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Description

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

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Listen

The

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

Development Status Gem Version Code Climate Coverage Status

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.).
  • Specs suite on JRuby and Rubinius aren't reliable on Travis CI, but should work.
  • Windows and *BSD adapter aren't continuously and automatically tested.
  • OSX adapter has some performance limitations (#342).
  • FreeBSD users need patched version of rb-kqueue (as of 2020/11). See #475 for the issue, mat813/rb-kqueue#12 for the patch, and Bug 250432 in bugzilla.
  • 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: ```ruby 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 / unpause / stop

Listeners can also be easily paused/unpaused:

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.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 DEFAULTIGNOREDDIRECTORIES and DEFAULTIGNOREDEXTENSIONS 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_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

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=
. For
, 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')

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 :waitfordelay 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

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

Author

Thibaud Guillaume-Gentil (@thibaudgg)

Contributors

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

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