TweetFollow Us on Twitter

File Mode Idioms

Volume Number: 19 (2003)
Issue Number: 6
Column Tag: Section 7

Section 7

File Mode Idioms

by Rich Morin

Which modes are used for what?

Each Mac OS X file system node has a 16-bit mode word, as described in chmod(1,2) and stat(2). This word specifies the node's type, what access modes are allowed, and some specialized handling. 16 bits provides 64 K possible variations, but only a relatively small number of "file mode idioms" are found with any frequency. By learning some of these idioms, you can make your system more secure and understand existing configuration decisions.

File Types and Modes

Before we get into the idioms, however, let's review the basics of file types and modes. The file system recognizes many types of "files", including a number of things (e.g., directories) that aren't really files, at all. The most common file type, however, is the "regular file", followed by the "directory" and the "symbolic link" (aka symlink(7)).

There are dozens of device files (see mknod(1,2,8) for details), but they are almost always segregated into the /dev directory. Sockets and named pipes can be used to enable interprocess communication between arbitrary processes (see mkfifo(2) for details). Finally, the "whiteout" type is used for the (ever-experimental) Union File System, described in FreeBSD's mount_union(8) man page.

The node's type is specified by the top four bits in the mode word, available via the stat(2) system call:

0160000  whiteout
0140000  socket
0120000  symbolic link
0100000  regular file
0060000  block special device
0040000  directory
0020000  character special device
0010000  named pipe (fifo)

The next three bits specify some forms of "special handling". If the node is a regular file, the bits are interpreted as follows:

0004000  set user  id on execution
0002000  set group id on execution
0001000  save swapped text, even after use

The first two bits are described in the setuid(2) man page. Briefly, they allow a program to run with the permissions of its owner (or group), rather than those of the user who started it. This is used to provide controlled access to elevated privileges in (carefully constructed!) system commands.

You can get a list of your system's setuid and setgid commands with the following C-shell command (use whereis(1) or which(1) to find the full path name of a specific command). In the output below, df is setgid to "operator" and rcp is setuid to "root":

% ls -l /{,usr/}{,*/}{,s}bin | grep '[r-]-s'
-r-xr-sr-x  1 root  operator  ...  df
-r-sr-xr-x  1 root  wheel     ...  rcp

The third bit, described in sticky(8), tells the system to retain the read-only parts of a program's image in memory, after the program has terminated. This can be used to reduce the start-up time for frequently-run programs. Whether your OS honors the request is, of course, up to the vendor (:-).'

If the node is a directory and the system is SysV-ish (e.g., Red Hat Linux), the setgid bit may be interpreted as forcing "BSDish" behavior in setting the group for a new file. That is, a new file will get the enclosing directory's group, rather than the user's. On BSDish systems (e.g., OSX), this bit has no effect.

Sticky directories are a bit more complex; here's a snippet from the manual: "A file in a sticky directory may only be removed or renamed by a user if the user has write permission for the directory and the user is the owner of the file, the owner of the directory, or the super-user. This feature is usefully applied to directories such as /tmp, which must be publicly writable but should deny users the license to arbitrarily delete or rename each others' files."

The bottom nine bits are divided into three sets of permissions (for the file's owner, members of the file's group, and everyone else); each set specifies read, write, and execute permission:

0000400  read  permission, owner
0000200  write permission, owner
0000100  execute/search permission, owner
0000040  read permission, group

The meanings of read, write, and execute are a bit strained, when it comes to directories. Read permission allows the user to "read" the directory, looking for file names, etc. Write permission allows the user to "write" the directory, creating or removing files, etc. Finally, execute permission allows the user to access an item contained within the directory.

File Idioms

Most files are readable and writable by their owners. If nobody else is expected to access the file, no other permissions are needed. However, it is common to allow group access, as well:

% touch 0600 0660
% chmod 0600 0600
% chmod 0660 0660
% ls -l 0*
-rw-------  ...  0600
-rw-rw----  ...  0660

Obviously, executable files need to have the appropriate "execute" bits set. Less obviously, the "read" bit must be set for scripts (so the interpreter can read them!). In practice, even binary executables tend to have read access turned on; for one thing, this allows debuggers to inspect the binary.

