Summary
Now that you have completed this chapter, you should have a firm understanding of the following:

UNIX initiates, manages, and terminates programs using system processes. When a program such as the sort command or the vi editor starts, the operating system kernel assigns it a unique Process Identification Number or PID. The kernel also allocates CPU time and RAM space for the program to run. UNIX processes are hierarchical similar to the UNIX file system. Every process has a parent process except the init process, which is process number one.

There are five types of system processes on a UNIX system:

  • Daemon - Processes started by the UNIX Kernel
  • Parent - The process that spawned another
  • Child - The process spawned by the parent
  • Orphan - Result of parent process terminating before child returns
  • Zombie - Child process which does not return to the parent

The ps command is used to display processes and has several options. The command ps -ef will display every process with full information.

  • -e - Every process
  • -f - Full listing of information
  • -u - User processes

The output of the ps command can be piped to the more command to see a screen at a time or to the grep command to search for specific strings such as program names.

Long running or unresponsive processes can be terminated using the kill command and while specifying the PID of the process. Most processes can be terminated with the basic kill PID# command or soft kill. To kill a shell process requires the kill -9 PID# or sure kill command.

Computers have physical memory which is known as Random Access Memory or RAM. During installation, the UNIX operating system sets aside an area on the hard disk called swap space. As programs are executed, they are copied from the hard disk into system RAM and divide into pages. In order to make the best use of RAM, portions of programs that have not been used recently are paged out to the swap area of the hard disk. This allows more programs to fit into RAM. If pages that are on the hard disk are needed, they can be paged back into RAM. The combination of physical RAM and the swap space on the hard disk is known as virtual memory.