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

A beginner's guide to Spellbinders
Spellbinderslooks a heck of a lot like Clash Royale, but has an entirely different pace to it. As such, it can be difficult to adjust to. Just in case you've been playing way too much Clash Royale (and who hasn't), we've put together a bunch of... | Read more »
Camping With Grandpa (Education)
Camping With Grandpa 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Black Hole Joyrider (Games)
Black Hole Joyrider 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Guide your spacecraft through the gravity well of a powerful black hole using only your retro-thrusters and dwindling fuel... | Read more »
My Koi (Games)
My Koi 1.0 Device: iOS iPhone Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: My Koi is a beautiful and relaxing fish pond app. Customise and name each fish. Feed them daily. Watch them grow. Collect new fish.... | Read more »
Never Gone (Games)
Never Gone 1.0.2 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.2 (iTunes) Description: ###IMPPORTANT### Never Gone's HD art resources require devices with more than 1GB RAM, so please note that iPhone 4/4s, iPad 2/... | Read more »
INKS. (Games)
INKS. 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers of BAFTA-winning Lumino City comes INKS. INKS updates pinball for a new generation. It combines the joy of pinball with... | Read more »
How to maximise your profits in Bakery B...
Running a bakery can be an expensive venture. You’ll need to continuously upgrade your oven, your kitchen supplies, and even your ingredients to keep customers happy. Most of these renovations in Bakery Blitz cost a pretty penny, but we have a few... | Read more »
How to manage your time in Bakery Blitz
It can be tricky, especially when you risk burning your kitchen to the ground if you forget a cake in the oven, so make sure to use these time management tricks to keep your bakery running smoothly. Don’t collect the money right away [Read more] | Read more »
Model 15 (Music)
Model 15 1.0 Device: iOS iPhone Category: Music Price: $29.99, Version: 1.0 (iTunes) Description: The Moog Model 15 App is the first Moog modular synthesizer and synthesis educational tool created exclusively for iPad, iPhone and... | Read more »
How to deal with wind in Angry Birds Act...
Angry Birds Action! is a physics-based puzzler in which you're tasked with dragging and launching birds around an obstacle-littered field to achieve a set objective. It's simple enough at first, but when wind gets introduced things can get pretty... | Read more »

Price Scanner via MacPrices.net

Hybrid Tablets Meet Rising Demand for a Singl...
Rising demand for a new breed of hybrid tablets — 2 in 1 devices that function as either a laptop or a tablet — is shaking up the tablet market and contributing to a shift in the way owners are... Read more
Bausch + Lomb and IBM Collaborate on iPhone...
Global eye health company Bausch + Lomb, and IBM on Thursday announced a collaboration to develop the first app of its kind for iPhone and iPad for surgeons who perform cataract surgery. The... Read more
Apple & SAP Partner to Deliver New iOS Ap...
Apple and Walldorf, Germany based SAP have announced a partnership to revolutionize the mobile work experience for enterprise customers of all sizes, combining powerful native apps for iPhone and... Read more
New 12-inch 1.1GHz Rose Gold Retina MacBook o...
B&H Photo has the new 2016 12″ 1.1GHz/256GB Rose Gold Retina MacBook (sku MMGL2LL/A) on sale for $1199 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest... Read more
12-inch 128GB iPad Pros on sale for up to $10...
B&H Photo has 12″ 128GB WiFi iPad Pros on sale for up to $100 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 128GB WiFi iPad Pro: $899 $50 off MSRP... Read more
Global Tablet Sales Slump Continues, iPad’s F...
Another miserable showing for the global slate tablet category in calendar Q1/16, with global tablet shipments falling another 1ten percent to 46.5 million units during the according to Strategy... Read more
Revel Systems to Showcase iPad POS Platform w...
Revel Systems, specialists in iPad Point of Sale management solution for brick-and-mortar retail, food businesses and more, today announced that it will showcase its innovative iPad Point of Sale... Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Apple refurbished 2015 iMacs available for up...
Apple now has a full line of Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are... Read more
Indian Smartphone Market Grows Annually by 12...
India’s smartphone market grew by 12 percent year-over-year, with 24.4 million units shipping in Q1 2016. The top five vendors stayed the same, with Samsung in the lead, followed by Micromax, Intex... Read more

Jobs Board

*Apple* Support Technician II - Worldventure...
…global, fast growing member based travel company, is currently sourcing for an Apple Support Technician II to be based in our Plano headquarters. WorldVentures is 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
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.