TweetFollow Us on Twitter

File System Metadata

Volume Number: 18 (2002)
Issue Number: 12
Column Tag: Section 7

What's in there, beside the data...

by Rich Morin

One of Apple's larger challenges, in merging Mac OS and BSD, has been the need to reconcile differences between their respective ways of handling files. Previous columns have touched on some of these issues; this month, we'll go into the topic in a bit more depth.

In order to keep both BSD and Mac OS program(mer)s happy, Apple has had to implement nearly every feature found in either HFS+ (Mac OS's "Extended Hierarchical File System") or UFS (BSD's "Unix File System"). This is a challenging task; the systems evolved independently and a variety of decisions were made in different manners.

A look at links

To take just one example, consider file links (BSD) and aliases (Mac OS). Conceptually, these are very similar: they provide ways to have multiple names for a single file. The devil, of course, lies in the details.

Hard links have been around since the earliest days of Unix. A hard link is a directory entry, within the same file system, to the same item. So, for example, the "." and ".." entries that you see in an "ls -al" directory listing are actually implemented as hard links.

Because all of BSD's file metadata is stored in the file's inode (information node), anything that affects any hard link to a file affects every hard link to the file. So, for example, you can't change the ownership or permissions on just one of a file's hard links.

Symbolic links (i.e., symlinks) were invented by the BSD developers, but other versions of Unix quickly picked up on the idea. A symlink is a specially-interpreted a file, containing the name of some other file, directory, etc. When the kernel is asked to access a symlink, it grabs the contained file name, then wanders off to resolve it. The referenced file may be anywhere on the system; in fact, it may not even exist!

Aliases are quite a bit like symlinks, but they differ in some subtle ways. For instance, Mac OS will try to find the file that an alias references, even if it has been moved or renamed. As long as the referenced file stays in the same file system, Mac OS will typically succeed in finding it. Symlinks, in contrast, either match completely or not at all.

These issues aside, symlinks act (mostly) like aliases, when viewed from the Finder. An alias, however, is unusable from the BSD command line; it simply appears to be a zero-length file. In short, if you need a link that is visible everywhere, use either a hard link or a symlink...

File System Metadata

Apple has done a very creditable job in merging file system metadata. Both Mac OS and BSD programs have access to the sorts of information they need; users and even programmers can largely ignore the underlying issues. To take full advantage of the combined environment, however, we need to know what kinds of metadata it supports.

The tables below list a variety of file system metadata and ways for users to get access to their values. Programmers will have to look up the appropriate libraries and/or frameworks, but the discussion below may provide some useful hints.

Perl programmers should look at Dan Kogai's MacOSX::File module, which provides access to a wealth of file metadata. Perl versions of several Mac OS X file access commands are included in the distribution (available on the CPAN: cpan.perl.org).

My table entries are a bit cryptic, so some explanation may be in order. Get Info (GI) is part of the standard Mac OS X distribution. File Buddy (FB) and Super Get Info (SGI) are available from SkyTag Software (www.skytag.com) and Bare Bones Software (www.barebones.com), respectively.

As their terse names hint, du and ls have been in the Unix lexicon for a number of years. GetFileInfo (and a companion command, SetInfo) are part of the Developer Tools and may be found (along with several other file-related utilities) in /Developer/Tools.

I realize that I may have left out some interesting data items (not to mention useful tools!); please drop me a line if you spot an egregious error or omission. As Apple continues to evolve the file system, I'm sure that a follow-on article will be needed...

Access
   Ownership     FB, GI, SGI   ls -l
   Permissions   FB, GI, SGI   ls -l
   set[gu]id                   ls -l
   sticky                      ls -l

Prior to OSX, Mac OS didn't have much in the way of access control. In fact, the only limitations were on file sharing. As convenient as this may have been, it did not protect users from each others' mistakes, nor protect the system files from user errors. BSD, as a multi-user system, has long had the notions of file ownership and permissions. OSX now has these, as well, but you can expect to see other (e.g., capability-based) forms of access control being added over time.

The setgid and setuid bits allow programs to run with the gid (group id) or uid (user id) of the file, rather than that of the user running the command. This is a convenient way to let programs do privileged things, in a controlled manner.

The "sticky" bit has different meaning, depending on the item involved. Adding the sticky bit to a directory makes the directory "append only"; see sticky(8) for details. On some systems (but not OSX), adding the sticky bit to an executable file causes its binary image to stay in memory after the program has finished.

The first character of the permission string printed by "ls -l" indicates the type of the file. This is not the same as the Type (four-character code) shown by some Mac utilities. In fact, it is more similar to the "Kind" field: it specifies whether the item is a plain file, a directory, a symlink, etc.

