acpid Linux Service

acpid service:

acpid service is listening daemon. ACPId stands for Advanced Configuration and Power Interface event daemon. It listens on a file and when an event occurs, executes programs to handle the event. It also dispatch acpi events from the kernel.
It is more flexible daemon for delivering ACPI events. It shutdown the applications if the power fails.

Now lets see the manual of acpid service.
Manual acpid:
NAME
acpid - Advanced Configuration and Power Interface event daemon
SYNOPSIS
acpid [options]
DESCRIPTION
acpid is designed to notify user-space programs of ACPI events. acpid should be started during the system boot, and will run as a background process, by default. It will open an events file (/proc/acpi/event by default) and attempt to read whole lines. When a line is received (an event), acpid will examine a list of rules, and execute the rules that match the event.
Rules are defined by simple configuration files. acpid will look in a configuration directory (/etc/acpi/events by default), and parse all files that do not begin with a period ('.'). Each file must define two things: an event and an action. Any blank lines, or lines where the first character is a pound sign ('#') are ignored. Extraneous lines are flagged as warnings, but are not fatal. Each line has three tokens: the key, a literal equal sign, and the value. The key can be up to 63 characters, and is case-insensitive (but whitespace matters). The value can be up to 511 characters, and is case and whitespace sensitive.
The event value is a regular expression (see regcomp(3)), against which events are matched.
The action value is a commandline, which will be invoked via /bin/sh whenever an event matching the rule in ques- tion occurs. The commandline may include shell-special characters, and they will be preserved. The only special characters in an action value are '%' escaped. The string '%e' will be replaced by the literal text of the event for which the action was invoked. This string may contain spaces, so the commandline must take care to quote the '%e' if it wants a single token. The string '%%' will be replaced by a literal '%'. All other '%' escapes are reserved, and will cause a rule to not load.
This feature allows multiple rules to be defined for the same event (though no ordering is guaranteed), as well as one rule to be defined for multiple events. To force acpid to reload the rule configuration, send it a SIGHUP.
In addition to rule files, acpid also accepts connections on a UNIX domain socket (/var/run/acpid.socket by default). Any application may connect to this socket. Once connected, acpid will send the text of all ACPI events to the client. The client has the responsibility of filtering for messages about which it cares. acpid will not close the client socket except in the case of a SIGHUP or acpid exiting.
acpid will log all of it's activities, as well as the stdout and stderr of any actions to a log file (/var/log/acpid by default).
All the default file and directories can be changed with commandline options.
OPTIONS
-c, --confdir directory
This option changes the directory in which acpid looks for rule configuration files. Default is /etc/acpi/events.
-d, --debug This option increases the acpid debug level by one. If the debug level is non-zero, acpid will run in the foreground, and will log to stdout/stderr, rather than a log file.
-e, --eventfile filename
This option changes the event file from which acpid reads events. Default is /proc/acpi/event.
-g, --socketgroup groupname
This option changes the group ownership of the UNIX domain socket to which acpid publishes events.
-l, --logfile filename
This option changes the log file to which acpid writes. Default is /var/log/acpid.
-m, --socketmode mode
This option changes the permissions of the UNIX domain socket to which acpid publishes events. Default is 0666.
-s, --socketfile filename
This option changes the name of the UNIX domain socket which acpid opens. Default is /var/run/acpid.socket.
-S, --nosocket filename
This option tells acpid not to open a UNIX domain socket. This overrides the -s option, and negates all other socket options.
-v, --version
Print version information and exit.
-v, --version
Print version information and exit.
-h, --help Show help and exit.
EXAMPLE
This example - placed in /etc/acpi/events/power - will shut down your system if you press the power button.
event=button/power.*
action=/usr/local/sbin/power.sh '%e'
The script power.sh gets called and will see the complete event string as parameter $1.
DEPENDENCIES
Please make sure you are using the latest ACPI code possible. This is available from
http://developer.intel.com/technology/iapc/acpi/downloads.htm.
FILES
/proc/acpi/event
/etc/acpi/
/var/log/acpid
/var/run/acpid.socket
BUGS
There are no known bugs. To file bug reports, see AUTHORS below.
SEE ALSO
regcomp(3), sh(1), socket(2), connect(2)
AUTHORS
Tim Hockin <thockin@sun.com>