S6-LOG(8)

📦 version 2.15.1.0, in nixpkgs unstable

✨ NAME ✨

okay so this is s6-log — straight out of the System Manager's Manual, section 8 — and she is a reliable logging program with automated log rotation, similar to daemontools' multilog[1], but with full POSIX regular expression support 💅 like yes she does all that.

🎀 SYNOPSIS 🎀

s6-log [-d notif] [-q | -v] [-b] [-p] [-l linelimit]
       [-t lastlinetimeout] [--] logging script

💖 DESCRIPTION 💖

So first thing first: s6-log reads your logging script and compiles it into an internal form. Then it reads its standard input, line by line, and performs actions on it, following the script it's given. And bestie, it does its best to ensure there is never any log loss. fr fr. It exits cleanly when stdin closes, or when it receives a SIGTERM or a SIGHUP.

📁 logdirs 📁

A logdir (logging directory) is a place where logs are stored. s6-log can be scripted to write into one or more logdirs.

A logdir may contain the following files:

  • lock — this file is locked by s6-log at start, to make sure only one instance is running at the same time. no sharing, she's monogamous 🫶
  • current — the file where selected log lines are appended. If current has the executable-by-user flag, it means that no s6-log process is currently writing to it and the previous s6-log process managed to cleanly finalize it. If it does not, either an s6-log process is writing to it, or the previous one has been interrupted without finalizing it 👀
  • state — last processor's output, see below.
  • previous — a rotation is happening in that logdir.
  • processed, newstate — a rotation is happening in that logdir and the processor script is running.
  • timestamped files — those files are @timestamp.s or @timestamp.u, and they're old log files that have been processed and rotated (if they're ending in .s), or that were the current file when s6-log got interrupted (if they're ending in .u), in which case they have not been processed.

🔄 Rotation 🔄

In a logdir, selected lines are appended to the current file. When current becomes too big, a rotation happens ✨

The current file will be possibly processed, then it will become an archived log file named @timestamp.s, where timestamp, a TAI64N[2] timestamp, is the absolute time of the rotation. If there are too many archived log files in the logdir, the older ones are then deleted. Logging then resumes, to a brand new current file 💫

You can use this mechanism to ensure that your logs never fill up the available disk space, for instance — something that neither syslogd, nor syslog-ng, nor rsyslog offers. she ate and left no crumbs.

🧚 Processors 🧚

A processor script can be set for every logdir. When a rotation occurs, current (which has then been renamed previous) is fed to processor's stdin, and processor's stdout is saved and archived. processor can also read the state file on its fd 4; what it writes to its fd 5 will be saved as the next state file, for the next rotation. A processor script runs with the logdir as its working directory.

Processors should not background themselves: s6-log considers the processing done when its processor direct child dies. Processors should exit 0 on success and nonzero on failure; if a processor fails, s6-log will try it again after some cooldown time. 😭

Processors make s6-log Turing-complete by allowing you to use any external program to handle log files that are going to be archived. iconic behaviour honestly.

💅 Logging script syntax 💅

When starting up, s6-log reads its arguments one by one; this argument sequence, or directive sequence, forms a logging script which tells s6-log what to log, where, and how.

Every directive can be a selection directive, a control directive or an action directive. A valid logging script always contains at least one action directive; every action directive can be preceded by zero or more selection or control directives. s6-log will exit 100 if the script is invalid. If it can process the script but the last directive is not an action directive, s6-log will emit a warning. ⚠️

🦋 Selection directives 🦋

These directives tell s6-log whether to select or deselect lines it reads from stdin; actions will only happen on selected lines. By default, every line is selected.

  • + regexp — select yet-unselected lines that match regexp, which must be a POSIX Extended Regular Expression[3].
  • - regexp — deselect yet-selected lines that match regexp, which must be a POSIX Extended Regular Expression[3].
  • f — select exactly lines that have not yet been acted upon (i.e. that were always deselected when the script encountered an action directive). the forgotten ones 🥺

🌸 Control directives 🌸

These directives tune s6-log's behaviour for the next actions. slay, customization.

