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 which represent ACPI events. If the events file does not exist, acpid will attempt to connect to the Linux kernel via the input layer and netlink. When an ACPI event is received from one of these sources, acpid will examine a list of rules, and execute the rules that match the event. acpid will ignore all incoming ACPI events if a lock file exists (/var/lock/acpid by default).
Rules are defined by simple configuration files. acpid will look in a configuration directory (/etc/acpi/events by default), and parse all regular files with names that consist entirely of upper and lower case letters, digits, underscores, and hyphens (similar to run-parts(8)). Each file must define two things: an event and an action. Any blank lines, or lines where the first character is a hash ('#') 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 question 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.
The pseudo-action <drop> causes the event to be dropped completely and no further processing undertaken; clients connecting via the UNIX domain socket (see below) will not be notified of the event. This may be useful on some machines, such as certain laptops which generate spurious battery events at frequent intervals. The name of this pseudo-action may be redefined with a commandline option.
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.
For faster startup, this socket can be passed in as stdin so that acpid need not create the socket. In addition, if a socket is passed in as stdin, acpid will not daemonize. It will be run in foreground. This behavior is provided to support systemd(1).
acpid will log all of its activities, as well as the stdout and stderr of any actions, to syslog.
All the default files and directories can be changed with commandline options.
This example will shut down your system if you press the power button.
Create a file named /etc/acpi/events/power that contains the following:
- event=button/power action=/etc/acpi/power.sh "%e"
Then create a file named /etc/acpi/power.sh that contains the following:
- /sbin/shutdown -h now "Power button pressed"
Now, when acpid is running, a press of the power button will cause the rule in /etc/acpi/events/power to trigger the script in /etc/acpi/power.sh. The script will then shut down the system.
acpid is a simple program that runs scripts in response to ACPI events from the kernel. When there's trouble, the problem is rarely with acpid itself. The following are some suggestions for finding the most common sources of ACPI-related problems.
When troubleshooting acpid, it is important to be aware that other parts of a system might be handling ACPI events. systemd(1) is capable of handling the power switch and various other events that are commonly handled by acpid. See the description of HandlePowerKey in logind.conf(5) for more. Some window managers also take over acpid's normal handling of the power button and other events.
kacpimon(8) can be used to verify that the expected ACPI events are coming in. See the man page for kacpimon(8) for the proper procedure. If the events aren't coming in, you've probably got a kernel driver issue.
If the expected events are coming in, then you'll need to check and see if your window manager is responsible for handling these events. Some are, some aren't. (E.g. in Ubuntu 14.04 (Unity/GNOME), there are settings for the laptop lid in the System Settings > Power > "When the lid is closed" fields.) If your window manager is responsible for handling the problematic event, and you've got it configured properly, then you may have a window manager issue.
Lastly, take a look in /etc/acpi/events (see above). Is there a configuration file in there for the event in question (e.g. /etc/acpi/events/lidbtn for laptop lid open/close events)? Is it properly connected to a script (e.g. /etc/acpi/lid.sh)? Is that script working? It's not unusual for an acpid script to check and see if there is a window manager running, then do nothing if there is. This means it is up to the window manager to handle this event.