cron on android is awesome

Cron is a popular time based job scheduler on Unix/Linux, and is super useful as you can virtually run any command or script periodically at certain times or dates.

Even though Android is based on linux, it does not run crond (cron daemon) by default, well at least not on my Droid X.  Fortunately, you can run crond with busybox with a few hacks.  It's not the prettiest hack but it works.  Here's how you do it:

1. Make sure that your busybox supports crond.  Try typing "crond" in Terminal and see what it says.  I run busybox v1.17.1.
2. Run your favorite command line tool telnet, Terminal, adb, etc. to run the commands below.
3. Remount / and /system rw.
4. Create a /etc/passwd. (crond calls getpwnam to search user db)
   echo "root:x:0:0::/data/cron:/system/bin/bash" > /etc/passwd
5. Create a symlink for /system/bin in /bin.
   ln -s /system/bin/ /bin
6. Create your crontab file, and call it "root" (I created /data/cron/root).  Check the example section in this article to see what a crontab file should look like: http://en.wikipedia.org/wiki/Cron
7. Set your timezone.  I live in California so mine looks like this (apparently the new Olson style timezones do not work):
   TZ=PST8PDT
   export TZ
8. Run crond:
   crond -c /data/cron
9. To verify that crond is running succesfully run "pgrep crond".  If you get a number (PID) then you know it's running.

Obviously you don't want to do the above steps every time your phone boots, so you want to create an init.d script with the above commands to run at boot, or add the commands to the end of an existing init.d script.  I've added them at the end of my init.d script called /etc/init.d/99imoseyon:

# enable crond
# crond calls getpwnam (user database search)
mount -o remount,rw -t yaffs2 `grep /system /proc/mounts | cut -d' ' -f1` /system
echo "root:x:0:0::/data/cron:/system/bin/bash" > /etc/passwd
mount -o remount,ro -t yaffs2 `grep /system /proc/mounts | cut -d' ' -f1` /system
# crond has "/bin/sh" hardcoded
mount -o remount,rw rootfs /
ln -s /system/bin/ /bin
mount -o remount,ro rootfs /
# set timezone
TZ=PST8PDT
export TZ
# use /data/cron, call the crontab file "root"
crond -c /data/cron

So what can we use it for?  Lots of things.  I'm going to start using it to periodically drop page/filesystem caches, back up SMS database nightly, kill memory resident apps hourly, etc.  There are literally hundreds of use cases for android phones. Here's a sample crontab file (backups my init.d script at 8pm every night) at /data/cron/root:

0 20 * * * cp /system/etc/init.d/99imoseyon /sdcard/data/init.d

UPDATE: Added timezone step to the instructions.  Also added a command to verify that crond is running.