  • n number — next logdirs will contain up to number archived log files. If there are more, the oldest archived log files will be deleted, only the latest number will be kept. By default, number is 10.
  • s filesize — next rotations will occur when current log files approach filesize bytes. By default, filesize is 99999; it cannot be set lower than 4096 or higher than 268435455.
  • S totalsize — next logdirs will contain up to totalsize bytes of archived (and maybe processed) log files. If archived log files take more space than that, the older ones are deleted until the total size fits. A totalsize of zero means no such limit; use n0 instead if you don't want any archived log files. By default, totalsize is 0 (unlimited).
  • l tolerance — next rotations will be triggered when the size of current goes higher than filesize minus tolerance. tolerance cannot be more than half of filesize. By default, tolerance is 2000.
  • r cooldown — if an error occurs during operations on the next logdirs, retry every cooldown milliseconds. By default, cooldown is 2000; it's strongly discouraged to set it to a value under 50. seriously. don't. 💀
  • R period — if period is not zero, the next logdirs will be automatically rotated every period seconds, even if the current file has not reached the size limit. If period is zero (the default), the next logdirs will not be periodically rotated.
  • E alertsize — only the first alertsize bytes of the selected lines will be used in the next alerts. An alertsize of 0 means no limit. By default, alertsize is 200.
  • ^ statussize — only the first statussize bytes of the selected lines will be used in the next status file updates. If a line is shorter than statussize bytes, the status file will be padded with newlines so it is always statussize bytes long. 0 means an unpadded, unlimited status file. By default, statussize is 1001.
  • p prefix — set prefix as a prefix to be printed on every output line. For instance, a pfoobar: directive means that the next action directives should prepend every line with foobar: (plus a space) before outputting it. Note that a prefix is always printed after the timestamps, if any. To remove a prefix for the next action directives, use a standalone p.
  • ! processor — registers execlineb -Pc processor as a processor for the next logdirs; execlineb must be found in s6-log's PATH. This directive is only supported if s6 has been built with execline support; otherwise, it yields a syntax error at starting time. If processor is empty, no processor will be set for the next logdirs. By default, no processor is set.
  • ? processor — registers /bin/sh -c processor as a processor for the next logdirs. It is just like the ! directive, except that the processor string is interpreted by /bin/sh, not execlineb. It is useful for people who want to build s6 without execline support. 🫶
  • t — the logged line will be prepended with a TAI64N[2] timestamp (and a space) before being processed by the next action directive. Giving the t directive several times before an action directive has no effect.
  • T — the selected line will be prepended with a ISO 8601[4] timestamp for combined date and time representing local time according to the system's timezone, with a space (not a T) between the date and the time and two spaces after the time, before being processed by the next action directive. Giving the T directive several times before an action directive has no effect.
Note that unlike the other control directives, the t and T directives are not sticky: their effect will disappear after the next action directive, and they need to be reapplied if necessary. If both a t and a T directive are given before an action directive, the TAI64N[2] timestamp will always appear before the ISO 8601 timestamp. 😌 order is order.

👑 Action directives 👑

These directives determine what s6-log actually does with the selected lines. the main character moment.

  • 2 — Alert. s6-log will print s6-log: alert:, possibly prepended with a timestamp, followed by the first alertsize bytes of the line, to its standard error.
  • 1 — Forward to stdout. s6-log will print the selected line to its stdout. If any error occurs, e.g. if stdout was a pipe and the reading end closed, this directive will be ignored for the rest of s6-log's lifetime.
  • = statusfile — Status. s6-log will atomically update the statusfile file with the first statussize bytes of the line, and pad it with newlines. s6-log must have the right to write to statusfile and to statusfile's directory.
  • dir — logdir. Must start with / or .. s6-log will log the line into the logdir dir. s6-log must have the right to write to dir.

