TweetFollow Us on Twitter

Shell Game: Calling Shell Commands from Applications

Volume Number: 20 (2004)
Issue Number: 4
Column Tag: Programming

Mac OS X Programming Secrets

by Scott Knaster

Shell Game: Calling Shell Commands from Applications

The coolest thing about Mac OS X is the skillful grafting of the lovely and attractive Aqua graphical user interface onto the geeky but sturdy Unix stuff underneath. Applications in OS X run on a Darwinian layer of software that you can access via Unix shells in Terminal, also known as the command line. This gives you the ability to lift the hood on an application and see what's going on underneath. You can observe an application's behavior, change its settings, or even fool around with its workings, if you know what you're doing or you don't mind the possibility of breaking things.

The commands you can run in the shell are very powerful. Sometimes it's handy to use a shell command from within an application, giving you an ideal combination of friendly user interface and utter command line power. In this month's column, we'll explore how to run shell commands from a Cocoa application. By the time you finish this column, you will be a master (or at least someone capable) of using shell commands from your lofty Cocoa programs.

How Did I Get Here?

What are shell commands, and where do they come from? Shell commands are instructions you type in Terminal to make things happen. For example, you type ls when you want a directory listing, kill to end a process, cp to copy a file, and so on.

Shell commands aren't built into the shell, with very few exceptions. They're actually executable programs stored in well-known (to the shell) directories. When you type the name of a command, the shell searches this set of directories for a file with the same name as the command. When it finds an executable with the sought-after name, the shell runs it. If there's no file by that name, you get the disheartening message Command not found.

The list of directories that hold shell commands is kept in a shell variable name PATH, so named because it holds the search path for commands. You can see the value of the PATH variable by typing a shell command: echo "${PATH}". Here's what I got when I typed that command:

[neb:~] scott% echo ${PATH}
/bin:/sbin:/usr/bin:/usr/sbin:/Developer/Tools

The command output shows that the shell will look for commands in 5 directories: /bin, /sbin, /usr/bin, /user/sbin, and /Developer/Tools. If we take a look in those directories, we should see some familiar command names living there. And sure enough, we do. For example, here are the files in /bin:

[neb:~] scott% ls /bin
[         date        expr      mv        rmdir     test
bash      dd          hostname  pax       sh        zsh
cat       df          kill      ps        sleep     zsh-4.1.1
chmod     domainname  ln        pwd       stty
cp        echo        ls        rcp       sync
csh       ed          mkdir     rm        tcsh

There are 33 commands in this directory, and hundreds more in the others. The /usr/bin directory alone includes over 600 commands.

The search path is initialized when the shell starts. The initial value for this and other shell variables is set by a configuration file named .tcshrc. This file is used with the tcsh shell, which is the default shell in OS X 10.3. If you switch to a different shell, such as bash, you'll have a different configuration file. Commands in .tcshrc are executed when the shell starts up. Let's take a look and see what's in .tcshrc:

[neb:~] scott% cat .tcshrc
setenv PATH "${PATH}:/Developer/Tools"

In this case, the .tcshrc file adds /Developer/Tools to the search path. This line gets tacked on to your configuration file when you install Xcode. If you want to add other directories of commands to the search path, you can put more setenv commands in your shell configuration file.

Task It

A shell command is just an executable file. When the system runs an executable, it creates a process. Because Cocoa likes things best when they're objects, Cocoa provides the NSTask class in the Foundation framework for handling processes. A process has an execution environment that includes its current directory, environment variables, and other values. You can create an NSTask object in your application and use it to control a shell command process.

It's going to be darned easy to call our first shell command. We'll only need a few methods of NSTask:

  • setLaunchPath specifies the file path of the command we're going to call from our application.

  • setArguments supplies any arguments that we would normally pass via the command line. Technically, this method isn't always necessary, because some commands don't take any arguments. But most of them do, most of the time.

  • launch executes the command and creates a new process from it.

That's all there is to it. What could be simpler? Of course, you should call alloc and init when you create the NSTask, and release when you're done.

In order to keep our example as basic as possible, we'll choose the open command, which simply opens a file or directory. For our example, we'll have it open the Applications folder to remind us of all the cool apps we're privileged to have on our Macs.

