Category: Scripts

In the previous article we discussed using grep and awk to harvest information.  The final example in that article may have left us wanting.  In this article, we’ll discuss some additional options that the cut command can give us.

As we discussed, the following command:

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

would output the name of our boot volume, assuming there were no spaces in it.  However, if our target Mac had a factory standard boot volume called “Macintosh HD”, we’d need to change our command to:

diskutil info / | grep "Volume Name:" | awk '{print $3,$4}'

If there were more than one space in our volume names, well, it all becomes a bit much to manage. Unfortunately, awk doesn’t provide a method to display word X and all following words, so we will look to another command called cut.

According to its man page, cut is designed to “cut out selected portions of each line of a file”.  Cut can work with “words”, like awk, or it can work with characters.  We’ll look at working with words first.  Unless specified otherwise, cut assumes that words are delimited (separated) by tab characters.  If the delimiter is something other than a tab, the delimiter must be defined using the “-d” option.  After defining the delimiter, we must tell cut which words we would like to output.  We do this using the -f option, followed by a number indicating the word’s position.  Unlike awk, we do not need a “$” or other character to indicate our word selection, just the number.  Also unlike awk, we can specify a range, including “X-” which tells cut to return the word at position X and everything after it, which we will do below.

diskutil info / | grep "Volume Name:" | cut -d ' ' -f 19-

The “-d” option has indicated that our delimiter is a space.  The “-f” option has asked for words 19 through the end of the line.  This may seem a bit confusing because if you look at the output of the first two commands, it would seem that we would be interested in word number 3 and onward.  It would appear that diskutil’s output contains both spaces and tabs, and a bit of trial and error helped to arrive at the number 19.  This command will return our boot volume’s name regardless of the number of spaces in the name.

The other option when working with cut is to simply count characters.  This precludes a need to define a delimiter.  By counting characters, we can find the same information with the following command:

diskutil info / | grep “Volume Name:” | cut -c 30-

This line will return character number 30 and all characters that follow it.  Like the example using the -d and -f options, this will return our boot volume’s name regardless of the number of spaces in the name.

We see that cut can provide us with some capabilities that awk doesn’t.  Hopefully this examination will help you to capture data in your own scripts.

The commands in this article have been tested on Mac OS X versions 10.5.8 and 10.6.7 (build 10J869).  Thanks go to Lisa at lisacherie.com for assistance in testing the commands used in this article.

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.