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Logging

By default, every service 66 starts gets its own logger — a dedicated 66-log process that captures what the service writes and stores it, rotated and optionally timestamped, in a human-readable file. This guide explains how that wiring works and how to tune or disable it.

The pieces live in the frontend file: the Options key in [Main], the StdOut/StdErr keys in [Execute], and the whole [Logger] section.

How a service's log is wired

   service stdout ──▶ 66-log ──▶ /var/log/66/<service>/current
                       (logger)

Three conditions must all hold for the automatic logger to exist (they do by default):

  1. The log option is not disabled. A logger is created unless you write Options = ( !log ) in [Main].
  2. StdOut (and/or StdIn) is left at its default, or set explicitly to 66log. Routing stdout elsewhere (a file, the console…) bypasses the logger.
  3. The [Logger] section, if present, tunes it — it never creates it.

For a root service the log lands in /var/log/66/<service>/, for a regular user in .66/log/<service>/. The live file is current; rotated files sit beside it. File descriptors for the log pipes are kept alive by a small fd-holder daemon, so logs survive a service restart without losing their pipe.

Reading logs

66 status foo

The status of a service ends with its most recent log lines. To read the whole log, use 66 log — this is the native equivalent of journalctl:

66 log foo

It reads one service, the system catch-all (66 log system), or every source interleaved in time order (66 log with no operand). It also follows a log as it is written, and narrows by time or by pattern:

66 log --follow foo
66 log --since 2026-08-01 --grep 'fail' foo

Or follow the underlying file directly:

tail -F /var/log/66/foo/current

Tuning: the [Logger] section

Optional. It controls rotation and timestamping. It also accepts RunAs, Execute and a single Timeout (the logger's start timeout — it has no stop transition), behaving as in [Start]. Its way down waits for the general stop delay chosen at the build of 66, then SIGKILL.

[Logger]
Backup = 10
MaxSize = 1000000
Timestamp = iso
RunAs = user
Key Effect Default
Backup number of rotated files kept 3
MaxSize bytes before a file rotates (min 4096) 1000000
Timestamp tai, iso or none iso
RunAs user the logger runs as 66log for a system service, chosen at the build (-D 66-log-user=); the owner for a user service
  • tai prefixes each line with a TAI64N stamp (decode with a TAI64N timestamp converter for human-readable time).
  • iso prefixes a local ISO-8601 date and time.
  • none writes the line as-is.

Turning the logger off

Set the !log option in [Main]:

[Main]
Type = classic
Options = ( !log )

The service then inherits its parent's standard output, unless you redirect it yourself with StdOut. See standard I/O redirection for file:, console, syslog, null and the other targets.

Dropping the logger from the frontend of a service already parsed takes effect when the service is parsed again, with 66 reconfigure or 66 parse -f: the logger is stopped, unsupervised and removed.

The logger and the commands

The logger of foo is named foo-log. It is a hard dependency of its service: foo depends on foo-log.

  • It follows its service on start, free, remove and disable. 66 stop of the service leaves it running.
  • Under -P, a command acts on the named services only: their logger does not follow them.
  • Named alone, it is refused by free, reconfigure and remove, which handle it with its service. A command that brings down, signals, reloads or restarts acts on its service as well, except under -P: 66 signal -a -P foo-log rotates the log file of foo and leaves foo untouched. 66 start foo-log starts the logger only.
  • The suffix -log is reserved to the loggers: a frontend whose name ends with it is refused by the parse.

Custom loggers

With a custom (shebang) Execute in [Logger], the Backup, MaxSize and Timestamp keys have no effect: you write the logging command yourself in the Execute field. Use this only when you need a logging pipeline 66-log cannot express.

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