su
mkdir -p /data/adb/modules/mybin/system/bin
cd /data/adb/modules/mybin/system/bin

echo $0
cat > 
#!/system/bin/sh
echo Hello World

#on desktop
wget "https://busybox.net/downloads/binaries/1.21.1/busybox-armv7l" -O busybox
adb push busybox /sdcard/
adb shell

#back on device
su
cd /data/adb/modules/mybin/system/bin
mv /sdcard/busybox .
chmod +x busybox
busybox wget
busybox sh

#don't link all with busybox - system won't boot :)