TweetFollow Us on Twitter

Back to bash Basics: Part 2 Time to Advance Ourselves

Volume Number: 21 (2005)
Issue Number: 11
Column Tag: Programming

Mac In The Shell

Back to bash Basics: Part 2 Time to Advance Ourselves

by Edward Marczak

With any exercise, you need to continually push yourself. Without that extra effort, what once was a challenge becomes easy, something to drift through. At the same time, you may be missing advanced techniques that make other areas easier or more efficient. Similarly, shell scripting can go many layers deep, and you can exercise your knowledge in many ways. Last month in, "Back to bash Basics Part 1," we focused on flow control. You may have noticed some of the things I didn't cover explicitly. There's always more to learn! Let's tie up those loose ends.

More Looping

Since we discussed looping constructs so much last month, that's where we'll pick up. In the select example, you'll see a break statement - that could use some explanation. break simply terminates the current loop. If it were removed from the select example, you would be asked repeatedly which file you want to inspect. Let's see what that would look like:

#!/bin/bash

select theItem; do
        if [ $theItem ]; then
                file $theItem
        fi
done

When this is run, the output looks like this:

Jack-Kerouak:~/bin marczak$ ./st.sh *
1) BidToJob.dmg 
2) bomcheck.sh
3) cl.txt
4) createdmg
5) diskrep.sh
6) exscript
#? 3
cl.txt: ASCII text
#? 4
createdmg: ASCII text
#? 5
diskrep.sh: Bourne-Again shell script text executable
#? ^C

Notice that this time, we need to press ctrl-c to stop the program. break applies to any loop:

#!/bin/bash
for i in $*;
        do
        if [ ! -O $i ]; then
                echo You do not own $i!  I am outta here!
                break
        fi
        echo $i is your file!
done

Running as me, I'm shown:

$ ./bt.sh *
BidToJob.dmg is your file!
bomcheck.sh is your file!
bt.sh is your file!
cl.txt is your file!
[...clipped for brevity]

Running in that same directory as root gives us:

# ./bt.sh *
You do not own BidToJob.dmg! I am outta here!

AOT (or, how the shell will separate files)

Have you ever crossed your AOT (Acronym Overload Threshold)? "There's a problem with the RIP!" Raster Image Processor, or Routing Information Protocol? While there's only so many TLAs (Three Letter Acronyms) that you can deal with, I need you add one more: IFS (no, not Iterative Fractal Systems!). The shell uses the Internal Field Separator to determine how to break apart tokens, and how to separate incoming parameters. By default, IFS is equal to space, tab and newline.

When we discussed the for loop last month, several things were quickly touched upon that can be expanded. In addition to the $@ variable, which expands to individual double-quoted strings, there is the $* variable, which is a single string containing each positional parameter. How do you know where each parameter breaks? $* separates each parameter by using the first character of your IFS variable. We'll get back to how this can be very useful.

Also, last month showed an example that looked something like this:

FILES=`ls *.sh`

for i in $FILES
do
        ...
done

This example 'just works' because ls *.sh will separate its output with linefeeds. Hey, that's one of the characters in IFS by default! What good fortune! This same example will fall apart if you reassign the IFS variable prior to the loop:

IFS="-"
FILES=`ls *.sh`
for i in $FILES; do
...
done

$i will still hold $FILES, but it won't be tokenized - not the way you'd expect (the line feeds will still be in there, but $i won't break on them).

So, then, why would we ever touch IFS? Well, what if you wanted to search through something that is not broken up by a space, newline or tab? Like $PATH, for instance:

#!/bin/bash

IFS=:

for theDir in $PATH
do
        theLatest=`ls -lotr $theDir | tail -1`
        echo Newest file in $theDir:
        echo $theLatest
        echo
done

This simply goes through our $PATH and tells us the newest file in each directory. Could be useful.

Oh, and another thing

Like Apple, shell scripting always seems to have "one more thing." For this month, this thing comes in the form of being able to effectively handle parameters. Time to introduce shift and getopts.

When writing a script, parameters can be accessed a few ways. If you always rely on direct access ($1, $2...etc), you run into some limits. One way to simply loop through all parameters is to use shift. shift makes $1 = $2, $2 = $3...etc. You lose the first value that was assigned to $1. To look for a few specific parameters, you can loop through the values:

#!/bin/bash

while [ `echo $1 | grep "-"` ]; do
        case $1 in
                -a ) echo "You supplied the -a flag";;
                -b ) echo "You supplied the -b flag";;
                -c ) echo "You supplied the -c flag";;
                * ) echo "Usage: $0 -a -b -c";
                exit 1;;
        esac    
        shift
done

Run this code and you'll see:

