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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 2.69.0 - 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
Evernote 6.9.1 - 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
jAlbum Pro 13.5 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
jAlbum 13.5 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Google Chrome 53.0.2785.143 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
Chromium 53.0.2785.143 - 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 53.0.2785.143: [Security Fix] High CVE-2016-... Read more
QuickBooks 2015 16.1.7.1524 R8 - Financi...
Save 20% on QuickBooks Pro for Mac today through this special discount link QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track... Read more
Sierra Cache Cleaner 11.0.1 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Default Folder X 5.0.7 - Enhances Open a...
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
Safari Technology Preview 10.1 - The new...
Safari Technology Preview contains the most recent additions and improvements to WebKit and the latest advances in Safari web technologies. And once installed, you will receive notifications of... Read more

Pumped BMX 3: Beginner tips and tricks
There’s a whole lot more to Pumped BMX 3 than meets the eye. Your goal is to perform a wide array of sweet flips and tricks, but that’s easier said than done. It takes well practiced timing and coordination, and the game doesn’t really explain that... | Read more »
Cybird’s latest release - BFB Champions...
Launched in the UK in early September, BFB Champions’ newest update is loaded with great new features, and looks set to outshine the original version by taking it out of soft launch and giving it a new lease of life. | Read more »
3 apps to boost your focus
As someone who works from home, my workspace is a minefield of distraction. Cats, tasty snacks, the wind blowing past my window, that cleaning that I suddenly can’t put off any longer. If I let distraction takes its course, I find that soon half... | Read more »
Pumped BMX 3 (Games)
Pumped BMX 3 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: The final instalment of the smash hit #1 rated BMX game is here! Following on from the insane success of Pumped BMX 2, Pumped 3... | Read more »
4 games like Burly Men at Sea to inspire...
Burly Men at Sea is out today and it looks a treat. It tells the tale of three Scandinavian fishermen who leave the humdrum of their daily lives to go exploring. It’s a beautiful folksy story that unfurls as you interact with the environment... | Read more »
3 reasons you need to play Kingdom: New...
Developed by a tag team of indie developers - Thomas "Noio" van den Berg and Marco "Licorice" Bancale - Kingdom is a vibrant medieval fantasy adventure that casts players as a king or queen who must expand their empire by exploring the vasts lands... | Read more »
JoyCity have launched a brand new King o...
Great news for all of you Game of Dice fans out there - JoyCity have just released a brand new limited edition pack with a really cool twist. The premise of Game of Dice is fairly straightforward, asking you to roll dice to navigate your way around... | Read more »
Burly Men at Sea (Games)
Burly Men at Sea 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burly Men at Sea is a folktale about a trio of large, bearded fishermen who step away from the ordinary to seek adventure. | Read more »
3 tips for catching the gnarliest waves...
Like a wave breaking on the shore, Tidal Rider swept its way onto the App Store charts this week settling firmly in the top 10. It’s a one-touch high score-chaser in which you pull surfing stunts while dodging seagulls and collecting coins. The... | Read more »
The beginner's guide to destroying...
Age of Heroes: Conquest is 5th Planet Games’ all new turn-based multiplayer RPG, full of fantasy exploration, guild building, and treasure hunting. It’s pretty user-friendly as far as these games go, but when you really get down to it, you’ll find... | Read more »

Price Scanner via MacPrices.net

CAZE Annouces New Zero 5 Case for Jet Black i...
Hong Kong basd CAZE has announced Zero 5 case for iPhone 7/ 7 Plus, one of the world’s thinnest clear hard cases, measuring just 0.5 millimeters. CAZE has been producing and improving the Zero 5... Read more
Nest Egg Inventory App for iOS Offers Conven...
Campbell, California based Winprogger LLC has announced the release and immediate availability of Nest Egg – Inventory 4.1.22, an important update to their easy-to-use, yet comprehensive inventory... Read more
Factor4, LLC Launches Apple iOS and Android G...
Factor4, LLC, which offers gift and loyalty services to the SMB marketplace, has released free mobile applications that enable merchants to process via all Apple and Android devices. The Apple and... Read more
15-inch Retina MacBook Pros on sale for $200...
B&H Photo has 15″ Retina Apple MacBook Pros on sale for $200 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... 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
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Apple refurbished 2016 13-inch MacBook Airs a...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2016 13″ 1.6GHz/8GB/128GB MacBook... Read more
1.4GHz Mac mini on sale for $449, save $50
Adorama has the 1.4GHz Mac mini on sale for $50 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini (Apple sku# MGEM2LL/A): $449 $50 off MSRP To purchase a mini at... Read more
Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more

Jobs Board

*Apple* Retail - Multiple Positions- Akron,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Hardware Design Validation Engineer - *Apple...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
Systems Architecture Prototyping - *Apple*...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
*Apple* Retail - Multiple Positions- South B...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.