Listen

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

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
• Increased change detection accuracy on OS X HFS and VFAT volumes.
• Continuous Integration: tested on selected Ruby environments via Github Workflows.

Issues / limitations

• Limited support for symlinked directories (#279):
  • Symlinks are always followed (#25).
  • Symlinked directories pointing within a watched directory are not supported (#273.
• No directory/adapter-specific configuration options.
• Support for plugins planned for future.
• TCP functionality was removed in listen 3.0.0 (#319, #218). There are plans to extract this feature to separate gems (#258), until this is finished, you can use by locking the listen gem to version '~> 2.10'.
• Some filesystems won't work without polling (VM/Vagrant Shared folders, NFS, Samba, sshfs, etc.).
• Windows and *BSD adapter aren't continuously and automatically tested.
• OSX adapter has some performance limitations (#342).
• Listeners do not notify across forked processes, if you wish for multiple processes to receive change notifications you must listen inside of each process.

Pull requests or help is very welcome for these.

Install

The simplest way to install listen is to use Bundler.

gem 'listen'

Complete Example

Here is a complete example of using the listen gem:

require 'listen'

listener = Listen.to('/srv/app') do |modified, added, removed|
  puts(modified: modified, added: added, removed: removed)
end
listener.start
sleep

Running the above in the background, you can see the callback block being called in response to each command:

$ cd /srv/app
$ touch a.txt
{:modified=>[], :added=>["/srv/app/a.txt"], :removed=>[]}

$ echo more >> a.txt
{:modified=>["/srv/app/a.txt"], :added=>[], :removed=>[]}

$ mv a.txt b.txt
{:modified=>[], :added=>["/srv/app/b.txt"], :removed=>["/srv/app/a.txt"]}

$ vi b.txt
# add a line to this new file and press ZZ to save and exit
{:modified=>["/srv/app/b.txt"], :added=>[], :removed=>[]}

$ vi c.txt
# add a line and press ZZ to save and exit
{:modified=>[], :added=>["/srv/app/c.txt"], :removed=>[]}

$ rm b.txt c.txt
{:modified=>[], :added=>[], :removed=>["/srv/app/b.txt", "/srv/app/c.txt"]}

Usage

Call Listen.to with one or more directories and the "changes" callback passed as a block.

listener = Listen.to('dir/to/listen', 'dir/to/listen2') do |modified, added, removed|
  puts "modified absolute path array: #{modified}"
  puts "added absolute path array: #{added}"
  puts "removed absolute path array: #{removed}"
end
listener.start # starts a listener thread--does not block

# do whatever you want here...just don't exit the process :)

sleep

Changes Callback

Changes to the listened-to directories are reported by the listener thread in a callback. The callback receives three array parameters: modified, added and removed, in that order. Each of these three is always an array with 0 or more entries. Each array entry is an absolute path.

Pause / start / stop

Listeners can also be easily paused and later un-paused with start:

listener = Listen.to('dir/path/to/listen') { |modified, added, removed| puts 'handle changes here...' }

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.start       # resumes processing changes
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_FILES 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: :ignore regexp patterns are evaluated against relative paths.

Note: Ignoring paths does not improve performance, except when Polling (#274).

Only

Listen watches all files (less the ignored ones) by default. If you want to only listen to a specific type of file (i.e., 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.

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_FILES 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

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."

Logging and Debugging

Listen logs its activity to Listen.logger. This is the primary method of debugging.

Custom Logger

You can call Listen.logger = to set a custom listen logger for the process. For example:

Listen.logger = Rails.logger

Default Logger

If no custom logger is set, a default listen logger which logs to to STDERR will be created and assigned to Listen.logger.

The default logger defaults to the error logging level (severity). You can override the logging level by setting the environment variable LISTEN_GEM_DEBUGGING=<level>. For <level>, all standard ::Logger levels are supported, with any mix of upper-/lower-case:

export LISTEN_GEM_DEBUGGING=debug # or 2 [deprecated]
export LISTEN_GEM_DEBUGGING=info  # or 1 or true or yes [deprecated]
export LISTEN_GEM_DEBUGGING=warn
export LISTEN_GEM_DEBUGGING=fatal
export LISTEN_GEM_DEBUGGING=error

The default of error will be used if an unsupported value is set.

Note: The alternate values 1, 2, true and yes shown above are deprecated and will be removed from listen v4.0.

Disabling Logging

If you want to disable listen logging, set

Listen.logger = ::Logger.new('/dev/null')

Adapter Warnings

If listen is having trouble with the underlying adapter, it will display warnings with Kernel#warn by default, which in turn writes to STDERR. Sometimes this is not desirable, for example in an environment where STDERR is ignored. For these reasons, the behavior can be configured using Listen.adapter_warn_behavior =:

Listen.adapter_warn_behavior = :warn   # default (true means the same)
Listen.adapter_warn_behavior = :log    # send to logger.warn
Listen.adapter_warn_behavior = :silent # suppress all adapter warnings (nil or false mean the same)

Also there are some cases where specific warnings are not helpful. For example, if you are using the polling adapter--and expect to--you can suppress the warning about it by providing a callable object like a lambda or proc that determines the behavior based on the message:

Listen.adapter_warn_behavior = ->(message) do
  case message
  when /Listen will be polling for changes/
    :silent
  when /directory is already being watched/
    :log
  else
    :warn
  end
end

In cases where the Listen gem is embedded inside another service--such as guard--the above configuration can be set in the environment variable LISTEN_GEM_ADAPTER_WARN_BEHAVIOR=warn|log|silent.

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 you 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', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

On *BSD

If you 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?

If you see:

Listen will be polling for changes.