$ ./shifttest.sh -b -a
You supplied the -b flag
You supplied the -a flag

Now, shift is cool, and still comes in handy, but to truly handle command-line options smoothly, we have to employ getopts. Sure, you can roll your own each time, however, people have come to expect their options to behave in certain ways. One should be able to combine options, as with tar, for example: tar -xzvf blah.tar.gz. Basically, you don't need to roll your own because getopts exists.

getopts allows you to handle options in a standard way. Seeing it in action is the quickest way to get up to speed:

#!/bin/bash

while getopts ":xyz:t" theOption; do
        case $theOption in
                x ) echo "Option x chosen";;
                y ) echo "Option $theOption chosen";;
                z ) echo "Option z chosen with argument: $OPTARG";;
                t ) echo "Option t chosen";;
                \? ) echo "Unknown option chosen"
                        exit 1;;
                * ) echo "You need to supply an option!"
                        exit 2;;
        esac
done

getopts is designed to be dumped in a loop that will feed it arguments passed into the script. It accepts a string that defines the allowed options, followed by a variable that will hold the current option, sans the "-" or "+" (nicely, either are allowed). Using getopts will define two variables: $OPTIND, the current index and $OPTARG, the current argument passed with an option. Running this produces this output:

$ ./gotest.sh -yxz test
Option y chosen
Option x chosen
Option z chosen with argument: test

The string that getopts accepts can only contain letters and the colon character. Each letter is an option you wish to support. If a letter is followed by a colon, that tells getopts that an argument is required. By having a lead colon character in the parameter list string, you suppress the error message that getopts will print if an option is not recognized. In either case, an unrecognized option will set the variable to "?", so you can deal with it.

Put it all Together

I've gotten a request or two asking how to deal with math in the shell. While there are specialized CLI apps that will deal with arithmetic, the shell can do some basic functions, and sometimes, that's all you need. The trick is the underused declare statement. declare tells the shell how you want to treat variables, which are strings by default. So, this doesn't do what one would expect:

$ number1=7
$ number2=8
$ total=number1*number2

When you echo $total, you get "number1*number2": strings. We need to tell the shell that $total should be treated as an integer:

$ declare -i total
$ total=number1*number2
$ echo $total
56

Much better! declare can define several different types of variables:

-a   variable is an array
-i   treat as integer
-r   makes variable read-only
-x   automatic export (like the 'export' built-in)

There are some others, but this is all we need concentrate on for now. All of the usual suspects are available as mathematic operators:

+   Addition
-   Subtraction
*   Multiply
/   Divide
%   Remainder
<<  Bit-shift left
>>  Bit-shift right
&   Bitwise and
|   Bitwise or
~   Bitwise not
!   Bitwise not
^   Xor

In addition to declare-ing a variable to be an integer, you can use let to make the assignment:

let theTotal='5 * 7'

Ah, let....brings me back to my C64 BASIC days...

Now, you should be able to write fairly sophisticated shell script that includes slick input processing, good error handling and even some basic computations!

Make Yourself Useful...

...to everyone. Just remember that bash scripting will help you not only with OS X, but with Linux, IRIX, FreeBSD, and even Windows - if you install a Unix shell there (which can be had for free from Cygwin or Microsoft).

This month highlights the fact that shell scripting is relatively easy, can be fun and powerful. Even better, you'll find bash built-in to every OS X machine you touch! Let this all sink in: while I'll get back to bash scripting in future columns, more Unix detours next month!


Ed Marczak keeps it simple. Tech simplicity at http://www.radiotope.com

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

BusyContacts 1.0.2 - Fast, efficient con...
BusyContacts is a contact manager for OS X that makes creating, finding, and managing contacts faster and more efficient. It brings to contact management the same power, flexibility, and sharing... Read more
Capture One Pro 8.2.0.82 - RAW workflow...
Capture One Pro 8 is a professional RAW converter offering you ultimate image quality with accurate colors and incredible detail from more than 300 high-end cameras -- straight out of the box. It... Read more
Backblaze 4.0.0.872 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac.With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Little Snitch 3.5.2 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
Monolingual 1.6.4 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. If you use your computer in only one (human) language, you... Read more
CleanApp 5.0 - Application deinstaller a...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Fantastical 2.0 - Create calendar events...
Fantastical is the Mac calendar you'll actually enjoy using. Creating an event with Fantastical is quick, easy, and fun: Open Fantastical with a single click or keystroke Type in your event details... Read more
Cocktail 8.2 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Direct Mail 4.0.4 - Create and send grea...
Direct Mail is an easy-to-use, fully-featured email marketing app purpose-built for OS X. It lets you create and send great looking email campaigns. Start your newsletter by selecting from a gallery... Read more
jAlbum Pro 12.6 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more

