5.1.1. General features of Debian OS

When the computer starts up, after loading procedures kernel, scripts are executed sequentially, which contain groups of commands in order to carry out tasks necessary to bring the operating system to the appropriate level of usability (runlevel). Mention 'single-user mode' is one of the 8 possible levels of usability, in which a computer can work as follows:

• 0 - ' Halt the System ' - shutdown of the computer.
• 1 - ' Single user mode ' - a higher level of ' S ' level for the root user, some scripts are run.
• 2 - ' Multi-user runlevel ' - more limited user mode without networking capabilities.
• 3 - ' Multi-user runlevel ' - complete multi user mode with networking capabilities without the graphics.
• 4 - ' Multi-user runlevel ' - not used.
• 5 - ' Multi-user runlevel ' - complete multi user mode with networking capabilities and graphics.
• 6 - ' Reboot the System ' - re-start the computer, the repeated start without physical exclusion.
• S - ' Single user mode ' - the lowest initial level only for the root user, the scripts do not run.
• s - same as ' S '.

When you turn on the computer goes through a ' Single User Mode ' in which to test the integrity of the partition with ' fsck ' command. If you do not pass the test successful computer stays at that level. If the computer is used as a workstation user configurable to be activated so that the graphics support switch on, and if it works as a server is not essential to include graphics support to the computer, and service charges that are hardly used. To computer came to the fifth level of the work must be pre sequentially pass modes with a lower number. Who will be the last mode automatically achieved by the inclusion of a computer depends on its configuration, which can easily be changed because all ordinary text configuration files to ASCII code table. Therefore, the user should be familiar editor for this purpose (basically the Vi editor), although the graphical environment allows that some adjustments may be made through him, that looks like a Windows OS, but in essence once again intervene in the text configuration files.

Compared to the Windows XP system, for example, in Linux there is only one administrator (ROOT, SUPERUSER) and no user logged on it can not be declared for the administrator. Has been added to the system can not become ROOT but may only use powers that have ROOT. Can be define which users are allowed to get administrator privileges and with the safety aspect, this concept of the good. As the ROOT user system can be accessed only from the console.

If a user is accessing a computer in text mode, the intermediary between him and the operating system is a SHELL that is COMMAND INTERPRETER which are manifestations of view similar to those of DOS. The shell is not an operating system and can be necessary to install more than one system. Besides, it can be learned from one shell to another of the same or different characteristics and capabilities. Basically if you are working from the first shell starts to work in another shell first shell is inactive and waiting to return to her, or interruption in the second shell, leaving it and thus its extinction. Primary working Linux shell is BASH (Bourne Again SHell), and others if need appeared to be subsequently installed.

If you will be using console or terminal emulator to access the operating system regarding its monitoring and configuration should be able to use the operating system commands and know their syntax. The basic syntax of the command and its execution is identified by:

• PROMPT - A place where the user is currently residing (working directory) and where the command is issued:
  
  Computer_Name:/directory-1/directory-2/...../directory_N$
          or
  Computer_Name:/directory-1/directory-2/...../directory_N#
  
  Character ' $ ' in a prompt indicates that the system is active on a ordinary user with limited privileges on the system, created during the installation process or later OS (e.g. drago).
  Character ' # ' in the prompt indicates that the system is active ADMINISTRATOR-SUPERUSER (ROOT), which has unlimited power to the system.
  
  
  • Initial directory of ordinary user e.g. [COMPUTER_NAME:/home/drago/] or shorter [COMPUTER_NAME:-] with which always begins when a user log-in into the system.
    
  • Initial directory of administrator is [COMPUTER_NAME:/root] or shorter [COMPUTER_NAME:-].
  • Character ' - ' indicates the presence of a user or administrator to the associated their starting (home) directory.
  • Root directory of system is [COMPUTER_NAME:/], thus starting all distribution systems.
    
