guard/listen

9 years ago
admin
9 minutes

The Listen gem listens to file modifications and notifies you about the changes.

Known issues / Quickfixes / Workarounds

Just head over here: https://github.com/guard/listen/wiki/Quickfixes,-known-issues-and-workarounds

Tips and Techniques

Make sure you know these few basic tricks: https://github.com/guard/listen/wiki/Tips-and-Techniques

Features

  • OS-optimized adapters on MRI for Mac OS X 10.6+, Linux, *BSD and Windows, more info below.
  • Detects file modification, addition and removal.
  • You can watch multiple directories.
  • Regexp-patterns for ignoring paths for more accuracy and speed
  • Forwarding file events over TCP, more info below.
  • Increased change detection accuracy on OS X HFS and VFAT volumes.
  • Tested on MRI Ruby environments (1.9+ only) via Travis CI,

Please note that:

  • Some filesystems won’t work without polling (VM/Vagrant Shared folders, NFS, Samba, sshfs, etc.)
  • Specs suite on JRuby and Rubinius aren’t reliable on Travis CI, but should work.
  • Windows and *BSD adapter aren’t continuously and automaticaly tested.

Pending features / issues

Pull request or help is very welcome for these.

Install

The simplest way to install Listen is to use Bundler.

gem 'listen', '~> 2.7' # this prevents upgrading to 3.x

Usage

Call Listen.to with either a single directory or multiple directories, then define the “changes” callback in a block.

listener = Listen.to('dir/to/listen', 'dir/to/listen2') do |modified, added, removed|
  puts "modified absolute path: #{modified}"
  puts "added absolute path: #{added}"
  puts "removed absolute path: #{removed}"
end
listener.start # not blocking
sleep

Pause / unpause / stop

Listeners can also be easily paused/unpaused:

listener = Listen.to('dir/path/to/listen') { |modified, added, removed| # ... }

listener.start
listener.paused? # => false
listener.processing? # => true

listener.pause   # stops processing changes (but keeps on collecting them)
listener.paused? # => true
listener.processing? # => false

listener.unpause # resumes processing changes ("start" would do the same)
listener.stop    # stop both listening to changes and processing them

Note: While paused, Listen keeps on collecting changes in the background – to clear them, call “stop”

Note: You should keep track of all started listeners and stop them properly on finish.

Ignore / ignore!

Listen ignores some directories and extensions by default (See DEFAULT_IGNORED_DIRECTORIES and DEFAULT_IGNORED_EXTENSIONS in Listen::Silencer), you can add ignoring patterns with the ignore option/method or overwrite default with ignore! option/method.

listener = Listen.to('dir/path/to/listen', ignore: /\.txt/) { |modified, added, removed| # ... }
listener.start
listener.ignore! /\.pkg/ # overwrite all patterns and only ignore pkg extension.
listener.ignore /\.rb/   # ignore rb extension in addition of pkg.
sleep

Note: Ignoring regexp patterns are evaluated against relative paths.

Note: ignoring paths does not improve performance – except when Polling

Only

Listen catches all files (less the ignored once) by default, if you want to only listen to a specific type of file (ie: just rb extension) you should use the only option/method.

listener = Listen.to('dir/path/to/listen', only: /\.rb$/) { |modified, added, removed| # ... }
listener.start
listener.only /_spec\.rb$/ # overwrite all existing only patterns.
sleep

Note: ‘:only’ regexp patterns are evaluated only against relative file paths.

Changes callback

Changes to the listened-to directories gets reported back to the user in a callback. The registered callback gets invoked, when there are changes, with three parameters: modified, added and removed paths, in that particular order. Paths are always returned in their absolute form.

Example:

listener = Listen.to('path/to/app') do |modified, added, removed|
  # This block will be called when there are changes.
end
listener.start
sleep

or …

# Create a callback
callback = Proc.new do |modified, added, removed|
  # This proc will be called when there are changes.
end
listener = Listen.to('dir', &callback)
listener.start
sleep

Options

All the following options can be set through the Listen.to after the directory path(s) params.

ignore: [%r{/foo/bar}, /\.pid$/, /\.coffee$/]   # Ignore a list of paths
                                                # default: See DEFAULT_IGNORED_DIRECTORIES and DEFAULT_IGNORED_EXTENSIONS in Listen::Silencer

ignore!: %r{/foo/bar}                           # Same as ignore options, but overwrite default ignored paths.

only: %r{.rb$}                                  # Only listen to specific files
                                                # default: none

latency: 0.5                                    # Set the delay (**in seconds**) between checking for changes
                                                # default: 0.25 sec (1.0 sec for polling)

wait_for_delay: 4                               # Set the delay (**in seconds**) between calls to the callback when changes exist
                                                # default: 0.10 sec

force_polling: true                             # Force the use of the polling adapter
                                                # default: none

relative: false                                 # Whether changes should be relative to current dir or not
                                                # default: false

debug: true                                     # Enable Celluloid logger
                                                # default: false

polling_fallback_message: 'custom message'      # Set a custom polling fallback message (or disable it with false)
                                                # default: "Listen will be polling for changes. Learn more at https://github.com/guard/listen#listen-adapters."

