CS 497C – Introduction to UNIX Lecture 26: - The Process

1. CS 497C – Introduction to UNIXLecture 26: - The Process Chin-Chih Changchang@cs.twsu.edu

2. 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).

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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 &