TweetFollow Us on Twitter

July 02 Section 7

Volume Number: 18 (2002)
Issue Number: 07
Column Tag: Section 7

by Rich Morin

More Basic Commands

and a first look at “shell scripts”

Previous “Section 7” columns have looked at several BSD commands, but they haven’t said much about the ways in which commands get performed. Let’s do that now, starting with the Terminal application. Each time you start up a new Terminal window, Mac OS X has to start up a program to talk to it. The program it starts is a “command interpreter”, most often referred to as a “shell”.

Whenever a Terminal window is active, your keyboard is tied to the “standard input” of the program that is currently running “in” the window. Similarly, the window’s text area displays the program ‘s “standard output”. If the shell starts up another program (such as ls), the shell must “loan” these I/O connections to the other program. In short, a Terminal window is always connected to some program; most often, this will be the shell.

Each shell runs as a separate process, so the “state” that gets set in one “shell session” is not shared with other shell sessions. Thus, if you use cd to change the “current directory” in one Teminal window, the shell sessions in your other windows will not be affected.

Because cd affects information that the shell must maintain (and hand off to any programs it starts up), cd is actually “built in” to the shell itself. Most commands, however, are not built into the shell. Instead, they are implemented as:

  • aliases or shell functions – These are sequences of commands, kept in the shell’s memory and interpreted upon demand.
  • shell scripts – These are sequences of commands, kept in separate files and interpreted upon demand. Sometimes the shell itself will interpret the script; other times, as discussed below, it will start up a separate program to do so.
  • executable binaries – These are separate programs, kept in separate files and executed upon demand. To run them, the shell must fork(2), then exec(3) the relevant file.

In the case of built-ins, aliases, and shell functions, the shell “knows” what needs to be done. In the case of shell scripts and executable binaries, however, it must find the requested file, determine its nature, and proceed accordingly.

If a command is not a built-in, an alias, or a shell function, the shell looks for an executable file of that name in one of several directories. Because the shell may cache the contents of these directories in an internal data structure, some shells require the user to run a special command (“rehash”) when a new command is added. The list of these directories, called the “search path”, is contained in the $path variable:

[localhost:~] rdm% echo $path
/Users/rdm/bin/powerpc-apple-darwin /Users/rdm/bin ...

The which command can be used to predict the shells’ behavior for a given command:

[localhost:~] rdm% which cwd l 
cwd:     aliased to echo $cwd
l:       aliased to ls -lg
[localhost:~] rdm% which cd echo setenv 
cd: shell built-in command.
echo: shell built-in command.
setenv: shell built-in command.
[localhost:~] rdm% which apropos keytool
/usr/bin/apropos
/usr/bin/keytool

Finding and executing files (e.g., /usr/bin/apropos) takes time, but it allows us to add new commands quite trivially. Imagine how awkward and constraining it would be if each new command had to be linked into the shell!

Now that we have the full path names of the commands, we can examine them a bit:

[localhost:~] rdm% cd /usr/bin
[localhost:/usr/bin] rdm% file apropos keytool
apropos: Mach-O executable ppc
keytool: Bourne shell script text

As file informs us, /usr/bin/apropos is an executable binary file for the PowerPC, encoded in “Mach-O” format. /usr/bin/keytool, in contrast, is a text file which must be run interpretively by sh (the “Bourne shell”) as a “shell script”. But wait a second; how do file (and the shell) know that? Maybe it’s time to take a look at /usr/bin/keytool:

[localhost:/usr/bin] rdm% cat keytool
#!/bin/sh
#
# Shell wrapper to launch keytool.
#
...

The first two characters of the file (“#!”) tell the user’s shell that this is a script. The rest of the line gives the full path name (“/bin/sh”) of the program which must be used to interpret the script. Because each script can declare its own language (e.g., Bourne shell, C shell, Perl), new scripting languages can be added at any time.

If a script starts with a sharp sign (“#”), but not the “#!” sequence, it is treated as a C shell script. If a script begins with any other character, it is treated as a Bourne shell script. Finally, you may encounter scripts that start like “#!/usr/bin/env perl“. This tells the system to run /usr/bin/env, which will then go off and find the “appropriate” copy of perl.

The same header information is used by file to determine the type of a given file. To see file’s list of “magic numbers” (and interpretations), let’s look at /etc/magic:

[localhost:/usr/bin] rdm% cd /etc
[localhost:/etc] rdm% wc -l magic
    3837 /etc/magic
[localhost:/etc] rdm% grep ‘shell script’ magic
0  string  #!/bin/sh     Bourne shell script text
...