    
  Prompt likely not be the same for the working environment (working shell - SHELL)
  
  
• COMMAND_KEY_WORD (KW) - Unique word which calls for execute a binary file from the working directory or the directory where the file is located.
• OPTIONS - Separated by space and KW form controls behavior. Preceded by a ' - ' or ' + ' or ' -- '.
• ARGUMENTS - Options are separated by space and indicate to whom it applies KW.

Based on the above, the FILE SPECIFICATIONS in the Linux operating system can be defined as follows:

In contrast to the specifications in the DOS disk partitions are not relevant, separating directories is done with a ' / ' and the file may or may not have a label type. File_name should not have more than 100 characters, so you do not make a mistake with some commands. If the name of the file (or directory name) consists of several words instead of good spacing between them use the character ' _ '. Of compatibility with other operating systems are not recommended total length specification files larger than 255 characters. The file name (or folder name) must not be used to characterize uses file system ( /, :, *, ? ... ), And does not recommend the use of the grapheme of localities. The general recommendation is that the file names and directory names using only letters and numbers in the group of the first 128 characters of ASCII code.

Prompt and cursor, as well as command line syntax Windows system shown in Figure 4.5.28a are very similar to Linux. According to the above simplest syntax commands use the Linux operating system would be:

# KW -option argument

           or

# KW -option -option argument

           or

# KW --option argument

How Linux OS distinguish between uppercase and lowercase letters, it should be about how is written KW and how to write options because there are a small and capital letters but with different effects and take account of the intervals by the syntax. Options are usually supplemented by a letter KW although there are multi-letter options as one word. Single-letter options can be strung together one after the other immediately after a ' - ' but only if used letters do not match with the name of multi-letter options. Then the single-letter options must be written individually. Multiple-letter options markets that precedes ' -- '. Sign ' + ' has reversed the effect of the ' - '.

One of the important features of the Linux OS that controls may CONCATENATE, so the result of one command to the other, which again received from the output of the first command can send your score to the third and so on. More on that in the analysis of log data, and analysis of files in which records everything that happens on the system.

The following example is the default command ' df -k ', which gives an overview of the busy individual partitions corresponding to the planned table set in the installation process (if it is well planned?), And the command ' cat /etc/fstab ' displays the contents of the fstab file that describes that drive apart and the properties and the type of individual partitions, and are associated with anything (mount point) in order to be reachable.

Figure 5.1.1 Shows the disk partition in two ways.

It has already been stated, and with the previous image is evident that all the disks or their partitions 'hooked' directories, of course, with some of them associated subdirectories and files, and to 'move' the file system is done just by switching from one directory to another. Partition ' hda1 ' is virtual memory (swap), and after the first command does not display because it is not part of the file system. Partition ' hda3 ' primary partition which is 'container' for other logical partitions and neither it does not appear, as shown in Figure 5.1. In addition to the keywords section character ASCII table must not be used in placing the names of files and directories, as follows:

                     * ? < > / ; & ! | \ ` ' " [ ] ( ) { }

The names of files and directories usually are given to describe what they contain because it is possible to use up to 255 characters. One should bear in mind that instead of spaces between words using the character ' _ ' and that name does not begin with the characters ' + ' or ' - '. Uppercase and lowercase letters are different and hence the name 'Drago' and 'drago' are not the same concepts, and files starting with a dot ' . ' hidden meaning that they will not recognize any commands. Views that give the command ' dir ' and ' ls ', which are used to view the contents of the disc, best illustrate the diversity of views of certain controls and effect options for the same purpose.

More about each command can get quick help if it is provided by Form ' KW --help ', but much more to learn from the help pages per form ' man X KW '. Descriptions which can fetch command ' man ' is divided into nine sections that belong to the ' X ' in the above mentioned form:

Not all sections contain descriptions that refer to the command that is to be studied in greater depth. Additional very thorough description can be obtained by the command ' info ' pattern ' info KR '. In addition to the above documentation is available that describes some aspects of configuring an operating system that is located in the directory [/usr/share/doc/HOWTO/en-txt] in which the .gz format are text descriptions of specific procedures. But it would be very inconsiderate not to mention two important ways that enrich all of the user's understanding of the operating system in addition to the above, and that are INTERNET and GOOGLE, the inexhaustible source of new knowledge, not only in this matter but also in general.

