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 (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.

 
AAPL
$116.47
Apple Inc.
+0.16
MSFT
$47.98
Microsoft Corpora
-0.72
GOOG
$537.50
Google Inc.
+2.67

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cobook 3.0.7 - Intelligent address book....
Cobook Contacts is an intuitive, engaging address book. Solve the problem of contact management with Cobook Contacts and its simple interface and powerful syncing and integration possibilities.... Read more
StatsBar 1.9 - Monitor system processes...
StatsBar gives you a comprehensive and detailed analysis of the following areas of your Mac: CPU usage Memory usage Disk usage Network and bandwidth usage Battery power and health (MacBooks only)... Read more
Cyberduck 4.6 - FTP and SFTP browser. (F...
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Evernote 6.0.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
calibre 2.11 - Complete e-library manage...
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... Read more
Herald 5.0.1 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
Firetask 3.7 - Innovative task managemen...
Firetask uniquely combines the advantages of classical priority-and-due-date-based task management with GTD. Stay focused and on top of your commitments - Firetask's "Today" view shows all relevant... Read more
TechTool Pro 7.0.6 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PhotoDesk 3.0.1 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos! (NO Uploads! / Image Posting! Instagram forbids that! AND you *need* an *existing* Instagram account). But you can do... Read more

Latest Forum Discussions

See All

Ubisoft Gives Everyone Two New Ways to E...
Ubisoft Gives Everyone Two New Ways to Earn In-Game Stuff for Far Cry 4 Posted by Jessica Fisher on November 21st, 2014 [ permalink ] | Read more »
Golfinity – Tips, Tricks, Strategies, an...
Dig this: Would you like to know what we thought of being an infinite golfer? Check out our Golfinity review! Golfinity offers unlimited ways to test your skills at golf. Here are a few ways to make sure your score doesn’t get too high and your... | Read more »
Dark Hearts, The Sequel to Haunting Meli...
Dark Hearts, The Sequel to Haunting Melissa, is Available Now Posted by Jessica Fisher on November 21st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Meowza! Toyze Brings Talking Tom to Life...
Meowza! | Read more »
Square Enix Announces New Tactical RPG f...
Square Enix Announces New Tactical RPG for Mobile, Heavenstrike Rivals. Posted by Jessica Fisher on November 21st, 2014 [ permalink ] With their epic stories and gorgeous graphics, | Read more »
Quest for Revenge (Games)
Quest for Revenge 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: The great Kingdom of the west has fallen. The gods ignore the prayers of the desperate. A dark warlord has extinguished... | Read more »
Threadz is a New Writing Adventure for Y...
Threadz is a New Writing Adventure for You and Your Friends Posted by Jessica Fisher on November 21st, 2014 [ permalink ] In the tradition of round-robin storytelling, | Read more »
SteelSeries Stratus XL Hardware Review
Made by: SteelSeries Price: $59.99 Hardware/iOS Integration Rating: 4 out of 5 stars Usability Rating: 4.5 out of 5 stars Reuse Value Rating: 4.25 out of 5 stars Build Quality Rating: 4.5 out of 5 stars Overall Rating: 4.31 out of 5 stars | Read more »
ACDSee (Photography)
ACDSee 1.0.0 Device: iOS iPhone Category: Photography Price: $1.99, Version: 1.0.0 (iTunes) Description: Capture, perfect, and share your photos with ACDSee. The ACDSee iPhone app combines an innovative camera, a powerful photo... | Read more »
ProTube for YouTube (Entertainment)
ProTube for YouTube 2.0.2 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 2.0.2 (iTunes) Description: ProTube is the ultimate, fully featured YouTube app. With it's highly polished design, ProTube offers ad-free... | Read more »

Price Scanner via MacPrices.net

15″ 2.2GHz Retina MacBook Pro on sale for $17...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale today for $1749. Shipping is free, and B&H charges NY sales tax only. B&H will also include free copies of Parallels Desktop... Read more
27-inch 3.5GHz 5K iMac in stock today and on...
 B&H Photo has the new 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available... Read more
21-inch 1.4GHz iMac on sale for $979, save $1...
B&H Photo has the new 21″ 1.4GHz iMac on sale for $979.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. B&H will also include free copies of Parallels Desktop... Read more
13-inch 1.4GHz/256GB MacBook Air on sale for...
B&H Photo has lowered their price on the 13″ 1.4GHz/256GB MacBook Air to $1059.99 including free shipping plus NY sales tax only. Their price is $140 off MSRP, and it’s the lowest price for this... Read more
Save up to $400 with Apple refurbished 2014 1...
The Apple Store has restocked Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and shipping... Read more
New 13-inch 1.4GHz MacBook Air on sale for $8...
 Adorama has the 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. B&H Photo has the 13″ 1.4GHz/128GB MacBook... Read more
Apple Expected to Reverse Nine-Month Tablet S...
Apple and Samsung combined accounted for 62 percent of the nearly 36 million branded tablets shipped in 3Q 2014, according to early vendor shipment share estimates from market intelligence firm ABI... Read more
Stratos: 30 Percent of US Smartphone Owners t...
Stratos, Inc., creator of the Bluetooth Connected Card Platform, has announced results from its 2014 Holiday Mobile Payments Survey. The consumer survey found that nearly one out of three (30 percent... Read more
2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has lowered their price on the new 1.4GHz Mac mini to $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Store Leader Program - College Gradu...
Job Description: Job Summary As an Apple Store Leader Program agent, you can continue your education as you major in the art of leadership at the Apple Store. You'll Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.