TweetFollow Us on Twitter


Volume Number: 18 (2002)
Issue Number: 9
Column Tag: Mac OS X


Analyze a file's permissions, using Perl

by Rich Morin

In order to know who can do what to a file, you have to understand the permissions on the file itself and on each directory leading to it. Locking down write permission on a file, for instance, keeps miscreants from writing into the file, but it doesn't keep them from removing and replacing it. To prevent that, you have to set the right permissions on the enclosing directory.

Or, let's say that your file path contains some symbolic links. In order to reach the file, a program must traverse the path up to the symlink, then backtrack and traverse the path up to the symlink's target. If the path is /A/B/C and B is a symlink to /X/Y, the program will need access to /, /A, /A/B, / (again), /X, /X/Y, and /X/Y/C.

The BSD command "ls -dl" will show the permissions on a specified file or directory, but typing in a long sequence of commands is both tedious and error-prone. Consider:

% ls -ld /
drwxrwxr-t  49 root  admin  1622 Jul 29 11:11 /
% ls -ld /Applications
drwxrwxr-x  36 root  admin  1180 Jul 28 10:34 /Applications

Fortunately, it's quite possible to automate this procedure. My ckpath script examines each element in the requested file path, back-tracking as necessary to handle symbolic links. It handles "white space" in file names (uncommon in BSD, but common in Mac OS X) and fiddles a bit with the output format.. Here's some sample output:

% ckpath "/Applications/AppleScript/Example Scripts"
"/Applications/AppleScript/Example Scripts"
1775 drwxrwxr-t  49 root  admin  2002.07.29 /
0775 drwxrwxr-x  36 root  admin  2002.07.28 Applications
0775 drwxrwxr-x   5 root  admin  2002.02.14 AppleScript
0775 lrwxrwxr-x   1 root  admin  2002.02.14
    "Example Scripts" -> /Library/Scripts
1775 drwxrwxr-t  49 root  admin  2002.07.29 /
0775 drwxrwxr-x  28 root  admin  2002.07.16 Library
0775 drwxrwxr-x  12 root  admin  2001.09.14 Scripts

The first two output fields (e.g., 0775 and drwxrwxr-x) contain the octal and symbolic representations of the node's permissions. For a complete explanation of BSD permission codes, see the ls(1) manual page. Briefly, however, the story is that each entity in the file system has a type (e.g., directory, file, symlink) and three sets of permissions bits (for user, group, and other). Some ancillary bits control special features such as set[ug]id execution.

A string such as drwxrwxr-x indicates that this is a directory and that anyone can read and execute (pass through) it. Any "other" user (not the owner, nor in the directory's group) cannot write (i.e., create, remove, or rename files) in the directory.

The following three fields (links, owner, and group) are taken directly from the ls output. The date has been normalized into YY.MM.DD format, improving line-to-line consistency and easing date calculations. The remainder of the line contains the node name, quoted if it contains spaces. As in ls output, symlinks are listed with their targets.

Code Walkthrough

This walkthrough is neither an attempt to teach Perl in one sitting, nor a truly detailed explanation of the intricacies of ckpath. Instead, it touches on both language and design issues, trying to hit some of the high points of each. The references listed in this month's "Section 7" column can help you with the Perl issues; I hope to explain the program's general flow in the following text.

The first line of ckpath allows for the possibility that we may have installed a copy of the Perl interpreter in a non-standard location. /usr/bin/env walks down our search path, finding the same copy of Perl that the shell would.

If ckpath is run with no argument, it examines the current working directory. Otherwise, it uses the argument as a path name, prepending the current working directory unless the path begins with a slash. This is fairly traditional behavior for a BSD command.

Some advocates of structured programming entirely refuse to use gotos. I avoid them in general, but use them (as in this case) when the alternative would be even uglier. Interested readers are invited to attempt a goto-free formulation.

After tidying up the incoming path name, we print it out for the user (in quotes, if it contains any white space). We then create a "todo" list, containing the full path names for each node in the input path name. This is the putative task list, but it may be abandoned if we encounter a symlink or an error.

After formatting the node name and determining that the node actually exists, we examine it in two ways. First, we run "ls -ald", discarding everything but the symbolic permission information. We then use lstat to retrieve the rest of the information we want.

This isn't particularly elegant or efficient, but it's a lot easier than generating the symbolic permission codes ourselves or, worse, trying to parse the output of ls. Interested readers, again, are welcome to try coding alternative approaches.

Using getpwuid and getgrgid, we try for symbolic versions of the user and group names, falling back to numeric forms if need be. localtime gives us a printable list of time values, from which we grab the year, month, and day.

If the node is a symlink, we add the target to the output line, fudge the path name to reflect the symlink's target, and jump back to REDO. Otherwise, we simply print a closing newline and go back for the next node.


