

• The exposed expect method returns or yields the match data when a match is found, which makes it possible to e.g. use capturing in a meaningful way (see Usage below)
• It also returns the return value of the block, when a block is given
• Logging can be achieved by adding observers
• There is an alternate fexpect method (the f can be thought of as "fixed" or "fast"), which is useful for capturing large outputs from shell commands
• The default timeout is nil, which will cause expect to wait indefinitely (again, this merely makes more sense to someone reading the code)

Add this line to your application's Gemfile:

gem 'cexpect'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install cexpect

require 'cexpect'

# Same semantics as PTY.spawn
rd, wr, pid = CExpect.spawn('sh')

rd.fexpect('$ ')
wr.puts('ls -1')
md = rd.expect(/ls -1\r\n(?<files>.*)\$ /m)
files = md[:files].split

# Or, equivalently
rd.expect(/ls -1\r\n(?<files>.*)\$ /m) do |md|
  files = md[:files].split
  # do something with files
end

wr.puts('cat very-large-file')
contents = rd.fexpect('$ ')

# If CExpect.spawn doesn't suit you
orig_rd, wr = IO.pipe
rd = CExpect::Reader.new(orig_rd)

wr.puts "Hello, World!\n"
greeting = rd.fexpect("\n")

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/kondensatorn/cexpect.

The gem is available as open source under the terms of the BSD 2-Clause License.