“wc -l” (word count, in line mode) tells us that there are about 4K lines in /etc/magic, so we don’t try to list it. Instead, we use grep (global regular expression print) to display lines that contain the string “shell script”.

As discussed last month, most of BSD’s “system metadata” (e.g., control files, log files) is kept in ASCII format. This works well with BSD’s wide range of text processing tools, allowing us to extract and process information in a very flexible manner.

If /etc/magic had been encoded in some proprietary, binary format (e.g., Microsoft Word), the exercise above would have required us to start up a particular application. And, if the application wasn’t able to do what we wanted, we’d have been stymied.

Scripting languages

BSD provides a variety of scripting languages. You are free to use any, all, or none of them, as you prefer. Here are some (biased :-) notes that may help you in making your selections:

  • The Bourne shell (sh) is not particularly convenient as an interactive command interpreter, but it works well as a programming language. The Korn and Z shells (ksh, zsh) are upwardly-compatible versions of the Bourne shell. The former is not supplied as part of Mac OS X, but the latter is and may deserve a look.
  • The Bourne-Again shell (bash) is upwardly compatible with the Bourne shell, but it also borrows a variety of features from other shells. It is the default shell for most Linux distributions, but it is not supplied as part of Mac OS X.
  • The C shell (csh) is very convenient as an interactive command interpreter, but it loses badly as a programming language due to its lack of syntax recognition. For example, multi-line commands always require backslashes at the end of non-terminal lines, even when the shell should be able to recognize that the command isn’t finished. Most BSD systems, including Mac OS X, actually provide tcsh, rather than csh. As the tcsh man page says:
    “tcsh is an enhanced, but completely compatible, version of csh(1). It is a command language interpreter, usable both as an interactive login shell and as a shell-script command processor. It includes a command-line editor, programmable word completion, spelling correction, a history mechanism, job control, and a C-like syntax.”
  • Awk (awk) is a convenient and simple scripting language, suitable for simple formatting and calculation tasks.
  • If things start to get too complex for Awk or the shell to handle, you should probably turn to Perl (perl). Perl is an extremely powerful language, with a large and active user community.
  • Python and Ruby are object-oriented scripting languages with small but vocal followings. Neither language is supplied as part of Mac OS X, but (like the other languages mentioned above), they are readily available for you to download and install.
  • Finally, if you’re a Tcl fan, you might want to try out tclsh (“a shell-like application that reads Tcl commands“).

Rich Morin has been using computers since 1970, Unix since 1983, and Mac-based Unix since 1986 (when he helped Apple create A/UX 1.0). When he isn’t writing this column, Rich runs Prime Time Freeware (www.ptf.com), a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at rdm@ptf.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Hopper Disassembler 4.2.5- - Binary disa...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32-bit and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about... Read more
BetterTouchTool 2.261 - 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
Sketch 44.1 - Design app for UX/UI for i...
Sketch is an innovative and fresh look at vector drawing. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus, windows, and... Read more
BetterTouchTool 2.260 - 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
Chromium 59.0.3071.115 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 59.0.3071.115: This update has no Flash plug... Read more
SyncTwoFolders 2.2.3 - Syncs two user-sp...
SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more
Myriad 4.1 - $79.00
Myriad is, simply put, one of the best audio batch processors. Totally redesigned, it looks beautiful and delivers incredible performance. Let Myriad do the heavy lifting while you get back to doing... Read more
Suitcase Fusion 7 18.2.4 - Font manageme...
Suitcase Fusion 7 is the creative professional's font manager. Every professional font manager should deliver the basics: spectacular previews, powerful search tools, and efficient font organization... Read more
SoftRAID 5.6.1 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID allows the user to create and manage RAID 4 and 5 volumes, RAID 1+0, and RAID 1 (Mirror) and... Read more
BetterTouchTool 2.25 - Customize Multi-T...
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