Distributed executables often have write access turned off. This seems like a good idea, because it reduces the chance of inappropriate modification. An inspection of /usr/bin on my OSX system, however, shows that this practice isn't universal:

-rwxr-xr-x  ...  cscope
-r-xr-xr-x  ...  ctags

System-wide files, such as the executables in bin directories, generally need to be accessible by everyone on the system. Many system control files also need universal read access:

-r--r--r--  ...  /etc/crontab

sudo(8) allows any command to be run as if by any specified user, assuming that the actual user can supply the required password. Unfortunately, this requires passwords to be handed out, remembered, guarded, etc. Fortunately, the file system provides an elegant solution:

-rwsr-x---  1 root  wheel  ...  so

The "so" command (at least our version :-) gives root privileges to anyone who is in group wheel. If arguments are given, they are run as a command line; otherwise, the user is given a root-enabled shell.

Directory Idioms

The mode bits for home directories should keep each user's files safe from casual inspection (let alone modification). Depending on the environment, and your own level of paranoia, one of the following is probably appropriate:

drwx------  ...  abc  grp1  ...
drwx--x---  ...  def  grp2  ...
drwxr-x---  ...  ghi  grp3  ...
drwxr-x--x  ...  jkl  grp4  ...
drwxr-xr-x  ...  mno  grp5  ...

User abc doesn't want anyone else doing anything with his files. User def is willing to let members of group grp2 access files, but only if they know the file's name (removing read access from directories turns off ls access, wild cards, etc.). User ghi seems to trust his group fairly well, but still doesn't want them creating files in his home directory.

Users jkl and mno trust everyone on their computer as much as users def and ghi trust members of their groups. Because security tends to be antithetical to convenience, user ghi has the least problems sharing files, etc. For what it's worth, I use mno's mode on my desktop machine and jkl's mode on my server account.

Allowing directory execute access by others can be quite useful. Let's say that you want to set up a "drop box" where other users can leave files. Anyone should be able to drop stuff off, but only you should be able to look into the box, retrieve files, or (gasp!) remove files. Here's how:

% chmod 1733 drop_box
% ls -d drop_box
drwx-wx-wt  ... drop_box

As the owner, I am allowed to do anything to the directory. Others (including members of my group) are allowed to access and even create files in the directory, but they are not allowed to list its contents. Finally, the "sticky" bit (described above), keeps anyone but me from removing files from the directory.

I encourage you to set up a "testbed" directory and try out different modes within it. Try out different combinations to see how they might be useful; all of this will pay off some day when you're trying to figure out an obscure "file not found" or "permission denied" error message!

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 (, a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at


Community Search:
MacTech Search:

Software Updates via MacUpdate

Suitcase Fusion 6 17.3.0 - Font manageme...
Suitcase Fusion 6 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
Nisus Writer Pro 2.1.2 - Multilingual wo...
Nisus Writer Pro is a powerful multilingual word processor, similar to its entry level products, but brings new features such as table of contents, indexing, bookmarks, widow and orphan control,... Read more
calibre 2.40.0 - 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
Vivaldi - An advanced browser...
Vivaldi is a browser for our friends. In 1994, two programmers started working on a web browser. Our idea was to make a really fast browser, capable of running on limited hardware, keeping in mind... Read more
OmniPlan 3.0 - Robust project management...
With OmniPlan, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success,... Read more
Yummy FTP 1.11 - FTP/SFTP/FTPS client fo...
Yummy FTP is an FTP + SFTP + FTPS file transfer client which focuses on speed, reliability and productivity. Whether you need to transfer a few files or a few thousand, schedule automatic backups, or... Read more
Tweetbot 2.1 - Popular Twitter client. (...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
MacPilot 8.0 - Enable over 1,200 hidden...
MacPilot gives you the power of UNIX and the simplicity of Macintosh, which means a phenomenal amount of untapped power in your hands! Use MacPilot to unlock over 1,200 features, and access them all... Read more
Typinator 6.7 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
Adobe Lightroom 6.2 - Import, develop, a...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more

