Linux - Detecting idleness - c++

Question

I need to detect when a computer is idle for a certain time period. My definition of idleness is:
No users logged in, either by remote methods or on the local machine
X server inactivity, with no movement of mouse or key presses
TTY keyboard inactivity (hopefully)
Since the majority of distros have now moved to logind, I should be able to use its DBUS interface to find out if users are logged in, and also to monitor logins/logouts. I have used xautolock to detect X idleness before, and I could continue using that, but xscreensaver is also available. Preferably however I want to move away from any specific dependencies like the screensaver due to different desktop environments using different components.
Ideally, I would also be able to base idleness on TTY keyboard inactivity, however this isn't my biggest concern. According to this answer, I should be able to directly query the /dev/input/* interfaces, however I have no clue how to go about this.
My previous attempts at making such a monitor have used Bash, due to the ease of changing a plain text script file, howver I am happy using C++ in case more advanced methods are required to accomplish this.

Answer 1

From a purely shell standpoint (since you tagged this bash), you can get really close to what you want.
#!/bin/sh
users_are_logged_in() {
who |grep -q .
return $?
}
x_is_blanked() {
local DISPLAY=:0
if xscreensaver-command -time |grep -q 'screen blanked'; then
return 0 # we found a blanked xscreensaver: return true
fi
# no blanked xscreensaver. Look for DPMS modes
xset -q |awk '
/DPMS is Enabled/ { dpms = 1 } # DPMS is enabled
/Monitor is On$/ { monitor = 1 } # The monitor is on
END { if(dpms && !monitor) { exit 0 } else { exit 1 } }'
return $? # true when DPMS is enabled and the monitor is not on
}
nobody_here() {
! users_are_logged_in && x_is_blanked
return $?
}
if nobody_here; then
sleep 2m
if nobody_here; then
# ...
fi
fi
This assumes that a user can log in in two minutes and that otherwise, there is no TTY keyboard activity.
You should verify that the who |grep works on your system (i.e. no headers). I had originally grepped for / but then it won't work on FreeBSD. If who has headers, maybe try [ $(who |grep -c .) -gt 1 ] which will tell you that the number of lines that who outputs is more than one.
I share your worry about the screensaver part; xscreensaver likely isn't running in the login manager (any other form of X would involve a user logged in, which who would detect), e.g. GDM uses gnome-screensaver, whose syntax would be slightly different. The DPMS part may be good enough, giving a far larger buffer for graphical logins than the two minutes for console login.
Using return $? in the last line of a function is redundant. I used it to clarify that we're actually using the return value from the previous line. nobody_here short circuits, so if no users are logged in, there is no need to run the more expensive check for the status of X.
Side note: Be careful about using the term "idle" as it more typically refers to resource (hardware, that is) consumption (e.g. CPU load). See the uptime command for load averages for the most common way of determining system (resource) idleness. (This is why I named my function nobody_here instead of e.g. is_idle)