Unix / Linux Fundamentals

1. Linux Desktop Usage

2. Linux Desktop Usage
• Upon boot-up completion of a Linux system, a “login” prompt
will be displayed; username is to be input here and the
password in the “password” prompt.
• Note that no marks at all are written into the prompt line
when typing in the password, that is normal.
• A successful login will result in receiving the shell (Linux
command line interface) prompt; the default shell in many
modern Linux distributions is Bash (Bourne Again Shell) which
is a newer and improved version of ‘sh’.
• There are several other alternatives for Bash
 TCSH
 CSH
 KSH

3. Logging into Linux
• The login process is the identification and authorization of a
user in the system.
• Multiple users are allows to log into the same system
simultaneously.
• The username and password provided are checked, by
default, against these three files:
 /etc/passwd: this file keeps a listing of all the currently existing users
on the system; users and their properties, such as home-directory
location and default shell can also be edited in this file.
 /etc/shadow: this file encrypts and contains the passwords to all of
the system’s users; it can also be used to manage password properties
such as setting the interval in which the password needs to be
changed, make the password inactive, etc.
 /etc/group: this file lists and manages the groups on the system and
the assignment of users into groups.

4. Logging into Linux
• Logins to the system can be performed on the local machine
either in Command Line or GUI mode and Remote Terminal
software such as PuTTY
• When a user logs in from a terminal software, whether
remote or local on the machine, a “pseudo-terminal” is
activated and is represented in the system as pts/0, pts/1,
pts/2 and so forth, depending on the amount of sessions
open concurrently.
• The pseudo-terminal is a piece of software that emulates old
keyboard and screen terminals in order to allow full support
on both modern and old systems, such as UNIX.

5. Running Shell Commands
• Linux(UNIX) is case sensitive. Most command names are
lower case
• Commands starts at the beginning of the line
• Commands and arguments are separated by space(s)
Commands are terminated by <CR> (The value of Enter)
• In most cases, the location of each argument is not relevant
and single character arguments, can be merged
$ ls
$ ls –l -r -t
$ ls -t -r -l
$ ls -ltr

6. Running Shell Commands
• When a command is running, use the control sequences to
manage it’s runtime
– CTRL-c Terminate the running program
– CTRL-z Suspends the running program
– fg makes suspended program return
– bg makes suspended program return and send output to the back-
ground
– jobs list all currently running jobs on the current terminal

7. System information
• Several basic commands can provide information about which
system you are connected to, which account you are using
and what is your system type
– uname [option(s)] print system information, such as name, type,
version and hardware architecture
– id [option(s)] print information about the connected user account
– tty print the terminal name for this session
– who print the list of active user sessions

8. The Shell Prompt
• A standard shell prompt looks like this:
 [user1@CentOS-LAB ~]$
• The first section is the username of the user currently logged
into the shell, in this case it is “user1”.
• “@” separates the username and the hostname of the Linux
machine, in this case the hostname is “CentOS-LAB”.
• “~” report the current working directory
• “$” represents a regular user
“#” is used when you are logged-in as the ‘root’ user
• The prompt can be customized and even colored.

9. Switching Users
• In order to switch between users in an active system that we
have already logged-in to, we’d use the “su” command, for
example to switch from the user “user1” to the administrative
user “root”, we will run the following command:
 $ su -
Password:
#
• When running “su –” without specifying any other username
to switch to, the command defaults into switching to the user
“root” and prompts for the root password.
• Once the root password has been successfully provided, the
user has been switched to “root” and the user now has all the
administrative privileges of “root”.

10. Switching Users
• Switching to another regular user is done the same as
switching to root with the addition of the desired username in
the command line:
$su - test
Password:
$
• As the example above shows, the user “user1” has now
switched to “test”, any action performed on the system now
will be logged as user “test” and either allowed or denied,
depending on that user’s permissions and privileges.
Note: While logged-on as “root”, the “su” command can be used
to switch to any regular username in the system and no
password will be asked since “root” is the all-mighty.

