Category: Zero-Touch

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.

Here’s another script for zero-touch deployment. If you want to enable Remote Management for Apple Remote Desktop access, this script will get you there.

This script uses the kickstart command which is buried within the ARDAgent application. Entering the following command in Terminal will display the full listing of options and examples available with this tool.

/System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart help

I have most commonly enabled the Remote Management service, granted all privileges to a single local admin user used by IT staff, and restarted the agent and menu extra to allow the agent to read the new configuration. The script below will accomplish these items. Reviewing the kickstart help will give you the syntax to write your own script to accomplish different Remote Management configurations.

#!/bin/bash

##### HEADER BEGINS #####
# scr_sys_enableARDforAdminUser.bash
#
# Created 20081231 by Miles A. Leacy IV
# miles.leacy@themacadmin.com
# Modified 20090812 by Miles A. Leacy IV
# Copyright 2009 Miles A. Leacy IV
#
# This script may be copied and distributed freely as long as this header
# remains intact.
#
# This script is provided "as is". The author offers no warranty or
# guarantee of any kind.
# Use of this script is at your own risk. The author takes no responsibility
# for loss of use,
# loss of data, loss of job, loss of socks, the onset of armageddon, or any
# other negative effects.
#
# Test thoroughly in a lab environment before use on production systems.
# When you think it's ok, test again. When you're certain it's ok, test
# twice more.
#
# This script:
# - enables the Remote Management service (ARD)
# - grants access and all privileges to the user "admin"
# - restarts the ARD agent and the ARD menu extra
#
# This script must be run as root (done automatically when deployed with
# the Casper Suite).
#
# When using the script with a Casper Suite configuration for imaging,
# set it to run At Reboot.
#
##### HEADER ENDS #####




/System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -activate -configure -access -on -users admin -privs -all -restart -agent -menu