The Code

Let's write the code. First, we need to name a few things. SOURCE is the volume we are backing up—in this case, root. FILEDEST is the volume where the backup will be stored. All disk images require a VOLUMENAME; otherwise, they get called untitled, which is really not a lot of use to anyone, so we set it to backup_YYYY-MM-DD and use it as the basis of the IMAGENAME:

#!/bin/sh
       
############################################
Set some variables
############################################
       
### The name of the volume we wish to backup
SOURCE='/'
       
### The volume onto which we are putting the backup
FILEDEST='/Volumes/Overflow'
       
### Creating the backup disk image file and volume name
VOLUMENAME=backup_`date +%Y-%m-%d`
IMAGENAME=$FILEDEST/$VOLUMENAME.dmg

############################################
#Create the image
############################################
       
hdiutil create -srcfolder $SOURCE -fs HFS+ \
  -format UDRW -volname $VOLUMENAME $IMAGENAME

Scary, isn't it? hdiutil creates an image file called IMAGENAME on volume FILEDEST based on SOURCE. In this case, SOURCE is my root volume, but it could be a folder such as /Users/peterhickman if I wanted to back up only my user files. It took only 15 lines, including comments. This is the power of hdiutil, and it can do a whole lot more.

By default, this script creates an uncompressed image, but by removing -format UDRW, the script creates a compressed image. Compressed sounds good, but a compressed image backup takes in excess of two hours, whereas an uncompressed image backup takes 69 minutes, based on backing up a 10 GB root volume on my Mac. Additionally, mounting the compressed image takes an age (minutes as opposed to seconds), which reduces the utility of a backup. If getting the data out of the backup is too much trouble, you will wonder why you went to all the trouble of creating one.