⭐ Signals ⭐

  • SIGTERM — if s6-log has been run with the -p option, does nothing. Without this option, SIGTERM instructs s6-log to stop reading stdin after the next newline and exit after logging the last line. polite and thorough 💅
  • SIGALRM — triggers a rotation on every logdir s6-log is monitoring, as if the current file in those logdirs had reached the size limit. surprise rotation, no cap.

🌈 Why use execlineb to interpret the processor string? 🌈

Because it is exactly what execlineb(1) is for. like, that's the whole job description.

  • Directly executing processor is not flexible enough. We want to be able to run a complete command line, with an executable name and its arguments.
  • We could interpret the processor string via /bin/sh. This is what multilog[1] does. However, /bin/sh, despite being the traditional Unix interpreter, is overpowered for this. We don't need a complete shell script interpreter: most processor commands will be very simple, with only two or three words, and we only need a way to turn a string into an argv, i.e. a command line. 💜
  • execlineb(1) — this one was literally designed for exactly this job 🌟 it takes plain strings and turns them into command lines. It's a super fast, super lightweight script launcher that does zero heavy startup initialization the way /bin/sh does. It just so happens to be the perfect tool for the job, no cap.
  • Okay, to be perfectly honest: I also did this on purpose so people have a reason to use the execline[5] language 🙈 but seriously, it really is the perfect tool for the job.

💅 Why have another logging mechanism?

Because the syslog mechanism and all of its implementations (save one) suck. I'm not being judgmental, bestie, I'm just stating the obvious 💀

🚩 The syslog design is flawed from the start

When someone asked Rainer Gerhards why he started rsyslog[6], he came up with a whole lot of hand-waving and not a single word about technical points. There's a reason for that: rsyslog is forked from sysklogd! So no matter how many bells and whistles get bolted onto it, it still suffers from the exact same basic flaws 😭

The problem with syslogd does not come from such or such implementation. The problem comes from syslog's design in the first place.

  • syslog makes you send all your logs to the same place. Logs from a zillion processes get read by a single syslogd server. The server checks log lines against system-wide regular expressions to decide where to write them. And this raises the following issues, ngl:
    • Unless the client explicitly mentions its name in every log line, there is no way for log readers to know what process generated a given line 🙈 Some syslogd implementations can log the pid of the client — big deal.
    • Log lines from every client have to run through the same regular expression matching. That requires huge regular expression sets, and an obvious performance impact, just to do anything meaningful. And as a matter of fact, standard syslogd configurations don't do anything meaningful at all: they separate the logs into a few streams such as /var/log/messages, /var/log/auth.log, /var/log/daemon.log or /var/log/syslog with very vague semantics 🫠 All of syslogd's line processing power just sits there unused, because making real use of it would be too complex.
  • syslogd logs to files. This is wrong, because files grow and disks fill up. Sure, there are utilities such as logrotate to perform cleaning up, but as long as logging and log rotation are kept separate, there is a race condition: a log file can grow and fill up a disk before a rotation occurs 🚨 I am all for separating tasks that can be separated, but there is no choice here: logging and log rotation management must be done by the same tool. Only a few non-mainstream implementations of syslogd do this, including the Busybox[7] one — and that is a feature added by the Busybox developers, who are aware of the problem but want to maintain compatibility with the historical syslogd. Neither syslogd (-ng or not) nor rsyslogd manages its log files: that's a flaw that no amount of external tools is going to fix 💀
  • syslogd is a complex process that runs as root. We all know what complex processes running as root mean: bugs turning into security holes 😬
  • syslog requires a syslogd service, and fails otherwise. A syslogd service may not be present, it may fail… or it may want to log stuff. Who's going to take care of syslogd's error messages? 👀 like, hello??

syslog is slow, it's unsafe, and it's incomplete. The only reason people use it is because it's historical, it exists, and there hasn't been any serious alternative yet — except maybe multilog[1], which s6-log improves upon ✨

🌈 A not-so-modest proposal: the logging chain

Unix distributions already do this to some extent, but it's at best unclear where the logs go for any given program.

