A log directory log contains some number of old log files, and the current log file current. Old log files have a file name starting with @ followed by a precise timestamp (see the daemontools' tai64n program), indicating when current was rotated and renamed to this file.

A log directory additionally contains the lock file lock, maybe state and newstate, and optionally the file config. svlogd creates necessary files if they don't exist.

If svlogd has trouble opening a log directory, it prints a warning, and ignores this log directory. If svlogd is unable to open all log directories given at the command line, it exits with an error. This can happen on start-up or after receiving a HUP signal.

svlogd appends selected log messages to the current log file. If current has size bytes or more (or there is a new-line within the last len of size bytes), or is older than a specified amount of time, current is rotated:

svlogd closes current, changes permission of current to 0755, renames current to @timestamp.s, and starts with a new empty current. If svlogd sees num or more old log files in the log directory, it removes the oldest one. Note that this doesn't decrease the number of log files if there are already more than num log files.

If svlogd is told to process recent log files, it saves current to @timestamp.u, feeds @timestamp.u through ``sh -c "processor"'' and writes the output to @timestamp.t. If the processor finishes successfully, @timestamp.u is deleted and @timestamp.t is renamed to @timestamp.s, otherwise @timestamp.t is deleted and the processor is started again. svlogd also saves any output that the processor writes to file descriptor 5, and makes that output available on file descriptor 4 when running processor on the next log file rotation.

A processor is run in the background. If svlogd sees a previously started processor still running when trying to start a new one for the same log, it blocks until the currently running processor has finished successfully. Only the HUP signal works in that situation. Note that this may block any program feeding its log data to svlogd.

On startup, and after receiving a HUP signal, svlogd checks for each log directory log if the configuration file log/config exists, and if so, reads the file line by line and adjusts configuration for log as follows:

If the line is empty, less than two characters long, or starts with a ``#'', it is ignored. A line of the form

ssize

sets the maximum file size of current when svlogd should rotate the current log file to size bytes. Default is 1000000. If size is zero, svlogd doesn't rotate log files. You should set size to at least (2 * len).

nnum

sets the number of old log files svlogd should maintain to num. If svlogd sees more that num old log files in log after log file rotation, it deletes the oldest one. Default is 10. If num is zero, svlogd doesn't remove old log files.

Nmin

sets the minimum number of old log files svlogd should maintain to min. min must be less than num. If min is set, and svlogd cannot write to current because the filesystem is full, and it sees more than min old log files, it deletes the oldest one.

ttimeout

sets the maximum age of the current log file when svlogd should rotate the current log file to timeout seconds. If current is timeout seconds old, and is not empty, svlogd forces log file rotation.

!processor

tells svlogd to feed each recent log file through processor (see above) on log file rotation. By default log files are not processed.

ua.b.c.d[:port]

tells svlogd to transmit the first len characters of selected log messages to the IP address a.b.c.d, port number port. If port isn't set, the default port for syslog is used (514). len can be set through the -l option, see below. If svlogd has trouble sending udp packets, it writes error messages to the log directory. Attention: logging through udp is unreliable, and should be used in private networks only.

Ua.b.c.d[:port]

is the same as the u line above, but the log messages are no longer written to the log directory, but transmitted through udp only. Error messages from svlogd concerning sending udp packages still go to the log directory.

If a line starts with a -, +, e, or E, svlogd matches the first len characters of each log message against pattern and acts accordingly:

-pattern

the log message is deselected.

+pattern

the log message is selected.

epattern

the log message is selected to be printed to standard error.

Epattern

the log message is deselected to be printed to standard error.

Initially each line is selected to be written to log/current. Deselected log messages are discarded from log. Initially each line is deselected to be written to standard err. Log messages selected for standard error are written to standard error.