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Managing system processes. Unit objectives Categorize the various types of processes, view processes using utilities, and illustrate the differences between various kill signals Describe how binary programs and shell scripts are executed, create and manipulate background processes
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Managing system processes Unit objectives • Categorize the various types of processes, view processes using utilities, and illustrate the differences between various kill signals • Describe how binary programs and shell scripts are executed, create and manipulate background processes • Use utilities to modify the priority of a process, schedule commands using the at daemon, and schedule repetitive commands using the cron daemon.
Topic A • Topic A: Starting, viewing and killing processes • Topic B: Foreground and background processes • Topic C: Process priorities and scheduling
Programs and processes • Program • Executable file • May be executed to create a process • Process • Instance of an executing program
User and daemon processes • User process • Process begun by a user that runs on a terminal • Daemon process • System process that isn’t associated with a terminal
Processes • Process ID (PID) • Unique identifier assigned to every process • Child process • Process that was started by another process (parent process) • Parent process • Process that has started other processes (child processes) • Parent Process ID (PPID) • PID of the parent process that created the current process
Exercise A-1 Discussing processes
The ps command • Most versatile and common utility that can view processes • Without arguments, displays processes running in the current shell
Zombie process • Process that has finished executing, but whose parent hasn’t yet released its PID • Also known as a defunct process
Process priority and nice value • Process priority (PRI) • Number assigned to a process, used to determine how many time slices on the processor it receives • Nice value (NI) • Value that indirectly represents the priority of a process • The higher the value, the lower the priority
top command • Most common command used to display processes aside from ps • Display organized by processor time • Processes that use the most processor time are listed at the top of the screen continued
Rogue process • Process that has become faulty and consumes more system resources than it should • Can be used to change the priority of processes or kill them
Activity A-2 Using the ps command to view processes
Sending signals to processes • Kill signal • Type of signal sent to a process by the kill command • Various kill signals affect processes in various ways • kill command • Command that kills all instances of a process by command name
Trapping and killall • Trapping • Process of ignoring a kill signal • killall command • Command that kills all instances of a process by command name
Activity A-3 Using the kill command to terminate processes
Topic B • Topic A: Starting, viewing and killing processes • Topic B: Foreground and background processes • Topic C: Process priorities and scheduling
Process execution • The three main types of Linux commands that you may execute: • Binary programs • Shell functions • Shell scripts
Forking • Act of creating a new BASH shell child process from a parent • Carried out by the fork function in the Linux kernel
Background and foreground processes • Foreground process • Process for which the BASH shell that executed it must wait for its termination • Background process • Process that doesn’t require the BASH shell to wait for its termination • Upon execution, the user receives the BASH shell prompt immediately
Background processes • jobs command • Command used to see the list of background processes running in the current shell • foreground (fg) command • Command used to run a background process in the foreground • background (bg) command • Command used to run a foreground process in the background
Activity B-1 Running a process in the background
Topic C • Topic A: Starting, viewing and killing processes • Topic B: Foreground and background processes • Topic C: Process priorities and scheduling
Process priorities • Time slice • Amount of time a process is given on a CPU in a multiprocessing operating system • The more time slices a process has, the more time it has to execute on the CPU and the faster it executes • Time slices are usually measured in milliseconds
The nice value and command • Processes are started with a nice value of 0 by default • On some systems, background processes are given a nice value of 4 by default to lower the chance they receive time slices • nice command • Command used to change the priority of a process as it’s started
The renice command • renice command • Used to alter the nice value of a process currently running • As with the nice command, only the root user may change the nice value to a negative value using renice
Activity C-1 Experimenting with scheduling priority
Scheduling commands • at daemon (atd) • System daemon that executes tasks at a future time • Configured with the at command • cron daemon (crond) • System daemon that executes tasks repetitively in the future • Configured using cron tables
Scheduling commands with atd • /etc/at.allow • A file listing all users who can use the at command • /etc/at.deny • A file listing all the users who can’t access the at command
crond scheduling commands • Cron table • File specifying tasks to be run by the cron daemon • User cron tables and system cron tables • Six fields separated by space or tab characters
Location of cron tables • /var/spool/cron • Stores user cron tables • /etc/cron.d • Contains additional system cron tables • /etc/crontab • Default system cron table
User cron tables • /etc/cron.allow • File listing all users who can use the cron command • /etc/cron.deny • File listing all users who can’t access the cron command • crontab command • Command used to view and edit user cron tables
The system cron table • Systems are scheduled to run commands during non-business hours • Commands may perform system maintenance, back up data, or run CPU-intensive programs • Most commands are scheduled by the cron daemon from entries in the system cron table /etc/crontable
Activity C-2 Using at and crontab utilities to schedule processes
Chapter summary • Categorized different types of processes, viewed processes by using utilities, and learned the difference between kill signals • Described how binary programs and shell scripts are executed, created and manipulated as background processes • Used utilities to modify the priority of a process and schedule commands