TweetFollow Us on Twitter

Back to bash Basics

Volume Number: 21 (2005)
Issue Number: 10
Column Tag: Programming

Mac in the Shell

Back to bash Basics: Following Up on the bash Presented Thus Far

by Edward Marczak

Carpenters shape wood. Fletchers shape arrows. Programmers shape code. So far, we've been apprentices in the mastery of bash scripting. This month, we'll take another step up that ladder, and learn how to make our code flow. In the ever-blurring distinction between programming and scripting, let's look at some of the bash shell's better programming conventions and statements.

Too smart for me

I'm going to make several assumptions this month. Namely:

  • You know how to turn on a computer
  • You've been following my column for a little bit
  • You know what variables are
  • You're smart, and have been around this stuff for at least a little bit
  • You've seen some kind of flow control statements in some language, somewhere

Not too rough, right? Good. Let's go.

Why are you beating me?

Since bash is built-in to every OS X box, present and ready-to-go, it's a great tool for small, quick and dirty scripts, to more complex and full-featured (dare I say?) applications. While "application" may be a stretch, for any code to approach that, it has to be able to make decisions. The scripting 'techniques' presented thus far have really been pouring the foundation, which is important, but have really been little more than stringing commands together sequentially. Fine for basic automation, but limited in the grand scheme of things.

We need a way to introduce flow control into our code. This is a basic concept in all programming languages. Flow control is the classic, "if some condition exists, then follow these instructions." Or, "perform this set of statements x number of times." Of course, the syntax tends to be a little different between each language, and bash won't let you down. First, here's what bash gives us:

  • if/else
  • while/until
  • for
  • case
  • select

Why are you beating me?

if/else is about as basic as it gets. When you need to decide if your code should, or should not do something, you use if/else. You've probably seen this before - even Excel's VBA has if tests! Here's where bash is different: most languages test for some true Boolean condition. bash's if tests exit codes. That's it. Lodge that in your brain and you'll save yourself grief later on.

Every Unix program returns a code, in the form of an integer in the range 0-255, to its parent process when it quits - the exit code. Here's where your brain may spin: 0 is (typically) the "OK" code, meaning success. So, in an if statement, an exit code of "0" means, "true, run this code." However, this is a de facto standard, and the meanings of exit codes are up to the programmer.

A better way to think of this is that of "exit success." So:


if some command was successful
then
do this bit of code
else
yell and scream
fi

bash uses the variable "?" to store exit status of the most recently run program. To illustrate, follow this example:

$ touch example.txt
$ echo $?
0
$ grep asdf example.txt
$ echo $?
1

"?" shows the exit codes. "touch example.txt" was successful; it returned a "0" exit code. grep, however, couldn't find the string "asdf" in example.txt, so it was unsuccessful, and handed us back a "1" exit code. Of course, we can use this in a script.

ping, like most utilities, will hand us back a "0" on success, and some value greater than zero if there's a problem. Here's a cheap-o host monitor script:

#!/bin/bash

if ping -c 1 4.2.2.2
then
   logger -i -t pingscript Ping success
else
   mail -s "Can't ping 4.2.2.2" support@example.com < pingerror.txt
   pagetech.sh
fi

We try to ping a host at 4.2.2.2 (and if you're connected to the net, this should work). If the ping succeeds, we simply make a note of it. If the ping fails, we send mail to alert us to that fact, and run another script that pages a tech.

We can also test two conditions at a time with the "&&" (and) and "||" (or) operators. If we needed two pings to different hosts to succeed, we could use "&&". Changing our if line to read:

if ping 130.57.4.27 && ping 4.4.4.2 

will run the first statement. Only if it succeeds do we even try the second. Both the first and the second statement must succeed to enter the "then" section of the if statement. You can do something similar outside of a script:

copyfile.sh && alterfile.sh && ftpfile.sh 

This example will run three scripts in succession. The next in the series will only run if the previous script exited with a successful exit code.