Date/Time
   Access            FB            ls -lu
   Backup            FB
   Creation          FB, GI, SGI   ls -cl, GetFileInfo
   Modification      FB, GI, SGI   ls -l,  GetFileInfo

Size
   Data fork
      file length    SGI
      size on disk   SGI   ls -l
   Resource fork
      file length    SGI
      size on disk   SGI
   Total
      file length    FB, GI, SGI   ls -l
      size on disk   FB, GI, SGI   du -k

The access and modification times, as you might expect, are the last times that the file's contents were accessed or modified, respectively. The creation time, on the other hand, is quite inconsistent and potentially confusing.

In both BSD and Mac OS, the creation time is initially defined as the time that the file was created. In BSD, however, the creation time gets updated when the file's directory entry (e.g.., its permissions) has been changed. To get the latter version, use ls -cl.

Because BSD does not have the notion of file "forks", standard command-line tools do not deal with them in a particularly consistent manner. In particular, ls may list the size of a resource fork as zero.

Attributes
   Alias file          FB            GetFileInfo
   Bundle              FB            GetFileInfo
   Custom icon         FB            GetFileInfo
   Desktop                           GetFileInfo
   Extension hidden    FB, GI, SGI   GetFileInfo
   Inited              FB            GetFileInfo
   Invisible           FB, GI, SGI   GetFileInfo
   Locked              FB, GI, SGI   GetFileInfo
   No INIT resource    FB            GetFileInfo
   Shared              FB            GetFileInfo
   Stationery Pad      FB, GI, SGI   GetFileInfo
   System file                       GetFileInfo

Comments
   Attached            FB
   Finder              FB, GI, SGI

Creator, etc.
   Creator             FB, SGI       GetFileInfo
   Kind                FB, GI, SGI
   Open with           SGI
   Path                FB, SGI
   Type                FB, SGI       GetFileInfo

Miscellany
   Name locked         FB
   Package             FB
   Preview             FB, GI, SGI
   Version             FB, SGI

To the best of my knowledge, all of these items are irrelevant to most BSD commands. That is, BSD commands neither affect nor respond to these items. Consequently, I won't discuss them in any detail.

The "Comments" information is, however, an exception. OSX stores comments in a binary file called ".DS_Store", located in the same directory as the file being commented upon. It also performs some hidden magic, copying the comments to the appropriate copy of .DS_Store when a file is moved, etc.

Because the BSD commands know nothing about this mechanism, they do NOT perform the same magic. So, if you use "cp" or "mv" on a file, don't expect the comments to follow along!

In Other News

If you're reading this column, you should probably take a look at O'Reilly 's "Mac OS X for Unix Geeks" (Jepson & Rothman) and "Learning Unix for Mac OS X (Taylor & Peek). Both volumes are slim and unassuming, but contain valuable nuggets of OSX lore .

O'Reilly has just released the second (and substantially larger) edition of "Mac OS X: The Missing Manual", by David Pogue. This looks like a handy book for all sorts of Mac OS X users, whatever their background. If you want a quick reference to the parts of OSX that aren't in your area(s) of specialization, give this book a once-over.


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

BetterTouchTool 1.989 - 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
calibre 2.77.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Quicksilver 1.5.2 - Application launcher...
Quicksilver is a light, fast and free Mac application that gives you the power to control your Mac with keystrokes alone. Quicksilver allows you to find what you need quickly and easily, then act... Read more
Paperless 2.3.9 - $49.95
Paperless is a digital documents manager. Remember when everyone talked about how we would soon be a paperless society? Now it seems like we use paper more than ever. Let's face it - we need and we... Read more
Apple GarageBand 10.1.5 - Complete recor...
The new GarageBand is a whole music creation studio right inside your Mac -- complete with keyboard, synths, orchestral and percussion instruments, presets for guitar and voice, an entirely... Read more
Adobe Audition CC 2017 10.0.2 - Professi...
Audition CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Audition customer). Adobe Audition CC 2017 empowers you to create and... Read more
Adobe After Effects CC 2017 14.1 - Creat...
After Effects CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). The new, more connected After Effects CC... Read more
Adobe Premiere Pro CC 2017 11.0.2 - Digi...
Premiere Pro CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Premiere Pro customer). Adobe Premiere Pro CC 2017 lets you edit... Read more
WALTR 2 2.0.9 - $39.95
WALTR 2 helps you wirelessly drag-and-drop any music, ringtones, videos, PDF, and ePub files onto your iPhone, iPad, or iPod without iTunes. It is the second major version of Softorino's critically-... Read more
Yummy FTP Pro 1.11.14 - $14.99 (50% off)
Yummy FTP Pro is an advanced Mac file transfer app which provides a full-featured professional toolkit combined with blazing speeds and impeccable reliability, so whether you want to transfer a few... Read more