11. Editing Text Files
• Some of the common text editors for Linux are:
 VI
 VIM
 EMACS
 Nano
• VI is the default editor for Linux and UNIX systems
• On newer distributions we are likely to find VIM, which is “Vi
Improved”. Newer and better version of VI including features
like syntax highlighting

12. Editing Text Files
• For this example, we will edit a file named “testfile” which
resides under the “/tmp” directory; the example will also use
VIM as the text editor of choice.
• First, we run VIM while defining which file we wish to edit; if
the file does not exist in the specified location, VIM will create
a buffer in which all the new text we type is kept until we
actively save the contents into an actual file, however if we
do not save the contents, they will be lost upon quitting VIM.
• Run the following command to enter VIM:
 $ vim /tmp/testfile

13. VIM Command/Navigation
• VI has three operation modes
– Navigation: In this mode we are able to navigate between words and
lines in our text file; navigation in VIM is performed using the arrow
keys
– Insert: In this mode we can add, remove and edit text
– Command: VI Command Line
When in Navigation mode, use “CTRL” and the right or left arrow
keys to jump whole words right to beginning of the next or
previous word

14. VIM Command/Navigation
• VIM has many different commands and shortcuts that can be
used while editing to make the process faster and easier for
the user; here’s a short list of basic actions:
 e: jumps to the end of the current word.
 b: jumps to the beginning of the current word.
 0: jumps to the beginning of the current line.
 $: jumps to the end of the current line.
 H/M/L: jumps to the first, middle or last line of the currently viewed
page on screen, respectively.
 :<n> : jumps to line number <n> in the file, for example :10 will jump
to line number 10.
 u: undo the last action.
 CTRL-r: redo the last action.

15. VIM Insert
• Insert mode: accessing insert mode in VIM can be done in a
few ways:
 Hitting the “Insert” keyboard button.
 Hitting the “i” keyboard button.
• Once in insert mode, the word “– INSERT –” will be written at
the bottom of the screen, this is the marker you are now in
edit mode and can type in any text you wish as plain text.
• Insert mode has another sub-mode which is called “Replace”;
hitting the “Insert” key a second time while in insert mode
will go into “Replace” and “– REPLACE –” will appear in the
bottom of the screen, this mode allows overwriting any text
in the position of the cursor.

16. VIM Save / Quit
• After the file has been created or edited it needs to be saved;
in order to do that, we first must return to
command/navigation mode by hitting the “ESC” key twice,
the INSERT or REPLACE marker will disappear from the
bottom.
• While in command/navigation mode, hit : and a marker will
appear in the bottom, this initiates and command line mode
VI commands:
 w save changes to file
 q quit VI
 wq
In case you want to do something that VI recommends against, use the
optional ! to force your command

17. Linux Help & Documentation
• The vast majority of Linux packages/applications arrive with
built-in documentation.
• There are number of ways to view different types of
documentations of an application:
 The info command: displays general information about the
application, for example run: info ls
 The man command: displays the manual and usage document of an
application, this document usually describes what the command does,
breaks down and explains in-depth each of its options, flags and
arguments and in many cases also provides syntax examples to that
command; for example: man ls
 The –help / -H argument: almost every comamnd in Linux has a set of
options / arguments it can receive; one of these options is –help or –H
which will display a summary of all the possible options that command
can take, right in the command line. Try running: ls --help

18. Linux Help & Documentation
Directories and files under ‘/usr/share/doc/’. Most
application has many optional documentation, examples
and README files under this folder. This can help you find
answers you can not find in ‘man’ or ‘info’

19. Linux Help & Documentation
• Reading the manual and documentation usually includes
understanding the Syntax declaration of a specific command.
When reading a Syntax declaration, it is highly important to
notice the different signs and characters and understand their
meanings:
mkdir [-pv] [-m mode] directory_name ...
– The first words stands for the commands name.
– ‘[ ]’ - Optional flags/parameters to the command
– param - Mandatory parameters
– … - The last parameter can be repeated multiple times

20. Linux Help & Documentation
• ‘man’ is of the most power-full assistance a Linux user can
have. It has all of the information needed in order to run the
different commands, all updated to the current version of the
Operating System
• Use ‘man -k {keyword}’ to find all manual pages which
contains ‘keyword’ in their name or description
• When viewing a manual page, the most important thing to
notice is the ‘section’ in which this page is.
Review the first (upmost) line of the manual page to get the
section number and name. e.g.
“PASSWD(1) BSD General Commands Manual PASSWD(1)”

21. Linux Help & Documentation
• When running ‘man’ use the following syntax to choose a
section: ‘man [section_number] {manual_name}’
• The ‘whatis’ command, can be used to get the starting lead
when trying to figure out what a command does
(in case you know the exact command name)

22. Linux Help & Documentation
• Aside from the built-in documentation, there is a great deal of
information online on Google, Wikipedia and numerous other
websites, such as:
 www.linux.com
 www.linuxforums.org
 www.linuxquestions.org
 www.unix.com
 www.howtoforge.com
 www.justlinux.com
And many others.
• These websites contain information varying from general
explanations to detailed how-to guides and answers to real
problems other users have experienced and asked about.