The following command works on both OS X and Linux. It creates an image from the SD card called raspi.img which you can later use to clone to another SD Card or just keep as a backup. It is exceptionally useful for backing up a Raspberry Pi.
Replace “mmcblk0” with your SD cards name. Take a look at this post if you need help finding the name.
sudo dd if=/dev/mmcblk0 of=~/raspi.img