  • Every program, without exception, should send its logs (be it error messages, warning messages, or anything) to its standard error descriptor, i.e. fd 2. This is why it's open for 💖
  • When process 1 starts, the logging chain is rooted to the machine console: anything process 1 sends to its stderr appears, without modification, on the machine console, which should at any time remain the last resort place where logs are sent.
  • Process 1 should spawn and supervise a catch-all logging mechanism that handles logs from every service that does not take care of its own logging. Error messages from this logging mechanism naturally go to the machine console.
  • Process 1's own error messages can go to the machine console, or dirty tricks can be used[8] so they go to the catch-all logging mechanism 👀
  • Services that are spawned by process 1 should come with their own logger service; the supervision mechanism offered by s6-svscan(8) makes it easy. Error messages from the loggers themselves naturally go to the catch-all mechanism.
  • User login mechanisms such as getty, xdm or sshd are services: they should be started with their own loggers. Of course, when a user gets a terminal and a shell, the shell's stderr should be redirected to the terminal: interactive programs break the automatic logging chain and delegate responsibility to the user 🫶
  • A syslogd service may exist, to catch logs sent via syslog() by legacy programs. But it is a normal service, and logs caught by this syslogd service are not part of the logging chain. It is probably overkill to provide the syslogd service with its own logger; error messages from syslogd can default to the catch-all logger. The s6 package, including the ucspilogd(8) program, provides enough tools to easily implement a complete syslogd system, for a small fraction of the resource needs and the complexity of native syslogd implementations 🌸

So, given a program, where are its logs sent? 👑

  • Logs sent via syslog() will be handled by the syslogd service as usual. Smart administrators will make sure that those ones are as few as possible. The rest of this analysis is about logs sent to stderr.
  • If the program is descended from a user's interactive program, its logs are sent to the user's terminal or the user's choice redirection target.
  • If the program is descended from a logged service, its logs are naturally sent to the service's logger.
  • Else the logs are sent to the catch-all logger.
  • Only the catch-all logger's error messages, the kernel's fatal error messages, and maybe process 1's error messages, are sent to the system console.

🎀 What does s6-log have to do with all this?

In a logging chain situation, every service must have its own logger. To avoid syslogd's design mistakes, one logger process per service must be run. s6-log fits that role perfectly 💅 Using s6-log as your one-stop logger offers the following benefits:

  • Every instance of s6-log can run as a different user, so it's easy to give different access rights to different logs. It is also more secure not to have any logger process running as root 🔒
  • s6-log consumes very little memory per instance (unless it accumulates unflushed log lines, which you can avoid with the -b option). So launching a lot of s6-log processes does not waste resources 🍓
  • s6-log is vastly configurable via logging scripts; every instance is as powerful as a traditional syslogd ⭐
  • s6-log can log to a RAM filesystem and thus is suitable as a catch-all logger. Clever tricks like Upstart's logd or daemontools' readproctitle[9] are just that: tricks. s6-log gives a unified interface to all of your system's loggers 🌈
"You're wrong about being as powerful as syslogd: s6-log does not do remote logging."

Okay so — you mean you want to send, live, every log line over the network via UDP? You can't be serious 😭💀

Do yourself a favor and use s6-log to write log lines to a logdir, with a processor script that sends files-being-archived to the network, possibly after compressing them. More reliability, less log lines lost, less network traffic, better engineering ✨ If you have no disk to even write the current files to, write to a small RAM filesystem.