The "||" operator runs the first statement, if it succeeds, statement 2 never runs, and we enter the then clause. If statement 1 fails, statement 2 does run. If it succeeds, we enter the then clause. Either statement's success will get us into the then clause. Consider revising our example:

if ping 130.57.4.27 || ping 4.2.2.2

Perhaps we're just trying to see if this machine is connected to the Internet. Say host 130.57.4.27 goes down. As long as we can still ping 4.4.4.2, this script doesn't send mail to us, or try to page someone.

if/then may be basic, but just this simple decision scheme alone can create powerful scripts. Let's make it more so. if/then is powerful, but testing exit codes alone can get tedious. The shell provides the test command to test various conditions. To make the command more concise and visually pleasing, [ is aliased to test. So this:

if test -e /bin/bash

is the same as this:

if [ -e /bin/bash ]

Most people use the [...] variant. I will do the same. When I say, "test", I'm referring to test or [. What can test check for us? The man page is of great use here, but I'll give an example or two:

File tests:

-e test for a file's existence, no matter the type.

-d test for a directory

String comparisons:

s1 = s2 string 1 is equal to string 2.

-n string is not null

Integer comparisons:

-lt less than

-eq equal

-ne not equal

Other notes for inside the brackets: you can use "!" to negate a test:

if [ ! -e ~/secretplans.txt ]

"And" and "or" are available within the brackets:

and: if [ -e /bin/bash -a -e /bin/false ]

or: if [ -e /bin/bash -o -e /bin/tcsh ]

...and don't forget that all of this can be combined:

if [ ! -d /Users/Shared/reports -o ! -e /nightly/`date +%Y%m%d`.txt ] 
   && rsync -delete -av -e ssh repuser@host.example.com:reports/* /Users/Shared/reports ; then
logger -t repsync Reports are current
else
logger -t repsync Reports are out of sync/missing
fi

Whew! That is:

If the "reports" directory or the nightly file doesn't exist, we try to rsync them. If that goes well, success all around. If the directory or file do exist, we're done. If the rsync fails, we note it in the system log. That packs a lot in a single "if" statement.

Why are you beating me?

If you're an old Pascal or C hand, you'll recognize these two constructs: while and until. For the uninitiated, while repeats a block of statements for the duration of a condition being true. until runs a block of code until a condition is met. One look at them in action, and you'll get it:

while [ -e /run/statusflag.txt ]
do
   copyfiles.sh
   sleep 300
done

The until block is very similar:

until [ -e thecowscomehome.txt ]
do
   jump_over_moon.sh
done

Easy, right? Naturally, you can run all of the tests that the if statement allows.

Why are you beating me?

Sometimes, either you or a variable know how many times a loop should run. (Does that make anyone else think of Tron?). The constructs of for and forelse allow us to do just this. This is where you'll see a lot of difference from traditional languages. While you can pull off those kinds of loops:

#!/bin/bash
for ((i=1;i<=10;i+=1))
do        
        echo $i
done

...you'll very rarely see that done in practice. What happens to be much cooler than this, though, is the ability to loop through file lists, directories and command line arguments passed to your script. This is truly, as the kids say, da bomb. Look at this simple example:

#!/bin/bash

for i in "*.sh"
do
        echo $i
done

This script gives me something like this:

$ ./loopy.sh 
backup.sh createdmg.sh fctest.sh it.sh loopy.sh speakdate.sh spread.sh syncsm.sh

Well, gosh, I could have simply used the one-line echo *.sh to achieve the same thing. How about something that gives us info on each file? Line by line output can be achieved in a few different ways, but here's the way I'm choosing, for the moment:

#!/bin/bash

FILES=`ls *.sh`

for i in $FILES
do
        echo $i
        if [ -O $i ]; then
                echo "You own $i."
        fi
        if [ -G $i ]; then
                echo "You are a group owner of $i."
        fi
        echo
done

Running the above gives me something akin to this:

$ ./loopy2.sh 
backup.sh
You own backup.sh.
You are a group owner of backup.sh.

createdmg.sh
You own createdmg.sh.

fctest.sh
You own fctest.sh.

it.sh
You own it.sh.

