Linux

Coming from windows OS, I always got stumped when I had to find/search something in Linux/Unix. The importance of search cannot be overstated. So I decided to start my blog by discussing various ways to find/search/locate things in Linux.

Find:

The basic syntax is where to find and what to find.

find <where to find>*  <what to find>

*where to find can have multiple location

find / $HOME -name $nameOfFile

TIP: When using FIND command please direct all the errors to /dev/null so that you only see the expected result and not the errors.

maverick@maverick-G53SW:~$ find / -name bazinga 2>/dev/null

/home/maverick/findtest/level1/level2/level3/bazinga

There are couple of options in Find that I would like to point out: -mmin, -mtime, and -user. Using mmin we can find out which files where changed in the last X mins whereas mtime deals with days.Using -user enables us to search a file using the name of the owner of the file.

find $HOME -mmin -10 #Files that were modified within the last 10 mins

find / -mtime -1 #Files that have been modified in the last 24 hours.

find / -user $nameOfUser

Locate:

Easier (faster) way to search in linux. Look at the syntax:

locate $filename

Yup thats it. Nothing like find :).

Couple of interesting flags:

-i : Ignore case

-b : base name only

TIP: If you want to search only for specific file then use  locate -b '\$name'

After a short hiatus we are back and today the post is about Routing in Linux. The destiny of your packet is dependent on the the Routing table. Will the packet be forwarded out or will it be dropped, all (well not all) depends on the routing table. In this post we will see the command and some of it's flags that help us see/edit our routing table. netstat is the command that will give us all that we want and even more (much more). Some of the interesting flags are:--r Routing --a All interfaces --n do not resolve

$ netstat -ra Kernel IP routing table Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface default         192.168.0.1     0.0.0.0         UG        0 0          0 wlan0 192.168.0.0     *               255.255.255.0   U         0 0          0 wlan0

In the above output we will describe all columns from left to right:Destination Network  --> Where the packet wants to go                                             Gateway --> Where the packet will go to go to destination              networkGenMask --> Subnet mask to define the destination network         Flags: This characterize the route: U means active (UP), G means Gateway, and H is a Host Route                                                                                    iface --> interface that will be used to reach to the gateway

$ netstat -rn Kernel IP routing table Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface 0.0.0.0         192.168.0.1     0.0.0.0         UG        0 0          0 wlan0 192.168.0.0     0.0.0.0         255.255.255.0   U         0 0          0 wlan0               -n forces no name resolution and hence we see 0.0.0.0 instead of default. The different ways in which we can add routes to our device are static or dynamic. For dynamic routing entries we will have to run routing protocols like OSPF/BGP with router. Following are the static ways of doing adding a route:

route add -net $Network netmask $SubnetMask gw $Gateway $introute add default gw $Gateway $int                                                                                

Many of you would have faced this issue when you where not able to stream Netflix on your Linux machine. Woila! There is a solution.

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This has been a little frustrating for me as I have been born and bought up serching things on the GUI. But hey nothing is too difficult. So here we go;

find / -name $FileName

TIp: This command will throw error if it tries to find files in the location that do not allow find action to go through. Something like this:

find: `/proc/16428/ns': Permission denied

What really happens behind the screen when we power on the monster. (x86) Lets go through the overview!

1. BIOS (Basic Input output System)  intializes the hardware and tests the main hardware. This process is also called POST ( Power on Self Test). After this the OS takes over.

2. Bootloader comes into the picture which is usually stored in the boot sector in the hard disk. For linux, boot loader will install kernel image and initial RAM-based file sys into memory so it can be used directly by the kernel.

3.When the kernel is loaded in RAM it configures not only compture's memory but also all the hardware attached to the system. initramfs performs all the actions needed to mount proper root filesystem (FS) and drivers.

4.Mount program tells OS that FS is in place. If this is successful then initramfs is cleared from the RAM and init program is executed /sbin/init

5.Init starts other process and gets the system up and running. Except for kernel proceses most of the other processes find their origin ultimately to init. Init is not only used to start up the system but also to shut it down cleanly.

6. By the end of the boot process init start a number of text-mode login prompts.

Some Terminology

Kernel: Glue between hardware and applications . Brain of OS

   Various Linux Kernel can be found at Kernel.org

Distros: Collection of programs with linux Kernel to make Linux OS (Ubuntu/ Red Hat)

Bootloader: boots the OS  (grub/isolinux)

Service: program that runs at the background process (httpd/named)

filesys: organising files (ext3/4/fat)

xwindow: GUI (gnome/kde)

CLI: Interface to type commands on OS

Shell: command line interpreter that interprets the command line input  and tells OS to perform necessary tasks and cmds.

Disclosure:

find / -name <fileName>

1. Show line number   ----> :set number 

2 Undo is u redo is :redo

3. cw does dw and i

4. Move cursors by:

word : w/W begning of word -> b/B  End of word --> e/E

Next sentence/paragraph  -> )/} 

FIVE cool things that I learned:

1. Always use interactive mode -i while removing/copying/renaming files around

2.grep 'wordThatYouWantToFind' <filename> will get the line/s in the file with the word in it

3.Head and Tail of file. Very useful when we have to look for logs etc.

4.diff -u is awesome <Try it Out>  Also, file <nameOfFile> will tell you details about file.

5.Do you want to record a session and save all o/ps as file? Try script commmand

Good stuff!

ls has been one the most commonly used command. But today I discovered that ls with -h flag gives ouput in more humanly readable format. for instance

ls -al | grep words.txt 

-rw------- 1 root root 1130521 Feb 6 2013 words.txt

ls -alh | grep words.txt -rw------- 1 root root 1.1M Feb 6 2013 words.txt

This will help out a lot as I wont have to keep counting 3,6,9 aah thats a gb :)