what is process ID or PID of process in linux

Before discussing about What is process ID or PID of process in linux? Firstly, we should know that in linux, every running task is known as the 'process'.
And these processes are scheduled among different processors
by a program called as 'scheduler' which plays important role in process scheduling.

And each process is assigned a unique ID so that they can't be merged.
And this ID is known as process ID or in short PID.

Now let's learn how to print the PID of any running process.
We're going to use C program for this.

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Let's start with simple example

1. #include<stdio.h>
2. #include<sys/types.h>
3. int main()
4. {
5. printf("Process ID=%d\n",getpid());
6. return 0;
7. }

Here, getpid() function is the library function which returns the PID of the calling process
(Here, printf  is the calling process).

Mine output is: 
Process  ID=5739

Now, whenever program is terminated, new process ID is assigned to printf when it is created again.
So after executing this program again
The output is:   Process  ID=5745
Hence we always get different results when we execute the program.

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Now we see PID of parent process also

1. #include<stdio.h>
2. #include<sys/types.h>
3. int main()
4. {
5. printf("Process ID=%d\n",getpid());
6. printf("Parent process's PID=%d",getppid());
7. return 0;
8. }

Here, getppid() function is the library function which returns the PID of the parent process
(Here, shell process  is the parent process).

Mine Output is:
Process ID=6677
Parent process's PID =3832

Now after again running the program, the output is:
Process ID=6678
Parent process's ID =3832
Here, we can easily see that Parent process's PID isn't changed