Loopy2.sh
You own loopy.sh.
You are a group owner of loopy2.sh

As mentioned, for is a fantastic way for dealing with arguments passed into the script. Here's a slightly contrived example that will make a directory and copy the files and/or types (based on extension) into that directory. The new concept here are the parameter variables: @ and #. "$@" contains a list of each positional parameter, $1, $2...$N that is passed into the script. "$#" contains the number of parameters passed in. We can even include a little error correction this way:

#!/bin/bash

thedir=`date +%Y%m%d`

if [ $# -gt 0 ]; then
        mkdir $thedir
        for list in "$@"; do
                cp $list $thedir
        done
else
        echo "usage: $0 (files)"
fi

Fun and functional! Right?

Why are you beating me?

The last flow control statements we'll cover this month, case and select, bring their own power to bash, much like the previous flow control variants.

case is a neat alternative to a bevy of if/then/else statements. You immediately know the reason for this if you've used "case" in Pascal, or "switch" in C. If you've never seen those, one example and you'll be a pro:

#!/bin/bash

freespace=`df / | tail -1 | awk '{print $5}' | cut -d "%" -f1`

case $freespace in
[1-6]*) report="Plenty of room on /"
;;
[7-8]*) report="You might want to fire up du and take a look at /, it is $freespace percent full."
;;
9[0-9]) report="Can you order a bigger disk and overnight it?  / is at $freespace percent!"
;;
*) report="I can't determine the amount of space on /"
;;
esac

echo $report

Of course, the power in the bash version lies in its ability to process patterns and command-line arguments:

#!/bin/bash

for file in $*; do
case $file in
*.txt) echo "$file is a text file."
;;
*.pl) echo "$file is a perl file."
;;
*.sh) echo "$file is a shell script."
;;
*.gif | *.png | *.jpg | *.jpeg | *.tiff) what=`sips -g format $file | tail -1`
        echo "$file is a $what"
        sizeh=`sips -g pixelHeight $file | tail -1`
        sizew=`sips -g pixelWidth $file | tail -1`
        echo "It is $sizew x $sizeh"
;;
*.aiff | *.sd2 | *.wav | *.mp3) echo "$file is some kind of audio file."
;;
*) echo "Sorry, I don't know what kind of file $file is!"
esac

done

Save this in a new file, make it executable and pass it some files you want information about. I really think the way you can process multiple options on one line is really elegant.

select is really its own unique construct. It will take the list of items passed to it, create a numbered menu out of those lists and wait for input. Watch it in action (file gives us information about a file, as seen on line 5):

#!/bin/bash

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

With no in clause (select variable in list), select defaults to the list of command-line parameters. So, when we run this example, we need to pass in the list to process:

$ ./st.sh *
1) printaudit.pl
2) BidToJob.dmg
3) cl.txt
4) list.txt
5) loopy.sh
#?

Pressing "2", followed by enter, has the program output:

BidToJob.dmg: Macintosh HFS Extended version 4 data last 
mounted by: '10.0', created:Tue Dec  7 16:58:10 2004, last 
modified: Wed Jan  5 18:07:44 2005, last checked: Tue Dec  7 
16:58:10 2004, block size: 4096, number of blocks: 1024, 
free blocks: 727

A bit Spartan, perhaps, but certainly functional, and will save you a ton of work. Again, the power here lies in being able to build menus, completely ad-hoc, based on the contents of a directory.

It's all about your style

Next month, we'll get a little deeper into why some of the above works, as we probe deeper into the bowels of bash. Also, we'll expand on other loop constructs. Like any programming, there's typically more than one way to achieve the same thing. You can avoid all use of until by negating a while loop. You can split up lists with bash's processing, grep, sed, cut, and other utilities. I can only serve as a guide. Practice, err, correct; then the apprentice becomes the master.


Ed Marczak, owns and operates Radiotope, a technology consulting company that specializes in networking, workflow automation, teaching about technology, and helping out. Tech relief at http://www.radiotope.com

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Logic Pro X 10.1.1 - Music creation and...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more
VLC Media Player 2.2.0 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Sound Studio 4.7.8 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
LibreOffice 4.4.1.2 - Free, open-source...
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
Freeway Pro 7.0.3 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
Cloud 3.3.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
Cyberduck 4.6.5 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Firefox 36.0 - Fast, safe Web browser. (...
Firefox for Mac offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and... Read more
Thunderbird 31.5.0 - Email client from M...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
VOX 2.4 - Music player that supports man...
VoxIt just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features & support for all audio formats you should ever need... Read more