Perl is particularly facile at handling this sort of problem. It has good string-handling capabilities, powerful and convenient data structures, and access to assorted system calls and library functions. I can't see doing this program as a shell script; the shell isn't powerful enough. Nor would I want to try writing it in C (no string-handling, regular expressions, etc.).

The strict and warnings pragmas are a bit like using lint(1) on C code. They tell Perl to look for all sorts of incipient problems, such as variables which are only used once. I've started using these more frequently than I once did, partly as a consequence of writing larger scripts where the scope of variables can become a real issue. The extra typing (and, occasionally, redesign) that the pragmas require seems to be more than compensated by the problems they uncover.

#!/usr/bin/env perl
# Usage: ckpath [file node]             # defaults to .
# Rationale:
# Let's say that you have a file which is having permissions
# problems.  In order to find out ALL the relevant
# permissions, you will have to run "ls -ld" on each element
# of the path, then back-track for each symbolic link you
# encounter.  Not fun.  This script automates the process,
# allowing you to see the entire path's permissions at once.
# It also tweaks the output format a bit (e.g., printing the
# octal modes and making the date format consistent).
# Written by Rich Morin, CFCL, 2002.06
use strict;
use warnings;
  my(@stat, @todo,
     $cwd, $grp, $mday, $mode, $mon, $name, $node,
     $save, $sm, $tgt, $tmp, $todo, $usr, $year
  $cwd = `pwd`; chomp($cwd);
  if ($#ARGV == -1) {             # Get path, if any.
    $todo =  $cwd;
  } else {
    $todo = $ARGV[0];
    $todo = "$cwd/$todo" if ($todo !~ m|^/|);
  $todo =~ s|/[^/]+/\.\./|/|g;    # "/foo/../" -> "/"
  $todo =~ s|/\./|/|g;            # "/./"      -> "/"
  $todo =~ s|//+|/|g;             # "//"       -> "/"
  $todo =~ s|/$||;                # ".../foo/" -> ".../foo"
  $save = $tmp = $todo;           # Print current task.
  $tmp = "\"$tmp\"" if ($tmp =~ m|\s|);
  print "\n$tmp\n";
  undef @todo;                    # Get list of nodes.
  while ($todo ne '') {
    push(@todo, $todo);
    $todo =~ s|/[^/]+$||;
  push(@todo, '/');
  while ($name = pop(@todo)) {    # Print info on node.
                                  # Format node name.
    ($node = $name) =~ s|^.*/([^/]+)$|$1|;
    $node = "\"$node\"" if ($node =~ m|\s|);
    if (! -e $name) {
      printf("%-48s %s\n",
        'Warning!  No such file or directory:', $node);
                                  # Protect white space.
    ($tmp = $name) =~ s|(\s)|\\$1|g;
                                  # Get symbolic mode info.
    $sm   = substr(`ls -ald $tmp`, 0, 10);
                                  # Get info on node.
    @stat = lstat($name);
                                  # Get numeric mode info.
    $mode = $stat[2] &  07777;
                                  # Get user name.
    $usr  = (getpwuid($stat[4]))[0];
    $usr  = $stat[4] if ($usr eq '');
                                  # Get group name.
    $grp  = (getgrgid($stat[5]))[0];
    $grp  = $stat[5] if ($grp eq '');
                                  # Get modification time.
    (undef, undef, undef, $mday, $mon, $year,
     undef, undef, undef) = localtime($stat[9]);
    printf("%04o %10s %3d %-8s %-8s %s.%02d.%02d %s",
      $mode, $sm, $stat[3], $usr, $grp,
      $year+1900, $mon+1, $mday, $node);
    if ($sm =~ m|^l|) {           # Eeek, a symbolic link!
      $tmp = $tgt = readlink($name);
      $tmp = "\"$tmp\"" if ($tmp =~ m|\s|);
      printf(" -> %s\n", $tmp);
      ($todo = $save) =~ s|^$name|$tgt|;
      if ($tmp !~ m|^/|) {
        ($tmp  = $name) =~ s|^(.*/)[^/]+$|$1|;
        $todo  = "$tmp$todo";
      goto REDO;
} }

