Project: Screenshot diary

So to try something new, I’ll write about a little scripting project you can try for laughs and learning. If you find this too basic, you can browse the “Advanced Linux-related things” category (and there’s an RSS feed for just those posts as well).

Now, if you know that someone is taking your picture, you try to smile and look natural (but invariably fail, with a strained smile and rigid pose as if you were caught grave robbing). The equivalent in screenshots is to either close all your apps (if you like your background image) or run a bunch of random ones (if you don’t), and then opening the program menu two or three levels. Judging from most screenshots you see, people are constantly contemplating which of their many lovely apps to run next:
Screenshot just as described Screenshot just as described Screenshot just as described

How about this for an idea: Take a new screenshot at random intervals while you actually use the desktop.

Not only do you always have a natural looking screenshot if anyone should ask, but you get basically a little timelapse of your activity. I set up such a system in 2004, and it’s more fun than it should be to flip through them all!

To do this, we’ll make a script and stick it in the crontab. Since cron can only run things at fixed intervals, we’ll use short intervals and make the script randomly choose if it should take a screenshot or not. When it does, it’ll put it with a timestamp into some directory.

Open ~/bin/takeshot. First the shebang, and in 99 out of 100 cases, we’ll just exit:

#!/bin/bash 

if (( RANDOM % 100 ))   
then
	exit 0
fi

Let’s define a good place for our screenshots:

directory=~/screenshots

Since the crontab runs independently of our X11 session, we have to specify which display to use. :0 is the first display, which on a single user box is probably the only one:

export DISPLAY=:0

While the screensavers are very nice, I don’t really want screenshots of them. Xscreensaver comes with a tool that can be used to check if the screen is currently blanked. For simplicity we use the short-hand && notation rather than a full if statement:

xscreensaver-command -cycle 2>&1 | grep -q 'cycling' && exit 0

This only works for xscreensaver, not for other screen saver packages such as xlock or the KDE screensavers. Feel free to skip.

Now let’s create the output directory if it doesn’t exist, and define the filename to use:

mkdir -p "$directory"
output="${directory}/shot$(date +%Y%m%d%H%M%S).png"

This gives us a filename with the current date and time, such as ~/screenshots/shot20090314232310.png.

Now to actually take the screenshot. There are tons of utilities for this, but the two main ones are ‘import’ from ImageMagick; and xwd (from X.org) plus NetPBM to convert it. Import is simpler to use, but I’m a fan of NetPBM for its modularity. Plus NetPBM produces png files that are half the size of ImageMagick’s. Here are both ways:

# Using ImageMagick 
import -win root "$output"
## NetPBM Alternative: 
# xwd -root | anytopnm | pnmtopng > "$output"

Now chmod +x ~/bin/takeshot and try running it a few times (you might want to temporarily delete the zeroes in “100” to speed things up). Check that the screenshots are there.

Now add it to cron. Run crontab -e and add

*/10 * * * * ~/bin/takeshot &> /dev/null

Save and exit whichever editor crontab -e invoked for you.

The script should now be taking a screenshot on average every 100*10 minutes, or 17 hours of actual use time. You can adjust either factor up and down (or make an even more clever scheme) to get more or less screenshots.

To summarise the script:

#!/bin/bash 

if (( RANDOM % 100 ))   
then
	exit 0
fi
directory=~/screenshots
export DISPLAY=:0

xscreensaver-command -cycle 2>&1 | grep -q 'cycling' && exit 0

mkdir -p "$directory"
output="${directory}/shot$(date +%Y%m%d%H%M%S).png"

# Using ImageMagick 
import -win root "$output"
## NetPBM Alternative: 
# xwd -root | anytopnm | pnmtopng > "$output"

Here are some random screenshots of mine from different years and wms:

Bunch of terminals in Fluxbox Bunch of terminals in KDE Bunch of terminals in Ion3 Bunch of terminals in Ion3, now widescreen

Multithreading for performance in shell scripts

Now that everyone and their grandmother have at least two cores, you can double the efficiency by distributing the workload. However, multithreading support in pure shell scripts is terrible, even though you often do things that can take a while, like encoding a bunch of chip tunes to ogg vorbis:

mkdir ogg
for file in *.mod
do
	xmp -d wav -o - "$file" | oggenc -q 3 -o "ogg/$file.ogg"
done

This is exactly the kind of operation that is conceptually trivial to parallelize, but not obvious to implement in a shell script. Sure, you could run them all in the background and wait for them, but that will give you a load average equal to the number of files. Not fun when there are hundreds of files.

You can run two (or however many) in the background, wait and then start two more, but that’ll give terrible performance when the jobs aren’t of roughly equal length, since at the end, the longest running job will be blocking the other eager cores.

Instead of listing ways that won’t work, I’ll get to the point: GNU (and FreeBSD) xargs has a -P for specifying the number of jobs to run in parallel!

Let’s rewrite that conversion loop to parallelize

mod2ogg() { 
	for arg; do xmp -d wav -o - "$arg" | oggenc -q 3 -o "ogg/$arg.ogg" -; done
}
export -f mod2ogg
find . -name '*.mod' -print0 | xargs -0 -n 1 -P 2 bash -c 'mod2ogg "$@"' -- 

And if we already had a mod2ogg script, similar to the function just defined, it would have been simpler:

find . -name '*.mod' -print0 | xargs -0 -n 1 -P 2 mod2ogg

Voila. Twice as fast, and you can just increase the -P with fancier hardware.

I also added -n 1 to xargs here, to ensure an even distribution of work. If the work units are so small that executing the command starts becoming a sizable portion of it, you can increase it to make xargs run mod2ogg with more files at a time (which is why it’s a loop in the example).