Wait for a X window to appear/disappear (in a sane way)

Question

Inside a shell script, I need to wait for a window that has a string on its title to appear, do some action, and then wait for it to disappear, and do some other action.

Until yesterday, I had this simple code. The problem with it is that the disk can't be put in a power saving state while the script is left running, and it can be for many hours:

while :; do
    until wmctrl -l | grep -q "$string"; do   # until
        sleep 0.5
    done
    : do action 1

    while wmctrl -l | grep -q "$string"; do   # while
        sleep 0.5
    done
    : do action 2
done

Since I decided the mentioned code was insanely waking the disk, I went through the documentation of a few command line tools, and decided on xdotool to wait for the window to appear, and xprop to figure out when the window has vanished:

while :; do
    # we use `until' because sometimes xdotool just crashes
    until xdotool search -sync -all -onlyvisible -pid $pid -name "$string"; do
        :
    done

    # xdotool isn't trustworthy either, so check again
    wmctrl -l | grep -q "$string" ||
        continue

    : do action 1

    xprop -spy -root _NET_CLIENT_LIST_STACKING | while read line; do
        if [[ ! ${_line:-} || $_line = $line ]]; then
            _line=$line
            continue
        else
            _line=$line
            if wmctrl -l | grep -q "$string"; then
                continue
            else
                : do action 2
                break
            fi
        fi
    done
done

Now I have two new problems with the code above:

• xdotool not only crashes and gives strange results, as I have workarounded before, but it also sucks about 15% of CPU while left waiting for the window to appear. So that means I got rid of simple code that wakes the disk, to write code that is left wasting the CPU for hours, and my intention was saving power in the first place.
• xprop -spy will notify me every time I change focus (which I have workarounded through $_line) or create and destroy windows. That wakes the disk more frequently than xdotool.

I'm looking for a simple program that just waits for the window with the title $string to appear or disappear. It can be an existing command line tool, a python script, compilable C code..., but I should be able to integrate it somehow to my script (even if it just writes some information to a fifo)!

Answer 1

It might be simpler, and more reliable, to rely on your window manager or X11 to handle this by writing a "real" X11 application.

What you want from the shell is something that registers with the window manager and waits for a desired event type before returning to the shell... it's much more load-friendly if you can avoid looping inside the shell. (Your until xdotool... causes load because there's no delay (sleep) inside the loop.)

Ah... apparently xdotool had that feature added over year ago --sync. That's not available in my current Linux distro (Debian Squeeze), so I haven't tried it out.

The xdotool developer answering a similar question to yours: https://groups.google.com/d/msg/xdotool-users/7zfKTtyWm0Q/DM6TSOBUWZMJ

Answer 2

This should give you all (OK: most. What have I forgotten? Sockets?) file system activities which include writes:

strace -f command 2>&1 | 
  grep -e '^open.*O_CREAT' \
    -e ^write   \
    -e ^mkdir   \
    -e ^rmdir   \
    -e ^unlink  \
    -e ^rename  \
    -e ^chmod   \
    -e ^link    \
    -e ^symlink \
    -e ^mknod

With this information a working chroot environment can be created in tmpfs (as an action of last resort; maybe symlinks to tmpfs are enough). If the program is started in a RAM chroot then it does not have a chance to wake the disk directly. No write to its file system hierarchy is ever written to disk.