Systemd::Journal

Ruby bindings for reading from the systemd journal.

• gem documentation
• libsystemd-journal documentation

Add this line to your application's Gemfile:

gem 'systemd-journal', '~> 2.0'

And then execute:

bundle install

Obviously you will need to have systemd installed on your system (specifically libsystemd or the older libsystemd-journal) in order to use the gem. Currently we support systemd 208 or higher.

require 'systemd/journal'

Print all messages as they occur:

Systemd::Journal.open do |j|
  j.seek(:tail)
  j.move_previous
  
  # watch() does not return
  j.watch do |entry|
    puts entry.message
  end
end

Filter events and iterate:

j = Systemd::Journal.new

# only display entries from SSHD with priority 6.
j.filter(priority: 6, _exe: '/usr/bin/sshd')
j.each do |entry|
  puts entry.message
end
j.close # close open files

Moving around the journal:

j = Systemd::Journal.new

j.seek(:head)   # move to the start of journal
j.move(10)      # move forward by 10 entries
c = j.cursor    # get a reference to this entry
j.move(-5)      # move back 5 entries
j.seek(c)       # move to the saved cursor
j.cursor?(c)    # verify that we're at the correct entry
j.seek(:tail)   # move past the end of the journal
j.move_previous # move to last entry in journal
j.move_next     # move forward (fails since we're at the end)

j.current_entry # get the entry we're currently positioned at

# seek the entry that occured closest to this time
j.seek(Time.parse('2013-10-31T12:00:00+04:00:00'))

Waiting for things to happen:

j = Systemd::Journal.new
j.seek(:tail)
j.move_previous
# wait up to one second for something to happen
puts 'something changed!' if j.wait(1_000_000)
  
# same as above, but can be interrupted with Control+C.
puts 'something changed!' if j.wait(1_000_000, select: true)

Accessing the catalog:

j = Systemd::Journal.new
j.move_next
j.move_next until j.current_entry.catalog?

puts j.current_entry.catalog
# or if you have a message id:
puts Systemd::Journal.catalog_for(j.current_entry.message_id)

Writing to the journal:

# write a simple message
Systemd::Journal.print(Systemd::Journal::LOG_INFO, 'Something happened')

# write custom fields
Systemd::Journal.message(
  message: 'Something bad happened',
  priority: Systemd::Journal::LOG_ERR,
  my_custom_field: 'foo was nil!'
)

See the documentation for more examples.

After calling one of the below, the Journal read pointer might not point at a valid entry:

Journal#filter
Journal#clear_filters
Journal#seek(:head)
Journal#seek(:tail)

The solution is to always call one of move, move_next, move_previous and friends before reading after issuing one of the above calls. For most functions, call move_next. For seek(:tail), call move_previous.

This is caused by a bug in libsystemd v245 (and maybe earlier) which cannot be solved in this gem, sadly. It's fixed upstream in this commit, which you can ask your distribution to backport if necessary until v246 is released.

In ArchLinux, this patch is applied in systemd-libs 245.6-2.

The most likely cause of this is a version mismatch between libsystemd on the host and in the container, where the older version does not support features used by the newer version. If you run journalctl you might see:

Journal file ... uses an unsupported feature, ignoring file.

Sadly, this error case is not exposed via the libsystemd API so this gem does not know when this happens. There are two potential workarounds:

• Ensure that the version of libsystemd are compatible across host/container
• Disable the features that are not supported by the older version to cause the newer version to emit compatible journal files.

This gem has been tested primarily on MRI and Arch Linux running systemd version 208 and up. Please let me know if you have issues with other versions or distributions.

If you run into problems or have questions, please open an Issue or Pull Request.

1. Fork it
2. Create your feature branch
3. Commit your changes
4. Push to the branch
5. Create new Pull Request, targeting the master branch.
6. Wipe hands on pants, you're done!