Get The Whole Story – Lone Wolf Complete...
Get The Whole Story – Lone Wolf Complete is Now Available and On Sale Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Who Wore it Best? The Counting Dead vs....
Like it or not, the “clicker” genre, popularized by cute distractions like Candy Box and Cookie Clicker, seems like it’s here to stay. So Who Wore it Best? takes a look at two recent examples: The Counting Dead and AdVenture Capitalist. | Read more »
Card Crawl, the Mini Deck Building Game,...
Card Crawl, the Mini Deck Building Game, is Coming Soon Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Tinytouchtales and Mexer have announced their new game, | Read more »
Witness an all new puzzle mechanic in Bl...
Well, BlastBall MAX is not one of those games and is bucking trends such as timers, elements of randomness, and tacked-on mechanics in favor of pure puzzle gameplay. When you first boot up the game you’ll see a grid made up of squares that are each... | Read more »
This Princess Has a Dragon and She isn’t...
This Princess Has a Dragon and She isn’t Afraid to Useit. | Read more »
Mecha Showdown Review
Mecha Showdown Review By Lee Hamlet on February 27th, 2015 Our Rating: :: IN A SPINUniversal App - Designed for iPhone and iPad Mecha Showdown replaces traditional buttons with a slot machine mechanic in this robot fighting game,... | Read more »
Reliance Games and Dreamworks Unveil Rea...
Reliance Games and Dreamworks Unveil Real Steel Champions Posted by Ellis Spice on February 27th, 2015 [ permalink ] Reliance Games and Dreamworks have announced that a third game in | Read more »
Sum Idea Review
Sum Idea Review By Jennifer Allen on February 27th, 2015 Our Rating: :: TAXING NUMBERSUniversal App - Designed for iPhone and iPad Sum Idea is a fairly charming but taxing puzzle game.   | Read more »
A New Badland Update Brings Daydream Lev...
A New Badland Update Brings Daydream Levels to Co-Op Posted by Ellis Spice on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Slashing Demons Review
Slashing Demons Review By Lee Hamlet on February 27th, 2015 Our Rating: :: IT'S A LONG WAY TO THE TOPUniversal App - Designed for iPhone and iPad Slashing Demons lacks the depth or scope to take it beyond the point of being just... | Read more »

Price Scanner via MacPrices.net

Apple CEO Tim Cook to Deliver 2015 George Was...
Apple CEO Tim Cook will deliver the George Washington University’s Commencement address to GWU grads on May 17, at which time he will also be awarded an honorary doctorate of public service from the... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Save up to $50 on iPad Air 2s, NY tax only, f...
 B&H Photo has iPad Air 2s on sale for $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469.99 $30 off - 64GB iPad Air 2 WiFi: $549 $50 off - 128GB iPad Air 2... Read more
16GB iPad Air 2 on sale for $447, save $52
Walmart has the 16GB iPad Air 2 WiFi on sale for $446.99 on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only,... Read more
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: $1029 $70 off - 21″ 2.7GHz iMac: $1199 $100 off - 21″ 2.9GHz... Read more
Apple Takes 89 Percent Share of Global Smartp...
According to the latest research from Strategy Analytics, global smartphone operating profit reached US$21 billion in Q4 2014. The Android operating system captured a record-low 11 percent global... Read more
New Travel Health App “My Travel Health” iOS...
Rochester, Minnesota based Travel Health and Wellness LLC has announced that its new iOS app help safeguard the user’s health when traveling abroad — “My Travel Health” is now available on the Apple... Read more
Sale! MacBook Airs for up to $115 off MSRP
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more
15-inch 2.0GHz Retina MacBook Pro (refurbishe...
The Apple Store has Apple Certified Refurbished previous-generation 15″ 2.0GHz Retina MacBook Pros available for $1489 including free shipping plus Apple’s standard one-year warranty. Their price is... Read more
Wither The iPad mini? End Of The Road Imminen...
AppleDailyReport’s Dennis Sellers predicts that the iPad mini is going to be left to wither on the vine, as it were, and then just allowed to fade away — a casualty of the IPhone 6 Plus and other... Read more

Jobs Board

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
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand 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** 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
*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.