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There are 3 ids ASSOCIATED with the Unix process:

• PID - Process Id, which is unique in the range of 0 to 3000.
• PPID - Parent process id
• PGID - Process group id, which is used to group processes together.

getppid() retrieves the Parent Process ID, getpid() retrieves the Process ID and getpgrp() retrieves the process group ID. The process ALSO has a real user ID (the UID), an EFFECTIVE user ID (the EUID), a real user group ID (the GID), and an effective user group ID (the EGID).

The UNIX operating system job scheduler provides three different types of job classes

• Time-share Job Class - This is the default job class and has priority 0 through 59, with 59 being the HIGHEST priority. In this job class, each process is assigned a time slice where the time slice specifies the number of CPU clock ticks that a particular task can occupy. Once the process finishes its time slice, the process is placed on the sleep queue, and the priority decreases.
• System Job Class - This class is reserved for system daemon processes. The priority RANGE for the system job class is between 60 and 99. These are of higher priority than the time-share job class. This is to ensure that the system can provide services to the time-share job class processes when system services such as memory, and/or file I/O are requested.
• Real-Time Job Class - This is the highest priority job class and has no time slice. Real-time process priorities range between 100 and 159. This ensures that critical processes always acquire the CPU as soon as the process is scheduled to run. Because there is no time slice the number of context SWITCHES is reduced significantly.

It is the first THING when the system starts. It has majorly 5 phases

• Hardware - BIOS fired up and checks for the hardware connection
• Operating System LOADER - The OS loader is located in the initial 512-byte block of the boot device known as the MBR
• Kernel - It initializes various components of the computer, operating system and each PORTION of software responsible for task, usually consider a driver
• root user-space process (init and inittab) - This defines what should be run when the /sbin/init program is instructed to enter a particular run-level, giving the administrator an easy way to establish an environment for some usage
• Boot scripts - For each managed service (mail, nfs server, cron, ETC.), there is a single startup script located in a specific directory.

nslookup COMMAND is used to query internet domain name servers which can be used to find HOSTNAME from an IP ADDRESS and for reverse lookup also, SIMILARLY host command can also be used.

ZOMBIE processes are those child processes that get finished before the parent process. So after finishing up, the process structure and address space are removed and FREED back to the system but the ENTRY in the process table still exists. To be able to get the status of the child process the parent CALLS WAIT(). In the interval between the child terminating and the parent calling wait(), the child is said to be a 'zombie'.

ps -EF will not list the processes with a very long COMMAND line while ps -auxwww will list those processes as WELL. This SOMETIMES helps in debugging as we do not always want to list all the processes but if we are not getting any IDEA we should switch to the ps -auxwww command.

Command substitution is a method by which the command is substituted by its output. We can use this to assign the output to a VARIABLE.

Mount:  The mount command mounts a STORAGE device or file SYSTEM onto an existing DIRECTORY making it accessible to users.

UNMOUNT: This unmount command detaches the mounted file system safely. It also informs the system of any pending read and write operations.

The various COMMANDS that are USED for displaying the user information in Unix are:

• ID: This displays the active user id with LOGIN and group.
• last: This displays the last login of the user in the system.
• who: This determines who is logged onto the system.
• groupadd admin: This is used to add group ‘admin’.
• usermod –a: This is used to add an existing user to the group.

kill() system call sends signals to any process which further takes suitable action ACCORDING to the signal. It takes two arguments, first is PID, to which you WANT to send a signal and the signal you want to send is second. This method returns the following return values:

• 0 - means that the process exists with the GIVEN PID and the system allows sending signals to it.
• -1 and errno==ESRCH - means that the process/process group with SPECIFIED PID does not exist
• -1 and errno==EPERM - means that the sender has no PERMISSION to send a signal to the target process.
• EINVAL - means an invalid signal was specified.

Virtual Memory is a PROCESS or a system to ADDRESS more memory than physically exists. The model CONSISTS of a set of memory pages, usually 4 KB each. There is a TRANSLATION layer between virtual memory addresses and physical memory addresses. This translation layer is part of the kernel and is usually written in machine LANGUAGE(Assembly) to achieve optimal performance when translating and mapping addresses. Each time a user process accesses memory, the kernel uses the address translation layer to map the virtual memory address into a physical memory address. Virtual Addressing makes it possible to address larger amounts of memory than physically exist.