Files, Directories and inodes

Files

Linux is very similar in many ways to UNIX. One feature that it shares is that in general, everything in Linux is a file, although some of these files are a bit special. Most files, or regular files as they are called, just contain normal data like text, programs or images. Special files include

• Directories: files that contain lists of other files
• IO files: are used for input and output devices and are held in the /dev directory
• Links: are pointer files used to make a given file or directory visible in a different part of the file tree.
• Domain sockets: communication sockets that provide inter-process networking
• Named pipes: a bit like sockets but they provide an easier way for inter processes communication.

File names are best kept to 80 characters long or less though there is more or less no physical limit. File names are case sensitive, and can include special characters.

i-nodes

We normally think of the file system as being like a tree, with a single ‘root’ directory that has directories attached to it, then subdirectories to those and eventually terminating with lots of ‘leaves’ or files. This is a familiar concept from the DOS days, and carried on through UNIX, Windows and Netware. A Linux system is also usually drawn like a tree, but in actual fact it is just a collection of pieces of data with links between them. The data is identified by inodes, just like UNIX. An inode is a bit like a label that is attached to a file. An inode label has a unique number within each partition and also contains

• The file owner
• The file type as categorised above
• The access permissions on the file
• The creation, last update and last accessed date.
• The date and time of last change to inode information
• The number of links to this file
• The file size
• The physical location of the data.

You can display the inode number for a file with the ls command, usually by using a –i switch.

ls –i 

When you create a partition, you set aside part of that partition space for a fixed number of inodes, and the inode numbers are unique within that partition, but duplicate inode numbers can exist between partitions. Every time you create a file, you assign a free inode to that file. The file system can build up a tree picture of the file structure by piecing together the file names and inode numbers (the inode does not contain the file name and directory, these are held in the directory files).

Directories

Files are organised into directories to make it easier to find things, and to keep common files in a central location. The lowest level directory is the root directory, denoted by a / (forward slash). A number of sub-directories will exist below the root; the common or standard directories are shown in the table below, but be aware that the names of some of these directories can be changed.

You can add your own directories to the root, but its best to add them as subdirectories under the appropriate main directory. Directory names are case sensitive, must be unique within one directory and can contain almost any character.

There are lots of GUI style file managers available for Linux that have the same look and feel as Windows Explorer. Popular products include Nautilus, Conqueror and Midnight Commander. The Linux Command section discusses the line commands to display files and directories

Paths

Say you develop a program called ‘space.exe’ in your /usr/storage/bin directory that goes out and checks on how full all your disks and partitions are, and reports the results back to your terminal. To execute that program you might have to type /usr/storage/bin/space which is a pain because it requires extra typing and you have to remember where the program is. LINUX stores lists of program locations in a PATH variable. To see what PATHs are defined type

>echo $PATH

And the result will look like

/usr/local/bin:/usr/storage/bin:/usr/bin:/usr/sbin/:/bin

LINUX will search these directories in order when a command is entered, and will execute the first command it finds, so if you type ‘space’ on a command line and there is a space program in /usr/local/bin then that version of space.exe will be executed, not the version in /usr/storage/bin

Each user could have a different path variable. The ‘which’ command will display the full path name for a command,

Which space
Space is /usr/local/bin

Identifies that the wrong program is being called.

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