Scripting: Getting Volume Details Using grep and awk

As sysadmins, we often need to write scripts that will interact with hard disks or other volumes on a client computer.  These scripts usually need some information about the volume(s) being worked with, such as a device identifer, UUID, etc..

I often see my fellow sysadmins making assumptions such as a Mac’s boot volume will be known by the device identifier “disk0s2”.  While this is often the case, it is by no means guaranteed.  For my money, often being correct isn’t acceptable, especially when always being correct can be achieved with a relatively short command. In this vein, I will outline some commands to harvest various volume information below.

Device Identifier

Some disk management commands require a device identifier.  The device identifier is in the format diskXsY.  diskX refers to a physical device.  sY refers to a volume or “slice” of diskX.

diskutil info / | grep "Device Identifier" | awk '{print $3}'

Volume UUID

The Volume UUID, or Universally Unique IDentifier, is a unique ID code generated for every volume.  The UUID is required for some disk operations.  Volume UUIDs are persistent regardless of your currently booted system.

diskutil info / | grep "Volume UUID" | awk '{print $3}'

Volume Name

Sometimes we’ll need to know the name, also referred to as the “label”, of a volume.  This is the name we see displayed in Finder.

diskutil info / | grep "Volume Name:" | awk '{print $3}'

Breaking It Down

You may have noticed a pattern in the commands above.    All of the sample commands begin with “diskutil info.”  Simply executing “diskutil info” followed by a volume, will output a list of information about that volume.  In the example commands above, we use “/”, which refers to the current boot volume.  By replacing “/” with “/Volumes/<otherVolumeName>” we can retrieve information from other volumes mounted on the Mac.  The pipe or “|” character passes the output from this command to the next one.

The next command is “grep”, followed by a quoted term.  Grep is a very powerful UNIX tool, but here, we’re using one of its most basic functions.  Grep will look within the text that it receives as input, in this case the output of “diskutil info /”, for the search term we’ve provided.  If the term is found, grep returns the entire line (s) on which our search term appears.  The grep output is then piped to the next command.

awk is another powerful tool;  books with page counts in the hundreds have been written about it.  Like grep, we are using one of awk’s more simple functions here.”awk ‘{print $X}'” takes the input it is given, the output of the grep statements in these examples, and returns the “word” at position X.  I’ve put “word” in quotes because awk doesn’t define word the same way as the English language does.  To awk, a word is a string.  Words are separated by spaces.  If we run our last example command (diskutil info / | grep “Volume Name:”) on a system booted to a volume called “BootDrive”, the output from the first two parts of the command  is ”   Volume Name:      BootDrive”.  In this case, “Volume” is word 1, “Name:” is word 2, and finally “BootDrive” is word 3.  This is why we ask awk to return word 3.

Note that if your volume name has a space in it, such as the factory default “Macintosh HD”, the command listed above would only return the first word of your volume name, for example, “Macintosh”.  To get awk to return multiple words, multiple words can be referenced inside the brackets, separated by commas.  For example “awk ‘{print $3,$4}'” would return the words at positions 3 and 4, with a space between.  We could repeat this for as many words as you need.  such as “awk ‘{print $X,$Y,$Z…..<and so on>}'”.  It is not necessary to choose consecutive words either.  “awk ‘{print $1,$5}'” would work just as well.  Referencing empty word positions will not generate any output, meaning that if we executed “diskutil info / | grep “Device / Media Name” | awk ‘{print $3,4}'” on a system with a boot volume called “BootDrive”, our output would be simply “BootDrive”.

Well, I hope some of you have found this exploration useful.  Future articles will build on what we’ve discussed here.