Appy to Have Known You - Lee Hamlet Look...
Being at 148Apps these past 2 years has been an awesome experience that has taught me a great deal, and working with such a great team has been a privilege. Thank you to Rob Rich, and to both Rob LeFebvre and Jeff Scott before him, for helping me... | Read more »
MLB Manager 2015 (Games)
MLB Manager 2015 5.0.14 Device: iOS Universal Category: Games Price: $4.99, Version: 5.0.14 (iTunes) Description: Guide your favorite MLB franchise to glory! MLB Manager 2015, officially licensed by MLB.com and based on the award-... | Read more »
Breath of Light (Games)
Breath of Light 1.0.1421 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1421 (iTunes) Description: Hold a quiet moment. Breath of Light is a meditative and beautiful puzzle game with a hypnotic soundtrack by... | Read more »
WWE WrestleMania Tags into the App Store
Are You ready to rumble? The official WWE WrestleMania app, by World Wrestling Entertainment, is now available. Now you can get all your WrestleMania info in one place before anyone else. The app offers details on superstar signings, interactive... | Read more »
Bio Inc's New Expansion is Infectin...
Bio Inc., by DryGin Studios, is the real time strategy game where you infect a human body with the worst virus your evil brain can design. Recently, the game was updated to add a whole lot of new features. Now you can play the new “Lethal”... | Read more »
The Monocular Minion is Here! Despicable...
Despicable Me: Minion Rush, by Gameloft, is introducing a new runner to the mix in their latest update. Now you can play as Carl, the prankster minion. Carl has a few new abilities to play with, including running at a higher speed from the start.... | Read more »
Dungeon of Madness (Games)
Dungeon of Madness 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: Dungeon of Madness is an action game where you rotate tiles to create our own route. Help the hero by connecting the... | Read more »
Filters for iPhone (Photography)
Filters for iPhone 1.0 Device: iOS iPhone Category: Photography Price: $.99, Version: 1.0 (iTunes) Description: | Read more »
Jump'N'Shoot Attack (Games)
Jump'N'Shoot Attack 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: A mobile game for gamers! Join Louise Lightfoot, the legendary "Master of Jumping and Shooting", on her mission to save... | Read more »
Space Bounties Inc. (Games)
Space Bounties Inc. 1.4 Device: iOS Universal Category: Games Price: $1.99, Version: 1.4 (iTunes) Description: SuperGameDroid: 4/5 "Satisfying futuristic RPG combat, high replay value, and a heavy dose of nostalgia make Space... | Read more »

Price Scanner via MacPrices.net

iMacs on sale for up to $205 off MSRP
B&H Photo has 21″ and 27″ iMacs on sale for up to $205 off MSRP including free shipping plus NY sales tax only: - 21″ 1.4GHz iMac: $1019 $80 off - 21″ 2.7GHz iMac: $1189 $110 off - 21″ 2.9GHz... Read more
Färbe Technik Offers iPhone Battery Charge LI...
Färbe Technik, which manufactures and markets of mobile accessories for Apple, Blackberry and Samsung mobile devices, is offering tips on how to keep your iPhone charged while in the field: •... Read more
Electronic Recyclers International CEO Urges...
Citing a recent story on CNBC about concerns some security professionals have about the forthcoming Apple Watch, John Shegerian, Chairman and CEO of Electronic Recyclers International (ERI), the... Read more
Save up to $380 with Apple refurbished iMacs
The Apple Store has Apple Certified Refurbished iMacs available for up to $380 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 27″ 3.5GHz 5K iMac – $2119 $... Read more
Logitech Says MX Master Is Its Most Advanced...
Logitech’s new MX Master Wireless Mouse incorporates the best of Logitech’s many computer mouse innovations into a striking hand-sculpted design. The company claims that the MX Master creates a new... Read more
Save up to $300 on a new Mac, $30 on an iPad,...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
Apple refurbished 2014 MacBook Airs available...
The Apple Store lowered prices on Apple Certified Refurbished 2014 MacBook Airs recently, with models now available starting at $679. An Apple one-year warranty is included with each MacBook, and... Read more
Mac Notebook Evolution; A Desktop Replacement...
More often than not right from the beginning, Apple’s Macs have tended to skew toward small. The original Macs were called “compacts,”, and notwithstanding a few exceptions like the honking Big Mac... Read more
13-inch 1.4GHz/128GB MacBook Air (Apple refur...
The Apple Store has Apple Certified Refurbished 2014 13″ 1.4GHz/128GB MacBook Airs available for $759 including free shipping plus Apple’s standard one-year warranty. Their price is $240 off original... Read more
YEP! Alternative Browser for iOS Now Supports...
Pfaeffikon, Switzerland based Power App AG has announced the release of an update to their Yep! Web Browser (v1.3.0) for iOS8 iPhone and iPad. Yep! hit the App Store shortly after the release of iOS... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Lead *Apple* Solutions Consultant - Retail...
**Job Summary** Job Summary The Lead ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store Read more
*Apple* Pay - Site Reliability Engineer - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.