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
do this bit of code
yell and scream

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

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

"?" 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:


if ping -c 1
   logger -i -t pingscript Ping success
   mail -s "Can't ping" < pingerror.txt

We try to ping a host at (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 && ping 

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: && && 

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 || ping

Perhaps we're just trying to see if this machine is connected to the Internet. Say host goes down. As long as we can still ping, 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* /Users/Shared/reports ; then
logger -t repsync Reports are current
logger -t repsync Reports are out of sync/missing

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 ]
   sleep 300

The until block is very similar:

until [ -e thecowscomehome.txt ]

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:

for ((i=1;i<=10;i+=1))
        echo $i
done'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:


for i in "*.sh"
        echo $i

This script gives me something like this:

$ ./

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:


FILES=`ls *.sh`

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

Running the above gives me something akin to this:

$ ./
You own
You are a group owner of
You own
You own
You own
You own
You are a group owner of

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:


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

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

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:


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 /"

echo $report

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


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!"


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):


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

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:

$ ./ *
2) BidToJob.dmg
3) cl.txt
4) list.txt

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


Community Search:
MacTech Search:

Software Updates via MacUpdate

Planet Diver guide - How to survive long...
Planet Diver is an endless arcade game about diving through planets while dodging lava, killing bats, and collecting Starstuff. Here are some tips to help you go the distance. [Read more] | Read more »
KORG iDS-10 (Music)
KORG iDS-10 1.0.0 Device: iOS iPhone Category: Music Price: $9.99, Version: 1.0.0 (iTunes) Description: ** Debut Discount: 50% OFF! Sale Price US$9.99 (Regular price US$19.99). Other all Korg apps are also 50% OFF until Dec 28! **... | Read more »
World of Tanks Generals guide - Tips and...
World of Tanks Generals is a brand new card game by the developer behind the World of Tanks shooter franchise. It plays like a cross between chess and your typical card game. You have to keep in consideration where you place your tanks on the board... | Read more »
TruckSimulation 16 guide: How to succeed...
Remember those strangely enjoyable truck missions in Grand Theft Auto V whereit was a disturbing amount of fun to deliver cargo? TruckSimulation 16 is reminiscent of that, and has you play the role of a truck driver who has to deliver various... | Read more »
The best GIF making apps
Animated GIFs have exploded in popularity recently which is likely thanks to a combination of Tumblr, our shorter attention spans, and the simple fact they’re a lot of fun. [Read more] | Read more »
The best remote desktop apps for iOS
We've been sifting through the App Store to find the best ways to do computer tasks on a tablet. That gave us a thought - what if we could just do computer tasks from our tablets? Here's a list of the best remote desktop apps to help you use your... | Read more »
Warhammer 40,000: Freeblade guide - How...
Warhammer 40,000: Freebladejust launched in the App Store and it lets you live your childhood dream of blowing up and slashing a bunch of enemies as a massive, hulking Space Marine. It's not easy being a Space Marine though - and particularly if... | Read more »
Gopogo guide - How to bounce like the be...
Nitrome just launched a new game and, as to be expected, it's a lot of addictive fun. It's called Gopogo, and it challenges you to hoparound a bunch of platforms, avoiding enemies and picking up shiny stuff. It's not easy though - just like the... | Read more »
Sago Mini Superhero (Education)
Sago Mini Superhero 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: KAPOW! Jack the rabbit bursts into the sky as the Sago Mini Superhero! Fly with Jack as he lifts impossible weights,... | Read more »
Star Wars: Galaxy of Heroes guide - How...
Star Wars: Galaxy of Heroes is all about collecting heroes, powering them up, and using them together to defeat your foes. It's pretty straightforward stuff for the most part, but increasing your characters' stats can be a bit confusing because it... | Read more »

Price Scanner via

World’s First USB-C Adapter For MacBook Suppo...
Innergie, a brand of Delta Electronics, has announced its official release of the world’s first USB-C adapter supporting four DC output voltages, the PowerGear USB-C 45. This true Type C adapter... Read more
13-inch and 11-inch MacBook Airs on sale for...
B&H Photo has 13″ and 11″ MacBook Airs on sale for up to $120 off MSRP as part of their Holiday sale including free shipping plus NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $819 $90 off... Read more
13-inch MacBook Pros on sale for up to $150 o...
Take up to $150 off MSRP on the price of a new 13″ MacBook Pro at B&H Photo today as part of their Holiday sale. Shipping is free, and B&H charges NY tax only. These prices are currently the... Read more
13-inch 128GB MacBook Air now on sale for $79...
Best Buy has just lowered their price on the 2015 13″ 1.6GHz/128GB MacBook Air to $799.99 on their online store for Cyber Monday. Choose free shipping or free local store pickup (if available). Sale... Read more
Best Buy lowers 13-inch MacBook Pro prices, n...
Best Buy has lowered prices on select 13″ MacBook Pros this afternoon. Now save up to $200 off MSRP for Cyber Monday on the following models. Choose free shipping or free local store pickup (if... Read more
Cyber Monday: Apple MacBooks on sale for up t...
Apple resellers have MacBook Pros, MacBook Airs, and MacBooks on sale for up to $250 off MSRP for Cyber Monday 2015. The following is a roundup of the lowest prices available for new models from any... Read more
Cyber Monday: Apple Watch on sale for up to $...
B&H Photo has the Apple Watch on sale for Cyber Monday for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - Apple Watch Sport: $50 off - Apple Watch: $50-$100 off B... Read more
Cyber Monday: 15% off Apple products, and sto...
Use code CYBER15 on Cyber Monday only to take 15% on Apple products at Target, and store-wide. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-... Read more
iPad Air 2 And iPad mini Among Top Five Black...
Adobe has released its 2015 online shopping data for Black Friday and Thanksgiving Day. The five best selling electronic products on Black Friday were Samsung 4K TVs, Apple iPad Air 2, Microsoft Xbox... Read more
All-in-one PC Shipments Projected To Drop Ove...
Digitimes’ Aaron Lee and Joseph Tsai report that all-in-one (AIO) PC shipments may drop a double-digit percentage on-year in 2015 due to weaker-than-expected demand, although second-largest AIO make... Read more

Jobs Board

*Apple* New Products Tester Needed - Apple (...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, continues Read more
Software Engineer, *Apple* Watch - Apple (U...
# Software Engineer, Apple Watch Job Number: 33362459 Santa Clara Valley, Califo ia, United States Posted: Jul. 28, 2015 Weekly Hours: 40.00 **Job Summary** Join the Read more
SW Engineer - *Apple* Music - Apple (United...
# SW Engineer - Apple Music Job Number: 40899104 San Francisco, Califo ia, United States Posted: Aug. 18, 2015 Weekly Hours: 40.00 **Job Summary** Join the Android Read more
Sr Software Engineer *Apple* Pay - Apple (U...
# Sr Software Engineer Apple Pay Job Number: 44003019 Santa Clara Valley, Califo ia, United States Posted: Nov. 13, 2015 Weekly Hours: 40.00 **Job Summary** Apple Read more
*Apple* Site Security Manager - Apple (Unite...
# Apple Site Security Manager Job Number: 42975010 Culver City, Califo ia, United States Posted: Oct. 2, 2015 Weekly Hours: 40.00 **Job Summary** The Apple Site Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.