Also, setting the environment variable LISTEN_GEM_DEBUGGING=1 does the same as debug: true above.

Listen adapters

The Listen gem has a set of adapters to notify it when there are changes.

There are 4 OS-specific adapters to support Darwin, Linux, *BSD and Windows. These adapters are fast as they use some system-calls to implement the notifying function.

There is also a polling adapter – although it’s much slower than other adapters, it works on every platform/system and scenario (including network filesystems such as VM shared folders).

The Darwin and Linux adapters are dependencies of the Listen gem so they work out of the box. For other adapters a specific gem will have to be added to your Gemfile, please read below.

The Listen gem will choose the best adapter automatically, if present. If you want to force the use of the polling adapter, use the :force_polling option while initializing the listener.

On Windows

If your are on Windows, it’s recommended to use the wdm adapter instead of polling.

Please add the following to your Gemfile:

gem 'wdm', '>= 0.1.0' if Gem.win_platform?

On *BSD

If your are on *BSD you can try to use the rb-kqueue adapter instead of polling.

Please add the following to your Gemfile:

require 'rbconfig'
if RbConfig::CONFIG['target_os'] =~ /bsd|dragonfly/i
  gem 'rb-kqueue', '>= 0.2'
end

Getting the polling fallback message?

Please visit the installation section of the Listen WIKI for more information and options for potential fixes.

Issues and troubleshooting

NOTE: without providing the output after setting the LISTEN_GEM_DEBUGGING=1 environment variable, it can be almost impossible to guess why listen is not working as expected.

See TROUBLESHOOTING

Performance

If Listen seems slow or unresponsive, make sure you’re not using the Polling adapter (you should see a warning upon startup if you are).

Also, if the directories you’re watching contain many files, make sure you’re:

  • not using Polling (ideally)
  • using :ignore and :only options to avoid tracking directories you don’t care about (important with Polling and on MacOS)
  • running Listen with the :latency and :wait_for_delay options not too small or too big (depends on needs)
  • not watching directories with log files, database files or other frequently changing files
  • not using a version of Listen prior to 2.7.7
  • not getting silent crashes within Listen (see LISTEN_GEM_DEBUGGING=2)
  • not running multiple instances of Listen in the background
  • using a file system with atime modification disabled (ideally)
  • not using a filesystem with inaccurate file modification times (ideally), e.g. HFS, VFAT
  • not buffering to a slow terminal (e.g. transparency + fancy font + slow gfx card + lots of output)
  • ideally not running a slow encryption stack, e.g. btrfs + ecryptfs

When in doubt, LISTEN_GEM_DEBUGGING=2 can help discover the actual events and time they happened.

Forwarding file events over TCP

Listen is capable of forwarding file events over the network using a messaging protocol. This can be useful for virtualized development environments when file events are unavailable, as is the case with shared folders in VMs.

Vagrant uses Listen in it’s rsync-auto mode to solve this issue.

To broadcast events over TCP programmatically, use the forward_to option with an address – just a port or a hostname/port combination:

listener = Listen.to 'path/to/app', forward_to: '10.0.0.2:4000' do |modified, added, removed|
  # After broadcasting the changes to any connected recipients,
  # this block will still be called
end
listener.start
sleep

As a convenience, the listen script is supplied which listens to a directory and forwards the events to a network address

listen -f "10.0.0.2:4000" # changes in current directory are sent as absolute paths
listen -r -f "10.0.0.2:4000" # changes in current directory are sent as relative paths
listen -v -d "/projects/my_project" -f "10.0.0.2:4000" # changes in given directory are also shown

NOTE: if you are using a gem like guard and the paths on host and guest are not exactly the same, you’ll generally want to use the -r option for relative paths

To connect to a broadcasting listener as a recipient, specify its address using Listen.on:

listener = Listen.on '10.0.0.2:4000' do |modified, added, removed|
  # This block will be called
end
listener.start
sleep

Security considerations

Since file events potentially expose sensitive information, care must be taken when specifying the broadcaster address. It is recommended to always specify a hostname and make sure it is as specific as possible to reduce any undesirable eavesdropping.

Development

  • Documentation hosted at RubyDoc.
  • Source hosted at GitHub.

Pull requests are very welcome! Please try to follow these simple rules if applicable:

  • Please create a topic branch for every separate change you make.
  • Make sure your patches are well tested. All specs must pass on Travis CI.
  • Update the Yard documentation.
  • Update the README.
  • Please do not change the version number.

For questions please join us in our Google group or on #guard (irc.freenode.net).

Acknowledgments

Author

Thibaud Guillaume-Gentil (@thibaudgg)

Contributors

https://github.com/guard/listen/graphs/contributors