OK, let's make the application. In Xcode, choose New Project and create a Cocoa application. Name it whatever you like - a nice name, like Melanie, or Commando. Double-click MainMenu.nib to open it in Interface Builder. In the application window, add an NSButton object. Shrink the window down and put the button in the center. Name the button "Show Apps". Your screen should look something like Figure 1.


Figure 1. Laying out our modest application in Interface Builder.

NeXT, click the Classes tab, select NSObject, and press return to create a subclass. Rename the subclass AppController. In the Inspector (which you can open with Tools a Show Info), click the Actions tab, then click Add to create an action named doCommand. Create the files for the class (Classes a Create Files for AppController) and make an instance (Classes a Instantiate AppController).

Now it's time for the hookup. Connect the button to the AppController object by Control-dragging from the button to the AppController (in the MainMenu.nib window). Double-click the doCommand action to complete the wiring. Your screen in IB (which is what the cool kids call Interface Builder) should now look like Figure 2.


Figure 2. Connect the button to the application controller.

Turning our short attention span back to Xcode, it's time to write the code. Double-click AppController.m and type in the doCommand method.

#import "AppController.h"
@implementation AppController
- (IBAction)doCommand:(id)sender
{
   NSTask *theProcess;
   theProcess = [[NSTask alloc] init];
   
   [theProcess setLaunchPath:@"/usr/bin/open"];
      // Path of the shell command we'll execute
   [theProcess setArguments:
      [NSArray arrayWithObject:@"/Applications"]];
      // Arguments to the command: the name of the
      // Applications directory
   
   [theProcess launch];
      // Run the command
   
   [theProcess release];
}
@end

First, we create and set up the task object by calling NSTask alloc and init. Then, once we've instantiated the object, we tell OS X where to find the executable by calling setLaunchPath. In this case, it's in /usr/bin. (How did I know it was in that directory and not another? I looked for it. Remember from our earlier exercise that shell commands are generally in one of five directories: /bin, /sbin, /usr/bin, /usr/sbin, or /Developer/Tools. You can find the location of any shell command by calling the, er, shell command which. And to learn the details of using which, you of course go to the shell and type man which. Ah, Unix, where the fun never ends.)

After setting up the launch path, we call setArguments to say what should be passed to the command when it's called. Here, the only argument we need is what should be opened - the Applications folder - so we pass "/Applications". When we call launch, the command runs, the Finder comes to the front, and the Applications folder opens up. We did it!

Increasing Our Openness

The command we used, open, has a bunch of options that make it a very versatile tool, and you can use any of them when you call it from an application. For example, you can open any file just by specifying its path. If the file is a document, it will be opened with the default application. You can also use open to view a web page in the default browser. Just don't forget the protocol, such as http. Do it like this:

   [theProcess setArguments:
      [NSArray arrayWithObject: @"http://www.mactech.com"]];

The open command has an option, -a, that lets you open a file with a program that's not the default. You can use this when you call open from the command line or from an application. In the shell, it would look like this:

   open -a /Applications/TextEdit.app /Users/Scott/grape.doc

In this example, grape.doc will open with TextEdit instead of Microsoft Word as Bill intended. To achieve this, we're passing multiple arguments to open. The technique for calling setArguments is a little different when you have more than one argument. Because setArguments actually takes an NSArray, you have to do something like this:

   [task setArguments:[NSArray arrayWithObjects:
         @"-a", 
         @"/Applications/TextEdit.app",
         @"/Users/Scott/grape.doc", 
         nil]];  

Here we call arrayWithObjects - that's objects, plural - compared to earlier when we called arrayWithObject, singular, with only one parameter. The arguments are separated by commas. And surprisingly, the "-a" is a separate argument from the application name that follows it. Another wacky thing to remember about arrayWithObjects is that you have to add a nil after your last real object.

Users Can Be Choosers

Our example uses the open command to show the Applications folder in the Finder. That's a fine demonstration of the ability to call a shell command from a Cocoa application, but really, how many times can you look at the same folder? Let's make it slightly more interesting by allowing the user to decide what to open.

