Linux Fundamentals for Networking- Part 1

• Reasons for Windows popularity: 1. Dedicated support 2. Compatibility with legacy softwares 3. Established 4. Windows- specific development tools and applications 5. Windows is used for Server load-balancing
• Linux Architecture:
• 
• Fun fact: Arista's EOS is a pure Linux OS, we can enter linux shell using #bash command
• ps command gives us a list of process IDs
• To see all process ps -ef
• The UID  tells which user started the process
• PID is process ID
• PPID is parent process ID
• For example, BGP is PID 1768 and the parent PID is 1767 which is proc_mgr (which is init)
• Any application related configs are copied to /etc directory
• In proc directory, the state of running process including address space, memory, etc
• Devices are there in /dev directory
• To make directory, mkdir. To remove directory rm -rf
• To create a file, we use touch command $touch filename.doc
• > : redirection. Saves the o/p to file.
• >> : apphend
• Wildcard- * and ? . For example, if we want all files that end with .txt, we can use $ls *.txt
• Inside bash in EOS, to see the flash contents: $cd /mnt/flash. Then ls -l
• Linux Distributions:
• "Linux" properly refers to the OS kernel.
• By "different flavors of linux", you are referring to different Linux distributions. A "distribution" is a complete operating system -- a particular packaging of the kernel with various other utilities, services and shells that provide the rest of what users expect from an operating system.
• The differences between distributions are in the particulars of

• what a distribution is designed to accomplish,
• which versions of what tools it includes to achieve that goal, and
• how it chooses to handle configuration and packaging.

• A distribution aimed at desktop users typically includes the X-window system, a desktop environment, a printing subsystem, networking components, etc. One designed for servers often doesn't include X or a desktop environment, but adds services that aren't needed on a desktop (webserver, database, network filesystems, etc).
• Linux System calls:
• http://www.digilife.be/quickreferences/qrc/linux%20system%20call%20quick%20reference.pdf
• For eg, To kill a process, we use sigkill system call
• Kill a Process:
• Kill -<signal number> <pid>
• 
  

signal #	Usage
1	HUP (hang up)
2	INT (interrupt)
3	QUIT (quit)
6	ABRT (abort)
9	KILL (non-catchable, non-ignorable kill)
14	ALRM (alarm clock)
15	TERM (software termination signal)




• Ls command in Linus:
• ls syntax : $ ls [options] [file or dir]
• ls command options:

option	description
ls -a	list all files including hidden file starting with '.'
ls --color	colored list [=always/never/auto]
ls -d	list directories - with ' */'
ls -F	add one char of */=>@| to enteries
ls -i	list file's inode index number
ls -l	list with long format - show permissions
ls -la	list long format including hidden files
ls -lh	list long format with readable file size
ls -ls	list with long format with file size
ls -r	list in reverse order
ls -R	list recursively directory tree
ls -s	list file size
ls -S	sort by file size
ls -t	sort by time & date
ls -X	sort by extension name
ls -ltr	Sort by latest file created

• Ls Command Examples:
• You can press the tab button to auto complete the file or folder names.
• List directory Documents/Books with relative path:
$ ls Documents/Books
• List directory /home/user/Documents/Books with absolute path.
$ ls /home/user/Documents/Books
• List root directory:
$ ls /
• List parent directory:
$ ls ..
• List user's home directory (e.g: /home/user):
$ ls ~
• List with long format:
$ ls -l
• Show hidden files:
$ ls -a
• List with long format and show hidden files:
$ ls -la
• Sort by date/time:
$ ls -t
• Sort by file size:
$ ls -S
• List all subdirectories:
$ ls *
• Recursive directory tree list:
$ ls -R
• List only text files with wildcard:
$ ls *.txt
• ls redirection to output file:
$ ls > out.txt
• List directories only:
$ ls -d */
• List files and directories with full path:
$ ls -d $PWD/*


• PS Command:
• The ps command displays active processes.
• The syntax for the ps command is: ps [options]
• Various Options are:



-a	list the processes of all users on the system rather than just those of the current user (except group leaders)
-u	provide detailed information about each process
-x	adds to the list processes that have no controlling terminal, such as daemons
-c	Displays scheduler data.
-d	Displays all processes with the exception of session leaders.
-e	Displays all processes.
-f	Displays a full listing.
-glist	Displays data for the list of group leader IDs.
-j	Displays the process group ID and session ID.
-l	Displays a long listing
-plist	Displays data for the list of process IDs.
-slist	Displays data for the list of session leader IDs.
-tlist	Displays data for the list of terminals.
-ulist	Displays data for the list of usernames.



• A common and convenient way of using ps to obtain much more complete information about the processes currently on the system is to use the following:
ps -aux | less    
Note that each of the ‘a’ , ‘u’ , ‘x’ are separate options
The aux gives details of all process and less makes it scrollable
• An alternative set of options for viewing all the processes running on a system is
ps -ef | less
• Note that the ‘-’ is optional. So, we can also use ps aux | less
• The processes shown by ps can be limited to those belonging to any given user by piping the output through grep, a filter that is used for searching text.
For example, processes belonging to a user with a username adam can be displayed with the following:
ps -ef | grep adam
• The -l option generates a long listing, and when used together with the -e and -f options creates a table with 15 columns:
ps -efl
• Sorting the PS Output:
•      CODE NORMAL   HEADER
        %C pcpu     %CPU
        %G group    GROUP
        %P ppid     PPID
        %U user     USER
        %a args     COMMAND
        %c comm     COMMAND
        %g rgroup   RGROUP
        %n nice     NI
        %p pid      PID
        %r pgid     PGID
        %t etime    ELAPSED
        %u ruser    RUSER
        %x time     TIME
        %y tty      TTY
        %z vsz      VSZ
• Example: ps -ef --sort=user | less or ps -ef --sort=pid | less
• Example: to see by startup time ps -ef --sort=time | less


• Filters:
• Some useful ones are the commands awk, grep, sed, spell, and wc.
• Filters usually works with Linux pipes.
• http://ryanstutorials.net/linuxtutorial/filters.php
• Grep:
• grep scans the output line per line, searching for matching patterns.
• It is case-sensitive. To make it insensitive, use -i attribute
• Some examples: suppose we want to know which files in a certain directory have been modified in February:
jenny:~> ls -la | grep Feb
• Important options of the grep command
• -e (Used to specify a pattern)
• -i (Case insensitive search)
• -c (Print a count of matching lines)
• -v (invert search - Returns lines that do not match, instead of lines that match.)
• -w (Matches only when the input text consists of full words)
• -x (Should match the entire line)
• --color (Colorize output)
• -l (Instead of normal output, prints just the names of input files containing the pattern)
• -L (Prints the names of input files that contain no matches)
• -o (Prints only the text that matches, instead of the whole line of input)
• -q (Suppresses output. Useful in finding the exit status (0 for success if a match is found, 1 for no match found, 2 if the program cannot run because of an error)).
• "^" character is used as anchor, to find the lines which begin which the following text. Example – “^RedHat”
• "$" character is used as anchor, to find the lines which end with the preceding text. Example – "RHEL5$"
• To search for lines which contains numbers, use RegEx expression range "[0-9]"
• To search for lines which contain small case letters, use RegEx expression range "[a-z]"
• To search for lines which contain caps letters, use RegEx expression range "[A-Z]"