CS 497C – Introduction to UNIX Lecture 26: - The Process Chin-Chih Chang
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CS 497C – Introduction to UNIX Lecture 26: - The Process Chin-Chih Chang email@example.com firstname.lastname@example.org
Understanding the Process A process is defined as an instance of a running program. A process lives as long as the program is running. The kernel makes sure that processes get their due time slice and are swapped out to disk as and when needed. A process is identified by the process identifier (PID).
Understanding the Process A process has the following attributes: –The real user-id of the user who created the process. The user is said to be the owner of the process. –The real group-id of the owner process. Both user-id and group-id of the owner are stored in /etc/passwd. –The priority with which it runs. –The current directory from where the process was run. A process is said to be born when the program starts execution.
Understanding the Process After execution is complete, the process is said to die. Every process has a parent identified by the parent process identifier (PPID) except the ancestry of every process - the first process (PID 0) which is set up when the system is booted. A process can spawn (generate) multiple children.
How a Process Is Created Every process comprises code and data. The process image as viewed by the kernel runs in its own user address space. This address space has a number of segments: –Text Segment – This contains the executable code of a program. – Data Segment – All variables and arrays the program uses are held here. –User Segment – This contains all the process attributes.
How a Process Is Created There are three distinct phases in the creation of a process, using three important system calls – fork(), exec() and wait(): : –Fork – A process is created by the parent with the fork system call, which creates a copy of itself. –Exec – The parent then overwrites the copy with the exec system call to create the child. –Wait – The parent then executes the wait system call to keep waiting for the child process to complete.
How a Process Is Created When a process is forked, it inherits the environment of its parent but changes in the child are not made available in the parent. Built-in shell command like pwd and cd don’t fork a separate process. The shell also spawns a separate sub-shell to execute a shell script.
The Login Shell: The First User Process When you log on to a UNIX system, the shell process is immediately set up by the kernel. This program may be sh (Bourne shell), csh (C Shell), ksh (Korn shell), or bash (Bourne again shell). The shell maintains a set of variables that are available to the user. The PID of the login shell is stored in the variable $$. It is the parent of all commands and scripts run from the shell.
The init Process The parent of all login shells is the init process with PID 1. init forks and execs a getty process which prints the login prompt and goes to sleep. When a user tries to log in, getty wakes up and execs the login program to verify the login name and password. On most systems, login forks and execs a shell.
Internal and External Commands The shell recognizes three types of commands: –External commands – These are the most commonly used utilities and programs such as cat, ls, and so forth. –Shell scripts – The shell executes these scripts by spawning another shell, which then executes the commands listed in the script. –Internal commands – The shell has a number of built-in commands such as cd and echo which don’t generate a process.
ps: Process Status The ps command is used to list the attributes of processes. By default, ps displays the processes associated with a user at the terminal: ps is a highly variant command. Its actual output depends on the version of UNIX as well as the hardware used. BSD and System V are two main brands of the ps command.
ps: Process Status Linux uses the BSD version of the ps command. ps in Linux uses two types of options – the BSD options that don’t use a dash and the GNU-style options which use - -- (2 hyphens). The -e option shows all process including user and system process. The -f (full) option gets a detailed listing showing the the parent of a process.
ps: Process Status The -u (user) option displays processes of a user. ps -f -u root The -a (all) option displays all processes run by the user. Linux uses the ax option to display system processes. The ps u command approximates the ps -f.
Running Jobs in Background A multitasking system lets a user do more than one job at a time. A job can be run in the foreground and the background. A job can be run in the background by affixing an & at the end of the command line or the nohup command. nohup ensures that a background job remains alive even after the user has logged out.
Running Jobs in Background The & is the shell’s operator used to run a process in the background. sort -o list list &; ps -f C shell and bash shell allows you to log out without aborting the job. The nohup (no hangup) command prefixing a command permits execution of the process even after the user logged out. nohup sort list &