Balloony Land offers a fresh twist on th...
Balloony Land by Palringo offers a fresh twist on the match three genre and is out now on iOS and Android. First-off, you'll be popping balloons instead of crushing candy and the balloons will float up and fill the empty spaces instead of dropping... | Read more »
Graphic - vector illustration and design...
Graphic - vector illustration and design 1.0 Device: iOS iPhone Category: Productivity Price: $2.99, Version: 1.0 (iTunes) Description: Autodesk Graphic is a powerful full-featured vector drawing and illustration application right in... | Read more »
Sago Mini Babies (Education)
Sago Mini Babies 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: Introducing the Sago Mini babies. Boys and girls love caring for these adorable characters. Feed Robin her favorite mush... | Read more »
PAUSE - Relaxation at your fingertip (H...
PAUSE - Relaxation at your fingertip 1.1 Device: iOS iPhone Category: Healthcare & Fitness Price: $1.99, Version: 1.1 (iTunes) Description: | Read more »
Super Sharp (Games)
Super Sharp 1.1 Device: iOS Universal Category: Games Price: $1.99, Version: 1.1 (iTunes) Description: Your finger has never been so sharp! Cut with skill to complete the 120 ingenious physics levels of Super Sharp and become a cut... | Read more »
Assembly - Graphic design for everyone...
Assembly - Graphic design for everyone 1.0 Device: iOS Universal Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Assembly is the easiest most powerful design tool on the App Store. Create anything you can... | Read more »
Dub Dash (Games)
Dub Dash 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: ARE YOU READY FOR THE ULTIMATE CHALLENGE? UNIQUE SYMBIOSIS OF MUSIC AND GRAPHICS | Read more »
Leave Me Alone (Games)
Leave Me Alone 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: 33% off launch sale!!! Somewhere between the 1980s and 1990s there exists a world that never was. A world of skatepunks,... | Read more »
YAMGUN (Games)
YAMGUN 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: The invasion has begun! Protect the walls of the citadel against waves of enemies! But watch out, you will soon run out of ammo...... | Read more »
Chesh (Games)
Chesh 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It’s like chess, only not at all. ***40% off for a limited time to celebrate our launch!*** Chesh is a game of skill, strategy, luck,... | Read more »

Price Scanner via

TP-LINK Next-Gen Routers Support a Large Numb...
TP-LINK, specialists in consumer and business networking products, have announced the availability of Archer C2600, the company’s next-generation router featuring wireless AC, multi-user MIMO, and 4-... 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, and... Read more
Save up to $350 with Apple refurbished iMacs
Apple has Certified Refurbished iMacs available for up to $350 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 27″ 3.5GHz 5K iMac – $1949 $350 off MSRP - 27... Read more
Mac Pros on sale for up to $300 off MSRP
B&H Photo has Mac Pros on sale for up to $300 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2818.99, $181 off MSRP - 3.5GHz 6-core Mac Pro: $3699... Read more
5K iMacs on sale for up to $150 off MSRP, fre...
B&H Photo has the 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. They have the 27″ 3.5GHz 5K iMac on sale for $2149, $150 off MSRP... Read more
Twelve South Redesigns BookArc For Today’s Sm...
Twelve South has announced a redesigned version of their very first product, BookArc for MacBook. Tailored specifically for the newest generation of MacBooks, BookArc holds the new, smaller Apple... Read more
Phone 6s Tips & Tricks – Tips Book For iP...
Poole, United Kingdom based Tap Guides Ltd. has announced the release and immediate availability of iPhone 6s Tips & Tricks, an in-depth eBook available in the iBookstore that’s priced just $2.99... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
13-inch 2.5GHz MacBook Pro on sale for $994,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $994.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $105 off MSRP. Price valid... Read more
Is The iPad Pro Really A Serious Laptop Repla...
Probably not, at least for productive professionals and other power users. Steve Jobs declared that we’d entered the “post-PC Era” with the advent of the original iPad in 2010, a phrase we don’t hear... Read more

Jobs Board

*Apple* Desktop Analyst - KDS Staffing (Unit...
…work with some of the brightest engineers. Job Description Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Points:Depth of Read more
*Apple* Retail Online, Customer Experience R...
**Job Summary** Apple Retail's Online Store sells Apple products to customers around the world. In addition to selling Apple products with unique services such Read more
Frameworks Engineer, *Apple* Watch - Apple...
**Job Summary** Join the team that is shaping the future of software development for Apple Watch! As a software engineer on the Apple Watch Frameworks team you will Read more
Senior Manager, Global Direct Marketing *App...
**Job Summary** Apple Retail is looking for an experienced Direct Marketing Leader to join its Marketing team. This position will take a leadership role in creating Read more
*Apple* Online Store Expansion - Apple (Unit...
**Job Summary** The Online Apple Store is seeking a person to lead its expansion into new countries. Based in Cupertino, CA, this person will develop and maintain an Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.