So that users can access system resources and use them by default is that the system must be made-open USER_ACCOUNT (account), or otherwise called ACCESS (Log_IN), which will create an administrator, which consists of:

• USER_NAME - User Identifier in the institution, which is associated with its unique UID (User Identifier), a numerical value of 16 bit (with some 32-bit system). User_name not contain name or surname and it's a TITLE that basically do not contain graphemes sites and can be any combination of letters and numbers. It is recommended that the TITLE not be longer than 8 characters.
• PASSWORD - Protection mechanism of USER_ACCOUNT which comprises 8 to 16 characters for the user who initially creates supeuser and this is a record that is kept encrypted. It's good to be a combination of uppercase and lowercase letters and at least two numbers and that does not exist in any dictionary. You need to use the names of sites, similar mix of letters and numbers similar to, for example '111keTOeat' (I like to eat).

The system can not be two users with the same USER_ACCOUNT. Password protection mechanism is reliable as long as you wish, it should be changed often, and can be used once and mechanism usable passwords like 'token'. This mechanism should be required to use SUPERUSER (su). Except users USER_ACCOUNT have some services on the computer even though not included in the 'regular' users. Most of the services has its own user_name that operate using (as mail, news, IRC ...). We do not need to live in delusion, nothing is perfect and neither the protection of the 'intrusion' naughty, but at least they should make life difficult, maybe give up.

The following Figure 5.1.2a shows which contains the initial (home) directory ROOT users and to see the contents of the directory command to display the directory structure with and without options, and present the ways to get to the root directory (cd /), or to the 'top' of file system, or to the user's initial directory (cd -)), which is typically composed of [home] directory. Figure 5.1.2b shows all of which made the Linux operating system. Some of the directories are related to the partition, and the rest in one of them. More on that in the following chapters.

Figure** 5.1.2 File list of user, ROOT and system / Graphical environment. ( + / - )

In Figure 5.1.2a shows the contents of the (home) directory of users 'drago' and ROOT. The command-line operation are two important concepts: PROMPT and CURSOR. The first shows where the user is currently located, and the other is flashing bars indicating willingness to accept (typing) controls. Figure 5.1.2b shows the composition system inspection from the roots of the tree structure of the operating system. Figure 5.1.2c not belong further described virtual machine has already been taken as an example of the virtual machine when you install all facilities labeled according to Figure 5.5c. Above leads to the conclusion that the installation of the graphics support significantly 'enriched' operating system, and if the server does not require graphics support does not need to even install it, but we should know the work in the command window.

Upon closer examination 5.1.2b image content can be seen that each file or directory name is preceded by 8 columns that describe the following:

