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6. UNIX TOOLS
6.1 ProcessesA process is the execution of a command by UNIX. Processes can also be executed by the operating system itself. Like the file structure, the process structure is hierarchical. It contains parents, children, and even a root. A parent can fork (or spawn) a child process. That child can, in turn, fork other processes. The first thing the operating system does to begin execution is to create a single process, PID number 1. PID stands for Process IDentification. This process will hold the same position as the root directory in the file structure. This process is the ancestor to all processes that each user works with. It forks a process for each terminal. Each one of these processes becomes a Shell process when the user logs on.
6.2 Executing a CommandWhen you give a command to the Shell, it will fork a process to execute the command. While the child process is executing the command, the parent will go to sleep. Sleeping means that the process will not use any CPU time. It remains inactive until it is awakened. When the child process has finished executing the command, it dies. The parent process, which is running the Shell, wakes up and prompts you for another command. When you request a process to run in the background (by ending the command line with an &), the Shell forks a child process that is allowed to run to completion. The parent process will report the PID of the child process and them prompts you for another command. The child and parent are now independent processes.
6.3 Process IdentificationThe Unix operating system assigns a unique process identification number (PID) to each process. It will keep the same PID as long as the process is in existence. During one session, the same process is always executing the login Shell. When you execute another command a new process is forked and a new PID is assigned to that process. When that child process is finished you are returned to the login process, which is running the Shell, and that parent process has the same PID as when you logged on. The Shell stores the PID in Shell variable called $$. The PID can also be shown with the process status (ps) command. The format for ps is as follows:
Command Format: ps [options] See on-line manual for options
Command Format: grep [options] limited_regular-expression [file] Use the man command for a complete list of options