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

Sound Studio 4.8.11 - Robust audio recor...
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
BetterTouchTool 2.291 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Carbon Copy Cloner 4.1.18 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Hopper Disassembler 4.2.14- - Binary dis...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
VOX 2.8.30 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Default Folder X 5.1.6b3 - Enhances Open...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
CleanMyMac 3.8.6 - $39.95
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
Postbox 5.0.17 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Amazon Chime 4.6.5852 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
coconutBattery 3.6.3 - Displays info abo...
With coconutBattery you're always aware of your current battery health. It shows you live information about your battery such as how often it was charged and how is the current maximum capacity in... Read more

The best 2v2 card combos in Clash Royale
2v2 is making it's grand return toClash Royalequite soon. 2v2 has quickly become one of the game's most popular gameplay modes, though they still have yet to make it a permanent fixture in the game. 2v2 is exciting and adds some new flavor to... | Read more »
The best games we played this week - Aug...
Another busy week has come to a close. We played a lot of excellent games this week and now it's time to look back and reflect on some our favorites. Here are our picks for the week of August 18. [Read more] | Read more »
War Wings beginner's guide - how to...
War Wings is the newest project from well-established game maker Miniclip. It's a World War II aerial dogfighting game with loads of different airplane models to unlock and battle. The game offers plenty of single player and multiplayer action. We... | Read more »
How to win every 2v2 battle in Clash Roy...
2v2 is coming back to Clash Royale in a big way. Although it's only been available for temporary periods of time, 2v2 has seen a hugely positive fan response, with players clamoring for more team-based gameplay. Soon we'll get yet another taste of... | Read more »
Roll to Win with Game of Dice’s new upda...
Joycity’s hit Game of Dice gets a big new update this week, introducing new maps, mechanics, and even costumes. The update sets players loose on an exciting new map, The Cursed Tower, that allows folks to use special Runes mid-match. If you feel... | Read more »
Bottom of the 9th (Games)
Bottom of the 9th 1.0.1 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0.1 (iTunes) Description: Play the most exciting moment of baseball in this fast-paced dice and card game! | Read more »
The best apps for viewing the solar ecli...
If you somehow missed the news, many parts of the United States will be witness to a total solar eclipse on August 21 for the first time in over 90 years. It'll be possible to see the eclipse in at least some capacity throughout the continental U... | Read more »
The 5 best mobile survival games
Games like ARK: Survival Evolved and Conan Exiles have taken the world of gaming by storm. The market is now flooded with hardcore survival games that send players off into the game's world with nothing but maybe the clothes on their back. Never... | Read more »
Portal Walk (Games)
Portal Walk 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Portal Walk is adventure and relaxing platform game about Eugene. Eugene stuck between worlds and trying to find way back home.... | Read more »
Technobabylon (Games)
Technobabylon 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: City of Newton, 2087. Genetic engineering is the norm, the addictive Trance has replaced almost any need for human interaction,... | Read more »

Price Scanner via MacPrices.net

Weekend sale: 13-inch MacBook Pros for up to...
Amazon has new 2017 13″ MacBook Pros on sale today for up to $200 off MSRP, each including free shipping: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (MPXV2LL/A): $1599.99 $200 off MSRP – 13″ 3.1GHz/... Read more
Back To School With The Edge Desk All-in-one...
Back to school is just around the corner, and the ergonomically correct Edge Desk all-in-one portable kneeling desk is ideal for students living in dorms and small apartments, Edge Desk features:... Read more
Norton Core Secure Wi-Fi Router Now Available...
First introduced at the 2017 Consumer Electronics Show (CES), Norton Core, a secure, high-performance Wi-Fi router, fundamentally changed the concept of Wi-Fi routers by making security the primary... Read more
ViewSonic Adds New 27-inch 4K UHD Monitor to...
ViewSonic Corp. has introduced the VP2785-4K, a 27-inch 4K UHD (3840×2160) monitor that delivers precise and consistent color representation and performance to ensure incredible image quality. Built... Read more
Apple now offering Certified Refurbished 2017...
Apple is now offering Certified Refurbished 2017 27″ iMacs for up to $350 off original MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: – 27″ 3.... Read more
13-inch 2.3GHz MacBook Pros on sale for $100...
Amazon has the new 2017 13″ 2.3GHz MacBook Pros on sale today for $100 off MSRP, each including free shipping: – 13″ 2.3GHz/128GB Space Gray MacBook Pro (MPXQ2LL/A): $1199.99 $100 off MSRP – 13″ 2.... Read more
Clearance 2016 13-inch MacBook Airs available...
B&H Photo has clearance 2016 13″ MacBook Airs available for up to $200 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: – 13″ 1.6GHz/128GB MacBook Air (MMGF2LL... Read more
Clearance 21-inch and 27-inch iMacs available...
B&H Photo has clearance 21″ and 27″ Apple iMacs available for up to $500 off original MSRP, each including free shipping plus NY & NJ sales tax only: – 27″ 3.3GHz iMac 5K: $1799 $500 off... Read more
New iOS 11 Productivity Features Welcome But...
The iOS community is in late summer holding mode awaiting the September arrival of the iPhone 8 and iOS 11. iOS 11 public betas have been available for months — number six was released this week —... Read more
Samsung Electronics Launches New Portable SSD...
Samsung Electronics America, Inc. has announced the launch of Samsung Portable SSD T5 – its newest portable solid state drive (PSSD) that raises the bar for the performance of external memory... Read more

Jobs Board

Map Quality Analyst Lead, *Apple* Maps Eval...
…a team of subject matter experts measuring the quality of a range of Apple Maps algorithms, such as Map search, recommendations, and routing. Our group works with Read more
Business Development Manager, *Apple* Pay -...
Job Summary Apple Pay is seeking an experienced Business Development professional to join the Apple Pay team to develop partnerships and strategic alliances with Read more
Information Systems Engineer, *Apple* Retai...
Job Summary Do you have a passion for the technology and systems that make Apple amazing? Are you interested in joining IS&T Retail on what they do best? This is an 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
Development Operations and Site Reliability E...
Development Operations and Site Reliability Engineer, Apple Payment Gateway Job Number: 57572631 Santa Clara Valley, California, United States Posted: Jul. 27, 2017 Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.