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

The best GIF making apps
Animated GIFs have exploded in popularity recently which is likely thanks to a combination of Tumblr, our shorter attention spans, and the simple fact they’re a lot of fun. [Read more] | Read more »
The best remote desktop apps for iOS
We've been sifting through the App Store to find the best ways to do computer tasks on a tablet. That gave us a thought - what if we could just do computer tasks from our tablets? Here's a list of the best remote desktop apps to help you use your... | Read more »
Warhammer 40,000: Freeblade guide - How...
Warhammer 40,000: Freebladejust launched in the App Store and it lets you live your childhood dream of blowing up and slashing a bunch of enemies as a massive, hulking Space Marine. It's not easy being a Space Marine though - and particularly if... | Read more »
Gopogo guide - How to bounce like the be...
Nitrome just launched a new game and, as to be expected, it's a lot of addictive fun. It's called Gopogo, and it challenges you to hoparound a bunch of platforms, avoiding enemies and picking up shiny stuff. It's not easy though - just like the... | Read more »
Sago Mini Superhero (Education)
Sago Mini Superhero 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: KAPOW! Jack the rabbit bursts into the sky as the Sago Mini Superhero! Fly with Jack as he lifts impossible weights,... | Read more »
Star Wars: Galaxy of Heroes guide - How...
Star Wars: Galaxy of Heroes is all about collecting heroes, powering them up, and using them together to defeat your foes. It's pretty straightforward stuff for the most part, but increasing your characters' stats can be a bit confusing because it... | Read more »
The best cooking apps (just in time for...
It’s that time of year again, where you’ll be gathering around the dinner table with your family and a huge feast in front of you. [Read more] | Read more »
Square Rave guide - How to grab those te...
Square Rave is an awesome little music-oriented puzzle game that smacks of games like Lumines, but with its own unique sense of gameplay. To help wrap your head around the game, keep the following tips and tricks in mind. [Read more] | Read more »
Snowboard Party 2 (Games)
Snowboard Party 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Crowned the best snowboarding game available on the market, Snowboard Party is back to fulfill all your adrenaline needs in... | Read more »
One Button Travel (Games)
One Button Travel 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: “To cut a long story short, If you like interactive fiction, just go buy this one.” - “Oozes the polish that... | Read more »

Price Scanner via

Holiday weekend Mac sales roundup: B&H Ph...
B&H Photo continues to have all new Macs on sale for up to $500 off MSRP as part of their Black Friday/Holiday weekend sale. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina... Read more
iMobie Releases its Ace iOS Cleaner PhoneClea...
iMobie Inc. has announced the new update of PhoneClean 4, its iOS cleaner designed to reclaim wasted space on iPhone/iPad for use and keep the device fast. Alongside, iMobie hosts a 3-day giveaway of... Read more
U.S. Cellular Offering iPad Pro
U.S. Cellular today announced that it is offering the new iPad Pro with Wi-Fi + Cellular, featuring a 12.9-inch Retina display with 5.6 million pixels — the most ever in an iOS device. U.S. Cellular... Read more
Newegg Canada Unveils Black Friday Deals for...
Newegg Canada is offering more than 1,000 deep discounts to Canadian customers this Black Friday, available now through Cyber Monday, with new deals posted throughout the week. “Black Friday is... Read more
Black Friday: Macs on sale for up to $500 off...
BLACK FRIDAY B&H Photo has all new Macs on sale for up to $500 off MSRP as part of their early Black Friday sale including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $... Read more
Black Friday: Up to $125 off iPad Air 2s at B...
BLACK FRIDAY Walmart has the 16GB iPad Air 2 WiFi on sale for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available): - 16GB iPad Air 2 WiFi: $399, save $... Read more
Black Friday: iPad mini 4s on sale for $100 o...
BLACK FRIDAY Best Buy has iPad mini 4s on sale for $100 off MSRP on their online store for Black Friday. Choose free shipping or free local store pickup (if available): - 16GB iPad mini 4 WiFi: $299.... Read more
Black Friday: Apple Watch for up to $100 off...
BLACK FRIDAY Apple resellers are offering discounts and bundles with the purchase of an Apple Watch this Black Friday. Below is a roundup of the deals being offered by authorized Watch resellers:... Read more
Black Friday: Target offers 6th Generation iP...
BLACK FRIDAY Save $40 to $60 on a 6th generation iPod touch at Target with free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: -... Read more
Black Friday: Walmart and Target offer iPod n...
BLACK FRIDAY Walmart has the 16GB iPod nano (various colors) on sale for $119.20 on their online store for a limited time. That’s $30 off MSRP. Choose free shipping or free local store pickup (if... Read more

Jobs Board

*Apple* Site Security Manager - Apple (Unite...
# Apple Site Security Manager Job Number: 42975010 Culver City, Califo ia, United States Posted: Oct. 2, 2015 Weekly Hours: 40.00 **Job Summary** The Apple Site Read more
WiSE *Apple* Pay Quality Engineer - Apple (...
# WiSE Apple Pay Quality Engineer Job Number: 44313381 Santa Clara Valley, Califo ia, United States Posted: Nov. 13, 2015 Weekly Hours: 40.00 **Job Summary** Join our Read more
Holiday Retail Associate with *Apple* Knowl...
…and assertive.Someone who can troubleshoot iOS devices (iPhone and iPad) and Apple Mail issues.Someone who can offer solutions.Someone who can work weekends.Someone with Read more
*Apple* Systems Engineer (Mclean, VA and NYC...
Summary:Assist in providing strategic direction and technical leadership within the Apple portfolio, including desktops, laptops, and printing environment. This person Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.