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PHP Foundations Working with Permissions in PHP, Part 1

by John Coggeshall

In the past few columns, I have been discussing using PHP's file I/O capabilities for manipulating both files and directories. This week, we'll take a slight detour from a strictly PHP-related subject and discuss file permissions in Unix systems. If you are using PHP in a Windows environment (or other environment without a permission system), this column may not apply to you.

How Permissions Work

Before we can explain how permissions can be used from within PHP applications, you'll need a little background on how permissions work in general. Although today's column only discusses Unix permission-related commands, these commands directly relate to their PHP counterparts discussed in my next column. If you haven't ever really worked with the permissions system in PHP (or need a refresher) read on.

In a Unix environment, all files and directories are owned by two different entities -- a user and a group. (A group represents multiple individual users.) Likewise, each file in the file system has three different permission sets which determine who can access a particular file or directory. Specifically, every file in a Unix system has the following permission sets: user-level, group-level, and global-level.

For each permission set, three different flags exist: read, write, and execute. If a particular user does not have the read flag set, he will be unable to read the desired file (or the files in a directory). Likewise, if a user does not have the execute permission on a file, she will be unable to execute that program. When a user creates a file, that file automatically is owned by the user and group to which the user belongs. In order to change the owner of a particular file the chown Unix command is used as follows:

[user@localhost]$ chown theuser thefilemask

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where theuser represents the username to change the file mask specified by thefilemask. Please note that this command can only be executed by a user who has super-user privileges (such as root).

Changing the group to which a file belongs to is done via the chgrp Unix command. Unlike chown, which requires super-user privileges, chgrp can be used by any user. The one restriction that applies is that chgrp will only allow the user to change the group of a file as long as the user belongs to that group. For example, a given user who belongs to the groups foo and bar can change the group of a given file to either foo or bar but not foobar -- because he does not belong to that group.

As I mentioned, for a given file there are three different permission levels that apply to each file and directory: the user-level, group-level, and global-level. Each level is independent of the other, and is used to permit read, write, or execution access for the given file. From a Unix console, one can see the owner, group, and permissions assigned to these three groups by executing the ls (list) command in a given directory and specifying the -l (long) tag as shown:

[user@localhost]$ ls -l
rwx-w-r--    4 php      mygroup          4096 Nov  7 15:52 mydirectory

In the above example, the directory mydirectory is owned by the user php and belongs to the group mygroup. The string drwx-w-r-- identifies the permissions.

If the permission has been granted (read, write, or execute) then that letter will be displayed for the particular group. Otherwise, a dash is shown. Thus, in the example above, this particular file has been given read, write, and execute permissions for the owner of the file (the user php). However, those who belong to the group mygroup can write to this file, while the remainder of people (global) can only read the file. The one flag that hasn't been identified yet (the first character, d) identifies this particular file as a directory.

Although permissions are fairly simple for normal files, they take on a slightly different meaning when applied to directories. Specifically, read permission is required in order for a user to view the contents of the directory. Write permission allows a user to create or remove files within the directory. Execute permission is required in order to access the directory at all. Note that a user with write permission to a directory will be able to delete any file in that directory, even if she lacks write permission for that file.

So how does one modify the permissions of a file? Unix permissions are handled through a command called chmod:

[user@localhost]$ chmod 755 thefilemask

In the above example, 755 is the numeric representation of the permissions to set, and thefilemask is the file mask of the affected files. Note that only the owner or a group member may modify the permissions of a file. There are two different ways to assign or to revoke permissions for a file -- one text-based and the other numeric-based. Because PHP does not provide means to modify permission values using the text-based method I will only discuss the numeric method.

The permissions of all of the permission groups can be represented by different numeric values. Added together, this represents the complete numeric permission value. The values of the different permission levels are:

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ValuePermission Level
400Owner Read
200Owner Write
100Owner Execute
40Group Read
20Group Write
10Group Execute
4Global Read
2Global Write
1Global Execute

In order to give read and execute permission to the file's owner, write permission to the group, and read permission to everyone else (global) the permission value would be:

  400Owner Read
+ 100Owner Execute
+  20Group Write
+   4Global Read
= 524Total Permission Value

Applying these permissions to the file is as simple as using the chmod command:

[user@localhost]$ ls -l
-rwx-w-r--    4 php      mygroup          4096 Nov  7 18:52 myfile
[user@localhost]$ chmod 524 myfile
[user@localhost]$ ls -l
-r-x-w-r--    4 php      mygroup          4096 Nov  7 18:60 myfile

PHP Returns Next Time!

That's it for today's column. Although no PHP commands were actually discussed, having a reasonable understanding of the Unix permission system (especially when working with files) is critical to PHP applications. Without being familiar with this subject it is very easy to open up your scripts to malicious users. In my next column, I'll take the Unix commands discussed today and apply them to the counterpart PHP functions.

John Coggeshall is a a PHP consultant and author who started losing sleep over PHP around five years ago.

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