Super Gridland (Entertainment)
Super Gridland 1.0 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 1.0 (iTunes) Description: Match. Build. Survive. "exquisitely tuned" - Rock Paper Shotgun No in-app purches, and no ads! | Read more »
Red's Kingdom (Games)
Red's Kingdom 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Mad King Mac has kidnapped your father and stolen your golden nut! Solve puzzles and battle goons as you explore and battle your... | Read more »
Turbo League Guide: How to tame the cont...
| Read more »
Fire Emblem: Heroes coming to Google Pla...
Nintendo gave us our first look at Fire Emblem: Heroes, the upcoming mobile Fire Emblem game the company hinted at last year. Revealed at the Fire Emblem Direct event held today, the game will condense the series' tactical RPG combat into bite-... | Read more »
ReSlice (Music)
ReSlice 1.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0 (iTunes) Description: Audio Slice Machine Slice your audio samples with ReSlice and create flexible musical atoms which can be triggered by MIDI notes or... | Read more »
Stickman Surfer rides in with the tide t...
Stickson is back and this time he's taken up yet another extreme sport - surfing. Stickman Surfer is out this Thursday on both iOS and Android, so if you've been following the other Stickman adventures, you might be interested in picking this one... | Read more »
Z-Exemplar (Games)
Z-Exemplar 1.4 Device: iOS Universal Category: Games Price: $3.99, Version: 1.4 (iTunes) Description: | Read more »
5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »
Last week on PocketGamer
Welcome to a weekly feature looking back on the past seven days of coverage on our sister website, PocketGamer. It’s taken a while for 2017 to really get going, at least when it comes to the world of portable gaming. Thank goodness, then, for... | Read more »
ROME: Total War - Barbarian Invasion set...
To the delight of mobile strategy fans, Feral Interactive released ROME: Total War just a few months ago. Now the game's expansion, Barbarian Invasion is marching onto iPads as a standalone release. [Read more] | Read more »

Price Scanner via MacPrices.net

B-Eng introduces SSD Health Check for Mac OS
Fehraltorf, Switzerland based independant Swiss company- B-Eng has announced the release and immediate availability of SSD Health Check 1.0, the company’s new hard drive utility for Mac OS X. As the... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
4-core 3.7GHz Mac Pro on sale for $2290, save...
Guitar Center has the 3.7GHz 4-core Mac Pro (MD253LL/A) on sale for $2289.97 including free shipping or free local store pickup (if available). Their price is a $710 savings over standard MSRP for... Read more
128GB Apple iPad Air 2, refurbished, availabl...
Apple has Certified Refurbished 128GB iPad Air 2s WiFis available for $419 including free shipping. That’s an $80 savings over standard MSRP for this model. A standard Apple one-year warranty is... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 13″ 2.7GHz/128GB Retina Apple MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $... Read more
Laptop Market – Flight To Quality? – The ‘Boo...
Preliminary quarterly PC shipments data released by Gartner Inc. last week reveal an interesting disparity between sales performance of major name PC vendors as opposed to that of less well-known... Read more
IBM and Bell Transform Canadian Enterprise Mo...
IBM and Bell Canada have announced they are joining forces to offer IBM MobileFirst for iOS market-ready enterprise applications for iPad, iPhone or Apple Watch. Bell, Canada’s largest communications... Read more
Otter Products is Closing… For a Day of Givin...
On Thursday, Feb. 9, Otter Products is closing doors to open hearts. In partnership with the OtterCares Foundation, the company is pausing operations for a day so all employees can volunteer with... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon has 2015 15″ 2.2GHz Retina MacBook Pros (MJLQ2LL/A) available for $1799.99 including free shipping. Apple charges $1999 for this model, so Amazon’s price is represents a $200 savings. Read more
Back in stock: Apple refurbished 13-inch Reti...
Apple has Certified Refurbished 2015 13″ Retina MacBook Pros available for up to $360 off original MSRP, starting at $1099. An Apple one-year warranty is included with each model, and shipping is... Read more

Jobs Board

*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Los Angeles, CA Introduction: We have immediate job openings for several Desktop Support Technicians with one of our Read more
*Apple* Retail - Multiple Positions - Apple,...
SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (Multi-L...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, 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
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Stamford, CT We have immediate job openings for several Desktop Support Technicians with one of our most well-known Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.