Files and Directories
Linux, like all other computer operating systems, maintains files within directories, but the hierachy is probably more structured than other operating systems and is rigidly enforced.

Directory	Definition
/bin	Operating System user commands.
/boot	Operating System boot files (including kernel).
/dev	Device files.
/etc	System configuration files, such as fstab.
/home	User directories.
/lib	Library files and directories.
/mnt	Local mounts.
/opt	Applications.
/proc	Operating System procedure files.
/sbin	Minimum commands needed to boot the system and mount other file systems.
/tmp	Operating System temporary files.
/usr	Shareable operating system commands, libraries and documentation.
/var	Dynamic information such as logs and spooler files.

Files and Directory Security
With Linux, all directories and the files within them have three levels of operation and three levels of access:

Operation		Access
Read	(r)	Owner
Write	(w)	Group
Execute	(x)	Everyone Else

The default depends on other environment variables but starts off as Read and Write for Owner and possibly Read for the Group. This means that the security flags for a folder or file must be set correctly before you will be allowed to access them. If you use the ls -l command you will see a list of files for the current directory, where each line begins with a collection of -, d, l, r, x or w values, which are the security rules for each file. Apart from the first character, the others are arranged in group of three pertaining to the owner, the group and everyone else.

For example:

 -rwxrw-r--     1 wally    users     175 November 25 2004 example.txt

• the first "-" indicates that it is a file; if this value was a "d" then it would be a directory (or folder)
• the next three characters "rwx" indicate that the owner, i.e. the name shown just to the right (wally), has read, write and execute/search privileges on the file
• the next three characters "rw-" indicate that the group, i.e. the name shown to the right of owner (users) has read and write privileges only on the file
• the last three characters "r--" indicate that everyone else has read only privileges
• the next value (number 1 in the above example) is the number of files - more meaningful for folders
• the next value (wally in this example) is the owner of the file as previously mentioned
• the next value (users in this example) is the group associated with the file
• the next value (175 in this example) is the size of the file in bytes
• the next value (November 25 2004 in this example) is the last modified date
• the last value (example.txt in this example) is the directory (folder) or file name

• Let's say we wanted to give the group execute access then we could type

  chmod g+x example.txt

  this says to add (+) execute (x) to the group (g) for file example.txt

• Let's try again, this time we want to remove read access from everyone else, in which case we would type

  chmod o-r example.txt

  this says to remove (-) read (r) from other (o) for file example.txt

• Or irrespective of what the value is at present, give read, write and execute access to the owner we could use

  chmod u=rwx example.txt

• Finally to give each level the same value, we could use

  chmod a=rw example.txt

    0400  (= u=r)  Read by owner
    0200  (= u=w)  Write by owner
    0100  (= u=x)  Execute (search in directory) by owner
    0040  (= g=r)  Read by group
    0020  (= g=w)  Write by group
    0010  (= g=x)  Execute/search by group
    0004  (= o=r)  Read by others
    0002  (= o=w)  Write by others
    0001  (= o=x)  Execute/search by others
    

• Instead of chmod g+x example.txt we could use chmod 774 example.txt - this effectively sets all of the values for file example.txt in one hit, so it is important to get it right.
• Similarly, instead of chmod o-r example.txt we could use chmod 760 example.txt
• Again, instead of chmod u=rwx example.txt we could use chmod 764 example.txt
• Finally, instead of chmod a=rw example.txt we could use chmod 666 example.txt

There MUST be an easier way!
Now that you know about all of the gory details, you need to know that there is an easier way. Probably the easier way to manipulate files and folders is to use Konqueror, which is a file browser installed with KDE, and can be run from the menu within KDE (Debian Menu > Apps > System > Konqueror. This can be run from any user, but you may need to enter the root password to use the program. When the program runs you will be greeted by a screen with a single icon labelled Desktop. If you click on one of the icons there using the right mouse button you will see "Properties" at the bottom of the menu. If you select this using the left mouse button another window will appear. The General tab is displayed which shows the file or folder name, it's size and location etc. If you click on the Permissions tab, you will see a matrix of boxes, some of which will have x's in them corresponding to the rwx values for each level as previously discussed. If you have the necessary permissions you can simply click on a box to turn the option on or off, and click on OK when you have finished. There are some boxes there you probably don't understand at present, so don't fiddle with them until you do. You have been warned! One extra box that is worth explaining though is the box at the bottom labelled "Apply changes to all subdirectories and their contents" - this does exactly what it says and can be very useful.

More to it than Meets the Eye!
As well as this structured approach to file and folder access, Linux holds a few Aces up its sleeve, in that some folders are restricted to root only, irrespective of what security options they have set, and these cannot be changed. Moreover some of the more powerful commands are stored in such directories so that they can only be accessed by the root user. This means that there are going to be times when you have to log in as the root user to execute such commands etc, for example anything stored in an sbin directory is restricted to the root user irrespective of where it lives, although they can be executed by other users using a pseudo login which requires the root password to be used as with kuser and konqueror above, but even then they will not necessarily have full rights.