This means the Listen gem can’t find an optimized adapter. Typically this is caused by:

• You’re on Windows and WDM gem isn’t installed.
• You’re running the app without Bundler or RubyGems.
• Using Sass which includes an ancient (the “dinosaur” type of ancient) version of the Listen gem.

Possible solutions:

1. Suppress the message by using the :force_polling option. Or, you could just ignore the message since it’s harmless.
2. Windows users: Install the WDM gem.
3. Upgrade Ruby (use RubyInstaller for Windows or RVM/rbenv for Mac) and RubyGems.
4. Run your apps using Bundler.
5. Sass users: Install the latest version of Listen and try again.

Simplified Bundler and Sass example

Create a Gemfile with these lines:

source 'https://rubygems.org'
gem 'listen'
gem 'sass'

Next, use Bundler to update gems:

$ bundle update
$ bundle exec sass --watch # ... or whatever app is using Listen.

Increasing the amount of inotify watchers

If you are running Debian, RedHat, or another similar Linux distribution, run the following in a terminal:

$ sudo sh -c "echo fs.inotify.max_user_watches=524288 >> /etc/sysctl.conf"
$ sudo sysctl -p

If you are running ArchLinux, search the /etc/sysctl.d/ directory for config files with the setting:

$ grep -H -s "fs.inotify.max_user_watches" /etc/sysctl.d/*
/etc/sysctl.d/40-max_user_watches.conf:fs.inotify.max_user_watches=100000

Then change the setting in the file you found above to a higher value (see here for why):

$ sudo sh -c "echo fs.inotify.max_user_watches=524288 > /etc/sysctl.d/40-max-user-watches.conf"
$ sudo sysctl --system

The technical details

Listen uses inotify by default on Linux to monitor directories for changes. It's not uncommon to encounter a system limit on the number of files you can monitor. For example, Ubuntu Lucid's (64bit) inotify limit is set to 8192.

You can get your current inotify file watch limit by executing:

$ cat /proc/sys/fs/inotify/max_user_watches

When this limit is not enough to monitor all files inside a directory, the limit must be increased for Listen to work properly.

You can set a new limit temporarily with:

$ sudo sysctl fs.inotify.max_user_watches=524288
$ sudo sysctl -p

If you like to make your limit permanent, use:

$ sudo sh -c "echo fs.inotify.max_user_watches=524288 >> /etc/sysctl.conf"
$ sudo sysctl -p

You may also need to pay attention to the values of max_queued_events and max_user_instances if Listen keeps on complaining.

More info

Man page for inotify(7). Blog post: limit of inotify.

Issues and Troubleshooting

If the gem doesn't work as expected, start by setting LISTEN_GEM_DEBUGGING=debug or LISTEN_GEM_DEBUGGING=info as described above in Logging and Debugging.

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

3 steps before you start diagnosing problems

These 3 steps will:

• help quickly troubleshoot obscure problems (trust me, most of them are obscure)
• help quickly identify the area of the problem (a full list is below)
• help you get familiar with listen's diagnostic mode (it really comes in handy, trust me)
• help you create relevant output before you submit an issue (so we can respond with answers instead of tons of questions)

Step 1 - The most important option in Listen For effective troubleshooting set the LISTEN_GEM_DEBUGGING=info variable before starting listen.

Step 2 - Verify polling works Polling has to work ... or something is really wrong (and we need to know that before anything else).

(see force_polling option).

After starting listen, you should see something like:

INFO -- : Record.build(): 0.06773114204406738 seconds

Step 3 - Trigger some changes directly without using editors or apps Make changes e.g. touch foo or echo "a" >> foo (for troubleshooting, avoid using an editor which could generate too many misleading events).

You should see something like:

INFO -- : listen: raw changes: [[:added, "/home/me/foo"]]
INFO -- : listen: final changes: {:modified=>[], :added=>["/home/me/foo"], :removed=>[]}

"raw changes" contains changes collected during the :wait_for_delay and :latency intervals, while "final changes" is what listen decided are relevant changes (for better editor support).

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=debug)
• 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=debug can help discover the actual events and time they happened.

Tips and Techniques

• Watch only directories you're interested in.
• Set your editor to save quickly (e.g. without backup files, without atomic-save)
• Tweak the :latency and :wait_for_delay options until you get good results (see options).
• Add :ignore rules to silence all events you don't care about (reduces a lot of noise, especially if you use it on directories)

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).

Releasing

Prerequisites

• You must have commit rights to the GitHub repository.
• You must have push rights for rubygems.org.

How to release

1. Run bundle install to make sure that you have all the gems necessary for testing and releasing.
2. Ensure all tests are passing by running bundle exec rake.
3. Determine which would be the correct next version number according to semver.
4. Update the version in ./lib/listen/version.rb.
5. Update the version in the Install section of ./README.md (gem 'listen', '~> X.Y').
6. Commit the version in a single commit, the message should be "Preparing vX.Y.Z"
7. Run bundle exec rake release:full; this will tag, push to GitHub, and publish to rubygems.org.
8. Update and publish the release notes on the GitHub releases page if necessary

Acknowledgments

• Michael Kessler (netzpirat) for having written the initial specs.
• Travis Tilley (ttilley) for this awesome work on fssm & rb-fsevent.
• Natalie Weizenbaum (nex3) for rb-inotify, a thorough inotify wrapper.
• Mathieu Arnold (mat813) for rb-kqueue, a simple kqueue wrapper.
• Maher Sallam for wdm, windows support wouldn't exist without him.
• Yehuda Katz (wycats) for vigilo, that has been a great source of inspiration.

Author

Thibaud Guillaume-Gentil (@thibaudgg)

Contributors

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