Linux Hierarchy in detail

1. What is linux? Linux is an operating system. An operating system is software that manages all of the hardware resources associated with your desktop or laptop. Thhe operating system manages the communication between your software and your hardware 7

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2. Linux hierarchy The following terms are encountered in describing file system directory contents: Static is content that remains unchanged until explicitly edited or reconfigured. Dynamic or variable is content typically modified or appended by active processes. Persistent is content, particularly configuration settings, that remain after a reboot. Runtime is processor system-specific content or attributes cleared during reboot. Important Red Hat Enterprise Linux directories: /usr: Installed software, shared libraries, include files, and static read-only program data. Important subdirectories include: /usr/bin: User commands. /usr/sbin: System administration commands. /usr/ local: Locally customized software. /tmp: 10 days validity /var Variable data specific to this system that should persist between boots. Files that dynamically change (e.g. databases, cache directories, log files. printer spooled documents. and website content) may be found under /var. subdirectories contain: /tmp: 30 days validity /etc Persistent system Configuration files specific to this system. /etc/fstab: persistent entry remains after reboot(static) /etc/seliux/config /run Runtime data for processes started since the last boot. This includes process ID files and lock files, among other things. The contents of this directory are recreated on reboot. /home Home directories where regular users store their personal data and configuration files. /root Home directory for the administrative superuser, root. /tmp A world-writable space for temporary files. Files which have not been accessed, changed, or modified for 10 days are deleted from this directory automatically. /boot Files needed in order to start the boot process. e.g /boot/grub  /dev Contains special device files which are used by the system to access hardware. Hidden “Dot” Files There are some files within the home directory that are ordinarily hidden.  Hidden files have names that begin with a period; hence, they have been given the nickname of dot files.  Hidden files are not displayed by the ls command unless the –a option is used in the format of ls –a. Since the default shell for Linux is the bash shell, the home directory contains the bash related scripts indicated below. Fle Description

File	Description
.bash_history	For users of the bash shell, a file containing up to 500 of the most recent commands available for recall using the up and down arrow keys.
.bash_logout	Script that is run by the bash shell when the user logs out of the system
.bash_profile	Initialization script that is run by the bash shell upon login in order to setup variables and aliases.  When bash is started as the default login shell, it looks for the .bash_profile file in the user’s home directory; if not found, it looks for .bash_login.  If there is no .bash_login file, it then looks for a .profile file.
.bashrc	Initialization script executed whenever the bash shell is started in some way other than a login shell. It is better to put system-wide functions and aliases in /etc/bashrc, which will be presented later in the book.
.gtkrc	GTK initialization file.  GTK+ is a multi-platform toolkit for creating graphical user interfaces, used by a large number of applications.  It is the toolkit used by the GNU project's GNOME desktop.
.login	The initialization script that is run whenever a user login occurs.
.logout	The script that is automatically run whenever a user logout occurs.
.profile	Put default system-wide environment variables in /etc/profile.
.viminfo	Initialization file for the Vim text editor that is compatible with vi.
.wm_style	Specifys the default window manager if one is not specified in startx
.Xdefaults & .Xresources	Initialization files for Xterm resources for the user. Application program behavior can be changed by modifying these files.
.xinitrc	The initialization file used when running startx, which can be used to activate applications and run a particular window manager.
.xsession	This file is executed when a user logs in to an X-terminal and is used to automatically load the window manager and applications.