If you have to log stuff live via the network, you do not need any local logging software. You don't even need syslogd. Just filter your stderr via some grep that selects lines for you, then sends them to a network socket. A trivial shell script, or execline script, can do that for you 🫶

Do not insist on using syslogd. It does nothing magical, and nothing that can't be done in a simpler way using simpler tools. Periodt 👑

✨ OPTIONS ✨

okay bestie, this is the part where you pick your vibe check 💅 every single one of these is a real flag, no cap:

  • -d notif — readiness notification, aka "hey girl, i'm ready" 💖 with this option, s6-log writes a newline character to file descriptor notif when it's ready — like, actually ready: it's successfully parsed its logging script, initialized all its necessary resources (the logdirs defined in the script, etc.), and is now listening on stdin to process log lines. that's the moment it taps you on the shoulder ✨
  • -b — blocking mode 🫶 with this one, s6-log stops reading its standard input while it has unflushed buffers. this ensures every log line has been fully processed before it reads the next one — which is also multilog[1]'s behaviour. by default, s6-log keeps reading from stdin even if its buffers still contain data. so: -b is safer, but it may slow down your service 😭 the default is faster, but may lead to unbound memory use if you have a lot of output going to a slow file system 💀
  • -p — protect against SIGTERM 🛡️ do not exit on receipt of a SIGTERM; only exit on a SIGHUP or when reading EOF on stdin. this is useful for a logger that you really do not want to lose, even if automated administration (e.g. the downing of a supervision tree) would kill it. main character logger energy, honestly 👑
  • -q | -v — quiet | verbose 🎀 decreases | increases s6-log's verbosity, i.e. which messages get sent to stderr. the default verbosity is 1. currently supported verbosity levels:
    • 0 — only write alerts and fatal errors 🙈
    • 1 — write alerts, warnings and fatal errors 🌸
  • -l linelimit — if a log line is longer than linelimit bytes, split it by inserting a newline after the linelimitth byte ✂️ after the split, the remainder of the line will also be processed, so it'll go through the selection process, timestamping, etc. linelimit cannot be less than 48, unless it is 0 (which means infinite). the default is 8192 bytes. setting linelimit to 0 ensures that lines will never be split — but heads up, this may cause important memory consumption by s6-log if it's fed extremely long lines, so use with caution ⚠️
  • -t lastlinetimeout — if s6-log receives a termination signal but has read a partial line in its buffer, it will wait for at most lastlinetimeout milliseconds for its service to send it the remainder of the line 🕐 if it still hasn't read a newline character by then, it will add a newline character itself, process the line, and then exit. by default, lastlinetimeout is 2000, which means s6-log will wait for at most 2 seconds for completion of its last partial line. if lastlinetimeout is given as ‘0’, then s6-log will wait forever — it won't exit until it actually reads a newline or EOF 😳

🌈 EXAMPLES 🌈

first one, a classic, she ate:

s6-log -b n20 s1000000 t /var/log/services/stuff

so what's happening here 👀 it logs all of stdin, prepending every line with a TAI64N[2] timestamp, into the /var/log/services/stuff logdir, with a maximum archive of 20 log files of 1 MB each; and it makes sure every line has been written before reading the next one 🎀

second one, and ngl this is the iconique one:

s6-log n30 E500 - +fatal: 2 - +^STAT =/var/log/foobard/status f
s10000000 S15000000 T !"gzip -nq9" /var/log/foobard

let's break it down bestie:

  • sends alerts to stderr with the 500 first bytes of lines containing “fatal:” 🔥
  • maintains the /var/log/foobard/status file at 1001 bytes, updating it when it finds a log line starting with “STAT” 📝
  • logs all other lines to logdir /var/log/foobard, prepending them with an ISO 8601 timestamp ⏰ when current reaches at least 9998 kB (i.e. 10 MB filesize minus 2kB tolerance), pipe it through ‘gzip -nq9’ and save the result into a timestamped archive file, with a maximum of 30 such files or a total of 15 MB of compressed archive files 🗜️💖

🦋 SEE ALSO 🦋

and the sources, in order, because citations are slay:

this man page is ported from the authoritative documentation at: https://skarnet.org/software/s6/s6-log.html 💜

👑 AUTHORS 👑

  • Laurent Bercot
  • Alexis <flexibeast@gmail.com> (man page port)
and that's the whole page, bestie — go forth and log responsibly 🫶✨

Debian put this one out on July 26, 2025, and it lives at S6-LOG(8) in the manual, iykyk 💅🎀

📦 where this page comes from