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

 
AAPL
$99.18
Apple Inc.
-1.57
MSFT
$45.90
Microsoft Corpora
-0.46
GOOG
$568.27
Google Inc.
-9.09

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cocktail Family License (5 Macs) 7.6.1 -...
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
Cocktail 8.0 Beta 2 - General maintenanc...
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
QuickBooks 2015 16.0.0.1352 R1 - Financi...
QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track money going in and out of your business, and spot areas where you can save.... Read more
Mac DVDRipper Pro 5.0.1 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
Apple OS X bash Update 1.0 - Fix for sec...
The OS X bash Update fixes a security flaw in the bash UNIX shell on OS X 10.9.5 (also on OS X 10.8 and 10.7 [see Related Links below]). OS X 10.9.5 or later Downloads for OS X 10.8 and OS X 10.7 in... Read more
SyncTwoFolders 2.0.5 - Syncs two user-sp...
SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more
FinderPop 2.5.7 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
VueScan 9.4.45 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
LibreOffice 4.3.2.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
calibre 2.4 - Complete e-library managem...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more

Latest Forum Discussions

See All

Banner Saga (Games)
Banner Saga 1.0.17 Device: iOS Universal Category: Games Price: $9.99, Version: 1.0.17 (iTunes) Description: NOTE: The Banner Saga does NOT run on iPhone 4. iPhone 4S and more modern devices recommended. | Read more »
Beatbuddy HD (Games)
Beatbuddy HD 1.0.2 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.2 (iTunes) Description: **IMPORTANT** Beatbuddy runs only on iPhone 6, 6 Plus, 5, 5c, 5s, iPad 4, iPad Mini Retina and iPad Air. | Read more »
Spirits of Spring (Games)
Spirits of Spring 1.0.1 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.1 (iTunes) Description: From the creators of the award-winning empathy game, Papo & Yo, comes a tale about finding strength. | Read more »
I am a brave knight (Games)
I am a brave knight 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Do you have ten minutes to live a life? | Read more »
iKeywi - Customizable 5-Row Keyboard (U...
iKeywi - Customizable 5-Row Keyboard 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: Want to add an extra row to your iPhone/iPad? One of the most popular keyboard extension in iOS... | Read more »
Manage Your Cloud – Wunderlist Now Suppo...
Manage Your Cloud – Wunderlist Now Supports Dropbox Posted by Jessica Fisher on October 1st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Nexticy Review
Nexticy Review By Jennifer Allen on October 1st, 2014 Our Rating: :: IDEAL FORM CREATIONiPad Only App - Designed for the iPad Nexticy allows you to make your own forms for research purposes or to organize your business better. It’s... | Read more »
Tiny Troopers: Alliance Marches onto the...
Tiny Troopers: Alliance Marches onto the App Store Tomorrow Posted by Jessica Fisher on October 1st, 2014 [ permalink ] Tiny Troopers: Alliance, by Kukouri, is a | Read more »
HeroCraft Introduces Unlimited Sequel to...
HeroCraft Introduces Unlimited Sequel to WW2: Sandbox. Strategy & Tactics Posted by Jessica Fisher on October 1st, 2014 [ permalink ] | Read more »
RGB Express Review
RGB Express Review By Jennifer Allen on October 1st, 2014 Our Rating: :: DELIGHTFUL PUZZLINGUniversal App - Designed for iPhone and iPad Guide trucks along their delivery routes in RGB Express, a testing but charming puzzle game... | Read more »

Price Scanner via MacPrices.net

ASUS, Lenovo, and Amazon Slug It Out For Tabl...
According to market intelligence firm ABI Research, Apple and Samsung have led the touchscreen tablet market by a substantial margin since Apple energized the category back in 2010. However, ABI says... Read more
Amazon offers 13-inch MacBook Air for $899, $...
Amazon.com has the 13″ 1.4GHz 128GB MacBook Air on sale for $100 off MSRP including free shipping: - 13″ 1.4GHz 128GB MacBook Air: $899.99 Read more
Apple resting On Its iPhone Laurels? – The ‘B...
Apple calls its new iPhone 6 and 6 Plus “The Biggest Advancements in iPhone History,” but does reality live up to the hype? “Seldom have so many waited so breathlessly for so little,” tweeted veteran... Read more
Roundup of Apple Mac and iPad Education disco...
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 Boycotts German Magazine Computer Bild...
Apple has revoked its PR accreditation of Germany’s Computer Bild, Europe’s best-selling PC magazine, in reaction to Bild’s posting of a “#Bentgate” YouTube video. Axel Telzerow, editor in chief of... Read more
iPhone 6 & iPhone 6 Plus Available in Chi...
Apple has announced that iPhone 6 and iPhone 6 Plus will be available in China beginning Friday, October 17 from the Apple Online Store (http://www.apple.com), Apple’s retail stores, and an expansive... Read more
MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only, in-... Read more
Apple Releases OS X Mavericks bash Update 1.0...
Apple has released a patch update for OS X Mavericks users to address the recently-detected “Shellshock” security bug in BSD UNIX’s bash shell. Apple says only a few Mac users who had manually... Read more
Pivotal Payments Ready for Apple Pay – FlexPo...
Pivotal Payments, a provider of merchant services and global payment processing solutions, has announced its proprietary FlexPoint platform will support credit and debit transactions through Apple’s... Read more
iStabilizer Announces Tabarm — First Friction...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, dollies, mounts, and remotes for smartphones, tablets, and cameras, announced today the iStabilizer tabArm, the first... 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
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.