How to ADB "reboot download" in bash script?

Question

I would like to reboot many Android devices to download mode simultaneously.

Both of the following work from the command line:

    adb reboot
    adb reboot download

The following bash script also works:

adb devices | while read line
do
    if [ ! "$line" = "" ] && [ `echo $line | awk '{print $2}'` = "device" ]
    then
        device=`echo $line | awk '{print $1}'`
        echo "$device $@ ..."
        adb -s $device "reboot" &
    fi
done

However, when I replace 'reboot' with 'reboot download', the script only provides ADB usage instructions:

user@user /home 
$ ./rebootToDownload.bsh 
29f7654c  ...

user@user /home 
$ Android Debug Bridge version 1.0.31

 -a                            - directs adb to listen on all interfaces for a connection
 -d                            - directs command to the only connected USB device
                                 returns an error if more than one USB device is present.
 -e                            - directs command to the only running emulator.
 ...

Could someone please explain why 'ADB reboot' executes successfully, but not 'ADB reboot download'?

Is there any way around this?

Any help is appreciated.