Table 1: Common dot files for the bash shell

The list below indicates the dot files installed in tclark’s home directory. -rw-------    1 roseat   roseat        3773 Jan 13 21:39 .bash_history -rw-r--r--    1 roseat   roseat        24 Aug 18 11:23 .bash_logout -rw-r--r--    1 roseat   roseat        191 Aug 18 11:23 .bash_profile -rw-r--r--    1 roseat   roseat        124 Aug 18 11:23 .bashrc -rw-r--r--    1 roseat   roseat        237 May 22  2003 .emacs
-rw-r--r--    1 roseat   roseat        120 Aug 24 06:44 .gtkrc The following is the content of the .bash_logout file in tclark’s home directory, which is executed whenever logging out of the system.  It does nothing more than clear the screen upon logout. # ~/.bash_logout clear The following is the content of the .bashrc file from tclark’s home directory.  Shell scripts will be presented in a later chapter, but for now suffice it to say that the script looks for a file in the /etc directory called bashrc and execute it if the file exists. # .bashrc # User specific aliases and functions # Source global definitions if [ -f /etc/bashrc ]; then         . /etc/bashrc fi These files primarily affect individual users on the Linux system.  Next we'll look at the files which affect the entire system. Important System Files Everything in the Linux environment exists in files.  It makes sense for users to become familiar with some of the more important Linux system files.  Some of these are configuration files, others are devices which Linux makes available through files and some are executable programs. Table 2 below is by no means intended to be an exhaustive list of Linux system files; however, it is a good representation to give some insight into the inner workings of Linux.
File	Description
/boot/vmlinuz	The Linux kernel file.  File naming conventions may include release information
/dev/fd0	Device file for the first floppy disk drive on the system
/dev/fd0H1440	Device driver for the first floppy drive in high density mode, commonly invoked when formatting a floppy diskette for that density
/dev/hda	Device file for the first IDE hard drive on the system
/dev/hdc	Commonly, the IDE CDROM drive device file which often is a symbolic link called to /dev/cdrom, the real CDROM driver file.
/dev/null	A dummy device which contains nothing.  It is sometimes useful to send output to this device to make it go away forever.
/etc/aliases	Contains aliases used by sendmail and other mail transport agents. Whenever this file is changed, the newaliases utility must be run to notify sendmail of the changes
/etc/bashrc	Contains global defaults and aliases used by the bash shell
/etc/crontab	A parent shell script to run commands periodically.  It invokes hourly, daily, weekly, and monthly scripts.
/etc/exports	Contains a list of filesystems which may be made available to other systems on the network via NFS.
/etc/fstab	The file system table contains the description of what disk devices are available at what mount points.
/etc/group	Holds information regarding security group definitions.
/etc/grub.conf	The grub boot loader configuration file
/etc/hosts	Contains host names and their corresponding IP addresses used for name resolution whenever a DNS server is unavailable
/etc/hosts.allow	Contains a list of hosts allowed to access services on this computer.
/etc/hosts.deny	Contains a list of hosts forbidden to access services on this computer.
/etc/inittab	Describes how the INIT process should set up the system in various runlevels
/etc/issue	Contains the pre-login message, often overwritten by the /etc/rc.d/rc.local script in Red Hat and some other rpm-based Linux distributions
/etc/lilo.conf	The lilo boot loader configuration file
/etc/modules.conf	Holds options for configurable system modules
/etc/motd	This is the ”message of the day” file which is printed upon login. It can be overwritten by /etc/rc.d/rc.local Red Hat on startup.
/etc/mtab	Status information for currently mounted devices and partitions
/etc/passwd	Contains information regarding registered system users. Passwords are typically kept in a shadow file for better security.
/etc/printcap	Holds printer setup information
/etc/profile	Contains global defaults for the bash shell
/etc/resolv.conf	A list of domain name servers (DNS) used by the local machine
/etc/securetty	This file contains a list of terminals where root can login
/etc/termcap	An extensive ASCII text file defining the properties of consoles, terminals, and printers
/proc/cpuinfo	Contains CPU related information
/proc/filesystems	Holds information regarding filesystems that are currently in use
/proc/interrupts	Stores the interrupts that are currently being used
/proc/ioports	A list of the I/O addresses used by devices connected to the server
/proc/meminfo	Contains memory usage information for both physical memory and swap
/proc/modules	Lists currently loaded kernel modules
/proc/mounts	Displays currently mounted file systems
/proc/stat	Contains various statistics about the system, such as the number of page faults since the system was last booted
/proc/swaps	Holds swap file utilization information
/proc/version	Contains Linux version information
/var/log/lastlog	Stores information about the last boot process
/var/log/messages	Contains messages produced by the syslog daemon during the boot process
/var/log/wtmp	A binary data file holding login time and duration for each user currently on the system

Table 2: Representative list of Linux files

Important Directories

/etc/rc.d	Contains subdirectories which contain run level specific scripts
/etc/rc.d/init.d	run level initialization scripts
/etc/rc.d/rc?.d	Where ‘?’ is a number corresponding to the default run level. Contains symbolic links to scripts which are in /etc/rc.d/init.d. for services to be started and stopped at the indicated run level.
/etc/skel	Holds example dot files used to populate a new user's home directory.
/etc/X11	Contains subdirectories and configuration files for the X Window system