To do this, we'll go back to Interface Builder and add an NSTextView to the window. We'll name the new text view fileName - a fine, functional name. Then we'll add an outlet to the AppController object for fileName and introduce the objects to each other by Control-dragging. We save our work in IB and then head over to Xcode.

We need to make a simple change to AppController.m. Find the line where we call setArguments and change it to:

   [theProcess setArguments:
      [NSArray arrayWithObject: [fileName string]]];

This line now extracts the text from fileName and uses it to set the arguments for our impending shell command call. Our remodeled application window is pictured in Figure 3.


Figure 3. Our application now allows the user to enter an item to be opened. This product is now almost ready to ship, and t-shirts will be available soon.

As we discussed previously, this technique works when you have one argument, but not multiple arguments. If you're looking for something to do, a fun exercise would be to add another text view to the window and use it to let the user specify the other argument, an application to use for opening the file.

Moving Right Along

We accomplished our goal of calling a shell command from Cocoa, but we only scratched the surface, with neatly trimmed and manicured nails, of what you can achieve with NSTask. Next month we'll continue messing around with NSTask to see what other fun we can have.


Scott Knaster attended Cocoa Bootcamp at Big Nerd Ranch (http://www.bignerdranch.com/classes/cocoa1.shtml). He did not go Cocoa Loco but he liked it very much. Scott counts the days until pitchers and catchers report.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

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 »
How to get three stars in every level of...
Angry Birds Action! is, essentially, a pinball-style take on the pull-and-fling action of the original games. When you first boot it up, you'll likely be wondering exactly what it is you have to do to get a good score. Well, never fear as 148Apps... | Read more »
The beginner's guide to Warbits
Warbits is a turn-based strategy that's clearly inspired by Nintendo's Advance Wars series. Since turn-based strategy games can be kind of tricky to dive into, see below for a few tips to help you in the beginning. Positioning is crucial [Read... | Read more »
How to upgrade your character in Spellsp...
So you’ve mastered the basics of Spellspire. By which I mean you’ve realised it’s all about spelling things in a spire. What next? Well you’re going to need to figure out how to toughen up your character. It’s all well and good being able to spell... | Read more »
5 slither.io mash-ups we'd love to...
If there's one thing that slither.io has proved, it's that the addictive gameplay of Agar.io can be transplanted onto basically anything and it will still be good fun. It wouldn't be surprising if we saw other developers jumping on the bandwagon,... | Read more »
How to navigate the terrain in Sky Charm...
Sky Charms is a whimsical match-'em up adventure that uses creative level design to really ramp up the difficulty. [Read more] | Read more »
Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »

Price Scanner via MacPrices.net

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
Get Notifications When Your Friend’s Phone Ba...
Calgary, Canada based Stonelight Pictures has announced the release of Battery Share 1.0.1, its new utility for iOS 9 supported devices. The company notes that people are spending more time on their... Read more
11-inch 1.6GHz/128GB MacBook Air on sale for...
Amazon has the current-generation 11″ 1.6GHz/128GB MacBook Air (sku MJVM2LL/A) on sale for $749.99 for a limited time. Their price is $150 off MSRP, and it’s the lowest price available for this model... Read more
Price drops on clearance 2015 13-inch MacBook...
B&H Photo has dropped prices on clearance 2015 13″ MacBook Airs by up to $250. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799, $200... Read more
Mac minis on sale for up to $100 off MSRP
B&H Photo has Mac minis on sale for up to $100 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP - 2.8GHz Mac mini... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for $130-$200 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1169 $130 off MSRP - 13″ 2.7GHz/... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
SanDisk Half-Terabyte SSD Optimized for Every...
SanDisk Corporation has announced the SanDisk Z410 SSD, a cost-competitive, half-terabyte solid state drive (SSD) that enables manufacturers to design for a broad range of desktop PCs and laptops.... Read more

Jobs Board

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
*Apple* Retail - Multiple Positions (US) - A...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform 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
*Apple* Subject Matter Expert - NTT Data, In...
…in Owings Mills, MD has a 6+ month contract position available for an Apple Subject Matter Expert. TITLE: Apple Subject Matter Expert LOCATION: Owings Mills, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.