Latest Forum Discussions

See All

Collect mini assassins in 'Assassin...
Assassin's Creed is traveling back in time to the Spanish Inquisition for its latest mobile entry, Assassin's Creed Rebellion. The game is giving the series a look that's a huge departure from its past design, recreating classic characters in a... | Read more »
Animal Crossing is still coming to mobil...
Animal Crossing is still coming to mobile in 2017, according to aWaypointinterview with Nintendo. Announced in 2016, the game was delayed without a defined release window. However, fans of Nintendo's fantasy slice of life game won't have to wait... | Read more »
Ravenscroft 275 Piano (Music)
Ravenscroft 275 Piano 1.0.0 Device: iOS Universal Category: Music Price: $35.99, Version: 1.0.0 (iTunes) Description: Experience the splendor of a Ravenscroft Grand with the most realistic sounding piano ever created for iOS. Launch... | Read more »
This War of Mine gets a new ending and m...
This War of Mine just got a big new update, featuring free DLC that adds a new ending to the game, among other exciting changes. The update is celebrating the game's two-year release anniversary. Apart from the new ending, which will be quite... | Read more »
Summon eight new heroes in Fire Emblem H...
Nintendo keeps coming at us with Fire Emblem Heroes updates, and it doesn't look like that trend is stopping anytime soon. The folks behind the game have just announced the new War of the Clerics Voting Gauntlet, expected to start next Tuesday. [... | Read more »
The best deals on the App Store this wee...
iOS publishers are pulling out all the stops this week -- there's a huge number of seriously great games at discounted prices this week. Let's not waste any time and get right down to business. [Read more] | Read more »
The House of da Vinci (Games)
The House of da Vinci 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Enter The House of Da Vinci, a new must-try 3D puzzle adventure game. Solve mechanical puzzles, discover hidden... | Read more »
Solve the disappearance of history’s gre...
Blue Brain Games invites you to indulge in an immersive hands-on 3D puzzle adventure in similar vein to The Room series, with its debut release The House of Da Vinci. Set during the historic period of the Italian Renaissance (when Leonardo himself... | Read more »
Age of Rivals (Games)
Age of Rivals 3.3 Device: iOS Universal Category: Games Price: $.99, Version: 3.3 (iTunes) Description: Deep civilization-building strategy in a fast-paced card game! | Read more »
Panthera Frontier (Games)
Panthera Frontier 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Apple Releases iOS 11 Public Beta; How To Get...
The official release of Apple’s latest mobile operating system iOS 11 is vaguely slated for the fall, but as of June 26, ordinary users can download an iOS 11 public beta. To download the iOS 11... Read more
Extend Life of MacBook Pro Retina 2.0TB With...
MacSales.com/Other World Computing has announced availability of the new OWC 2.0TB Aura Pro Solid State Drive for mid-2012 to early 2013 Apple MacBook Pro with Retina display. One of the fastest... Read more
BBEdit SummerFest 2017 Discount Ends Friday,...
You can get 20% off BBEdit for a limited time in Bare Bones Software’s http://www.eastgate.com/Tinderbox/Specials/SummerFest.html?mc_cid=f2101ca260&mc_eid=[UNIQID]SummerFest 2017 sale and... Read more
Use Apple’s Education discount to save up to...
Purchase a new Mac using Apple’s Education discount, and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: - 15″ 2... Read more
Clearance 27-inch 3.3GHz 5K iMac available fo...
Amazon clearance 27″ 3.3GHz 5K iMacs (MK482LL/A) available for $1799.90 including free shipping. Their price is $500 off original MSRP, and it’s the lowest price available for this model from any... Read more
13-inch 1.8GHz/256GB MacBook Air on sale for...
B&H Photo has the updated 2017 13″ 1.8GHz/256GB MacBook Air (MQD42LL/A) in stock and on sale for $1129 including free shipping plus NY & NJ tax only. Their price is $70 off MSRP. Read more
27-inch 3.4GHz iMac on sale for $1699, save $...
B&H Photo has the new 2017 27″ 3.4GHz iMac (MNE92LL/A) in stock and on sale for $1699 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
21-inch 2.3GHz iMac on sale for $1049, save $...
B&H Photo has the new 2017 21″ 2.3GHz iMac (MMQA2LL/A) in stock and on sale for $1049 including free shipping plus NY & NJ tax only. Their price is $50 off MSRP. Read more
ABBYY TextGrabber 6 for iOS Implements Instan...
ABBYY has announced the release of TextGrabber 6.0.0, an important feature update to the company’s productivity app developed for iOS and Android devices. TextGrabber 6.0 now offers Real-Time... Read more
vPhone, First Smartphone That Can’t Be Lost,...
Austin, Texas based Hypori has introduced the vPhone, a virtual smartphone that affords every business user the benefits of separate work and personal phones, conveniently delivered on a single... Read more

Jobs Board

*Apple* News Product Marketing Mgr., Publish...
…organizational consensus on strategy and vision for publisher tools, authoring, and Apple News Format.Carries this strategy and vision across the organization to 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
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject matter Read more
Lead *Apple* Solutions Consultant - Apple I...
…integrity, and trust.Success Metrics/Key Performance Indicators:Quantitative* Year over Year growth in Apple Product and Beyond the Box sales in the assigned Point of Read more
*Apple* Solutions Consultant till v%u00E5r...
…ethics, integrity, and trust.Success Metrics/Key Performance Indicators:QuantitativeYear over Year growth in Apple Product and Beyond the Box sales in the assigned Point Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.