switch desktops after 5 minutes idle (xprintidle): crontab or daemon? - linux

Question

on my raspberry pi (raspbian running) I would like to have the current desktop switched to desktop n#0 after 5 minutes of idle system (no mouse or keyboard action), through wmctrl -s 0 and xprintidle for idle time checking.
Please keep in mind I'm no expert...
I tried 2 different ways, none of them working and I was wondering which one of them is the best way to do have the job done:
bash script and crontab
I wrote a simple script which checks if xprintidle is greater than a previously set $IDLE_TIME, than it switches desktops (saved in /usr/local/bin/switchDesktop0OnIdle):
#!/bin/bash
# 5 minutes in ms
IDLE_TIME=$((5*60*1000))
# Sequence to execute when timeout triggers.
trigger_cmd() {
wmctrl -s 0
}
sleep_time=$IDLE_TIME
triggered=false
while sleep $(((sleep_time+999)/1000)); do
idle=$(xprintidle)
if [ $idle -ge $IDLE_TIME ]; then
if ! $triggered; then
trigger_cmd
triggered=true
sleep_time=$IDLE_TIME
fi
else
triggered=false
# Give 100 ms buffer to avoid frantic loops shortly before triggers.
sleep_time=$((IDLE_TIME-idle+100))
fi
done
script itself works.
Then I added it to crontab (crontab -e) for have it run every 6 minutes
*/6 * * * * * sudo /usr/local/bin/switchDesktop0OnIdle
not sure sudo is necessary or not.
Anyway It doesn't work: googling around I understood that crontab runs in its own environment with its own variables. Even though I don't remember how to access this environment (oops) I do remember that I get these 2 errors running the script in it (which correctly works in "normal" shell)
could not open display (is it important ?)
bla bla -ge error, unary operator expected or similar: basically xprintidle doesn't work in this environment a gives back an empty value
What am I missing ?
infinite-while bash script running as daemon
second method I tried to set up a script with an internal infinite-while checking if xprintidle is greater then 5 minutes. In this case desktop is switched (less elegant?). Saved also in /usr/local/bin/switchDesktop0OnIdle
#!/bin/bash
triggered=false
while :
do
if [ `xprintidle` -ge 300000 ]; then
if [ triggered == false ]
wmctrl -s 0
triggered = true
fi
else
triggered = false
fi
fi
done
again the script itself works.
I tried to create a daemon in /etc/init.d/switchDesktop0OnIdle (really not an expert here, modified an existing one)
#! /bin/sh
# /etc/init.d/switchDesktop0OnIdle
### BEGIN INIT INFO
# Provides: switchDesktop0OnIdle
# Required-Start: $all
# Required-Stop: $all
# Should-Start:
# Should-Stop:
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description:
# Description:
### END INIT INFO
DAEMON=/usr/local/bin/switchDesktop0OnIdle
NAME=switchDesktop0OnIdle
test -x $DAEMON || exit 0
case "$1" in
start)
echo -n "Starting daemon: "
start-stop-daemon --start --exec $DAEMON
echo "switchDesktop0OnIdle."
;;
stop)
echo -n "Shutting down daemon:"
start-stop-daemon --stop --oknodo --retry 30 --exec $DAEMON
echo "switchDesktop0OnIdle."
;;
restart)
echo -n "Restarting daemon: "
start-stop-daemon --stop --oknodo --retry 30 --exec $DAEMON
start-stop-daemon --start --exec $DAEMON
echo "switchDesktop0OnIdle."
;;
*)
echo "Usage: $0 {start|stop|restart}"
exit 1
esac
exit 0
I set it up
sudo update-rc.d switchDesktop0OnIdle defaults
and
sudo service switchDesktop0OnIdle start
(necessary?)
...and nothing happens...
also I don't find the process with ps -ef | grep switchDesktop0OnIdle but it seems running with sudo service switchDesktop0OnIdle status
can anyone please help?
thank you
Giuseppe

Answer 1

As you suspected, the issue is that when you run your scripts from init or from cron, they are not running within the GUI environment you want them to control. In principle, a Linux system can have multiple X environments running. When you are using one, there are environment variables that direct the executables you are using to the environment you are in.
There are two parts to the solution: your scripts have to know which environment they are acting on, and they have to have authorization to interact with that environment.
You almost certainly are using a DISPLAY value of ":0", so export DISPLAY=:0 at the beginning of your script will handle the first part of the problem. (It might be ":0.0", which is effectively equivalent).
Authorization is a bit more complex. X can be set up to do authorization in different ways, but the most common is to have a file .Xauthority in your home directory which contains a token that is checked by the X server. If you install a script in your own crontab, it will run under your own user id (you probabl shouldn't use sudo), so it will read the right .Xauthority file. If you run from the root crontab, or from an init script, it will run as the root user, so it will have access to everything but will still need to know where to take the token from. I think that adding export XAUTHORITY=/home/joe/.Xauthority to the script will work. (Assuming your user id is joe.)