How can I adb install an apk to multiple connected devices?

Here's a functional one line command tailored from kichik's response (thanks!):

adb devices | tail -n +2 | cut -sf 1 | xargs -iX adb -s X install -r *.apk

But if you happen to be using Maven it's even simpler:

mvn android:deploy

The other answers were very useful however didn't quite do what I needed. I thought I'd post my solution (a shell script) in case it provides more clarity for other readers. It installs multiple apks and any mp4s

echo "Installatron"

for SERIAL in $(adb devices | tail -n +2 | cut -sf 1);
do 
  for APKLIST in $(ls *.apk);
  do
  echo "Installatroning $APKLIST on $SERIAL"
  adb -s $SERIAL install $APKLIST
  done

  for MP4LIST in $(ls *.mp4);
  do
  echo "Installatroning $MP4LIST to $SERIAL"
  adb -s $SERIAL push $MP4LIST sdcard/
  done
done

echo "Installatron has left the building"

Thank you for all the other answers that got me to this point.

You can use adb devices to get a list of connected devices and then run adb -s DEVICE_SERIAL_NUM install... for every device listed.

Something like (bash):

adb devices | tail -n +3 | cut -sf 1 -d " " | xargs -iX adb -s X install ...

Comments suggest this might work better for newer versions:

adb devices | tail -n +2 | cut -sf 1 | xargs -iX adb -s X install ...

For Mac OSX(not tested on Linux):

adb devices | tail -n +2 | cut -sf 1 | xargs -I {} adb -s {} install ...