1. Description of the file type, and the rights to the file is shown as one-letter designation for the right to read (r), write (w) and executed (x) by the interest groups. If the mark shown right is feasible.
   • The first tag describes the type of file; regular file is labeled ' - ', the letter ' d ' indicates that it is a directory, the letter ' l ' indicates a symbolic link, and for various types of files are used to tag along with a description specified directory [/dev] .
   • I group rwx tags - refers to the file owner and what he can do with it.
   • II group rwx tags - refers to a group in which the user may or may not be the owner.
   • III group rwx tags - applies to all users who have an account on the server as the ANONYMOUS user, let's say that the file access from the Internet.
   So, for example, files can have an tag ' _rwxrwxrwx '. The colors show the extent to which it is subject to misuse. Why the user is susceptible to abuse? Because it is essentially a dowdy and does not lead to anything that is in its user space is not the occasional change of passwords and if a few of his data, there is something 'others', which allows a malicious intruder to take control over the system - become superuser. Some administrators to users who do not use the terminal-emulator simply disable SHELL.
2. The second column indicates with how many different places file (or directory) can 'fetch'. For example, in another directory is a link (link) that points to the physical location of the file shown in the review (something like SHORTCUT for Windows OS). Should distinguish symbolic link that is only a guideline and that the file does not have the same name as the file from direct connections (hard link), which is shown in the original file name. Deleting symlinks original file remains on the system, and in the second case, the file is deleted, which is very dangerous when it comes to connectivity to the directory. After deleting files no return, except for restoring data from BACKUP tapes or other media that are used for this purpose.
3. The third column lists the name of the file owner. The owner does not have to be ROOT or another user, but it can also be a SERVICE or DAEMON (a kind of program support) that uses some kind of name. The difference between them is that the DAEMON developed that basically has not been launched from the command interpreter (shell) and is not associated with a device in principle its work never stops.
4. In this column specifies the name of the group to which it belongs directory or file. Normally the group has the same name as the owner, but can be set differently. Group may have a name that is associated with a service that is not the name of a physical user does not have to have a name by service name but the name somewhere safe default. Group of users can join in the response configuration file.
5. File size is expressed in kB, but can be displayed and some other value in other units of measurement, which depends on the applied command options and arguments.
6. Month when the file (directory) created in the current year.
7. Date when the file (directory) created in the current year.
8. The time when the file (directory) created in the current year. If the file is created in one of the previous calendar year, then this is the place shows that year.

Example I

What rights do users of system directory [/etc] from the image file and 5.1.2b [~/Xauthority] at picture 5.1.2c?

Directory [/etc] in the first column has the tag 'drwxr--xr-x'. The first letter represents a directory in question and the third and the fourth column shows that its owner a superuser (root) and belongs to the group 'root'. Superuser has the rights to write per file and read the contents of the directory to run the execution (the next three letters - drwxr--xr-x). With superuser in his group, which means there is no one that he was the undisputed master of this directory. How are the configuration files have the right to read the other groups (three letters after superuser rights - drwxr--xr-x) and the users (the last three letters - drwxr--xr-x), and that there needs to be a single file in the directory must be able to perform, certain contents of the directory will be allowed to perform any of the group and the individual and from any group, and from a user. User group, and other users can not write anything on these facilities.

Xauthority file has a tag-attributes '-rw-------'. The first character has no meaning in terms of files, the user is the owner of her and belongs to his group while all the rest have no rights to it. The file is created in the user's initial directory [/home/drago/] which recognizes and record the prompt. Since this is the configuration file does not need to be executed by the user, since it serves to a certain software only read. None other than the operator of this file is not exercised any rights. No user settings are written in it at the sole discretion may change. Of course, the superuser can work with this file whatever he wants.

Directory Linux as opposed to Windows system has its own size. Directory and files are identified by their identifier that creates a file system which assigns a file name or directory. Directory is nothing more than a file that contains a list of identifiers which is attached to the file description, and that coupling to the files that are in the list. The figure in front of the book is not the sum of sizes of all files in a directory, but the file size that contains the list. Similarly, the files for the drivers. They do not contain some binary files but indicate a place where they call from the kernel and possibly the fact that the module connected. Executable file is in turn a set of binary records which processor or command interpreter can execute. So, in the Linux operating system everything is files. Disk devices and partitions are identified with a name and a directory structure for hierarchical distribution uses a ' / '.

Debian filesystem in core contains support for multiple file systems that are not subject to copyright other manufacturers: ext3, reiser, xfs and jfs. File systems of the same type configured with different parameters or different file systems by type, again by adjusting certain parameters that allow each partition by peculiarities largely adapted to the task for which it was intended. In this way, the operating system becomes more efficient. Due to copyright in the core can not be incorporated support for Microsoft FAT 32 or NTFS file system or a third-party software, but available as external modules that enable you to work with them.