Devops Engineer

An instance of a running program is called a process. Every time you run a shell command, a program is run and a process is created for it. Each process in Linux has a process id (PID) and it is associated with a particular user and group account.

Stages : 
The machine’s BIOS or boot microcode hundreds and runs a boot loader.
Boot loader finds the kernel image on the disk and loads it into memory, to start the system.
The kernel initializes the devices and their drivers.
The kernel mounts the basis filesystem.
The kernel starts a program referred to as init with a method ID zero
init sets the remainder of the system processes in motion.
For some purpose, init starts a method permitting you to log in, typically at the top or close to the top of the boot sequence.

inotifywait from inotify-tools is useful if you want to run a command every time a file (or any files in a directory) change.
For example:
inotifywait -r -m -e modify /var/log | 
while read path _ file; do 
echo $path$file modified
done

Use jps to list running java processes. The command returns the process id along with the main class. You can use kill command to kill the process with the returned id or use following one liner script.

kill $(jps | grep | awk '{print $1}')
MainClass is a class in your running java program which contains the main method.

Ruby provides File.directory?() method to check whether a directory exists or not. This function returns TRUE if the directory exists, otherwise, it returns FALSE.

#Ruby function to check directory existence
if(File.directory?('arjun')) 
puts 'Directory exists'
else
puts 'Directory not found'
end

1. LIMIT command
The limit command is used to control the number of rows to be displayed from the result set. The result set needs to display only those rows that are required. Therefore, one must use limit with the production dataset and provide an on-demand computation of rows for the production purpose

2. Inner joins vs WHERE clause
We should use inner join for merging two or more tables rather than using the WHERE clause. WHERE clause creates the CROSS join/ CARTESIAN product for merging tables. CARTESIAN product of two tables takes a lot of time.

3.Indexing
An index is a data structure used to provide quick access to the table based on a search key. It helps in minimizing the disk access to fetch the rows from the database. An indexing operation can be a scan or a seek. An index scan is traversing the entire index for matching criteria whereas index seek is filtering rows on a matching filter.