Swapping:
Whole process is moved from the swap device to the main memory for execution. Process size must be less than or equal to the available main memory. It is easier to implementation and overhead to the system. Swapping systems does not handle the memory more flexibly as compared to the paging systems.

Paging:
Only the required memory pages are moved to main memory from the swap device for execution. Process size does not matter. Gives the concept of the virtual memory. It provides greater flexibility in mapping the virtual address space into the physical memory of the machine. Allows more number of processes to fit in the main memory simultaneously. Allows the greater process size than the available physical memory. Demand paging systems handle the memory more flexibly.

The system calls used for low-level process creation are execlp() and execvp(). The execlp call overlays the existing program with the new one , runs that and exits. The original program gets back control only when an error occurs. execlp(path,file_name,arguments..); //last argument must be NULL A variant of execlp called execvp is used when the number of arguments is not known in advance. execvp(path,argument_array); //argument array should be terminated by NULL .

The standard command to see this is [ps]. But [ps] only shows you the snapshot of the processes at that instance. If you need to monitor the processes for a certain period of time and need to refresh the results in each interval, consider using the [top] command.
$> ps -ef
If you wish to see the % of memory usage and CPU usage, then consider the below switches
$> ps aux
If you wish to use this command inside some shell script, or if you want to customize the output of [ps] command, you may use "-o" switch like below. By using "-o" switch, you can specify the columns that you want [ps] to print out.
$>ps -e -o stime,user,pid,args,%mem,%cpu

Using command, we can do it in many ways. Based on what we have learnt so far, we can make use of [ls] and [$?] command to do this. See below:
$> ls -l file.txt; echo $?
If the file exists, the [ls] command will be successful. Hence [echo $?] will print 0. If the file does not exist, then [ls] command will fail and hence [echo $?] will print 1.

To check the status of last executed command in UNIX, you can check the value of an inbuilt bash variable [$?]. See the below example:
$> echo $?