CompTIA Linux+ Guide to Linux Certification Chapter 9

SIGQUIT

Also know as a core dump, this signal terminates a process by taking the process information in memory and saving it to a file called core on the hard disk in the current working directory. It uses the number 3.

Process priority

A number assigned to a process, used to determine how many time slices on the processor that process will receive; the higher the number, the lower the priority.

pstree command

A command that displays processes according to their lineage, starting from the init daemon.

kill command

A command used to stop or abort a process in a program or operating system.

/etc/at.allow

A file listing all users who can use the at command.

/etc/at.deny

A file listing all users who cannot access the at command.

cron table

A file specifying tasks to be run by the cron daemon; there are user cron tables and system cron tables.

user process

A process begun by a user and which runs on a terminal.

foreground process

A process for which the BASH shell that executed it must wait for its termination.

background process

A process that does not require the BASH shell to wait for its termination. Upon execution, the user receives the BASH shell prompt immediately.

rogue process

A process that has become faulty in some way and continues to consume far more system resources than it should.

zombie process

A process that has finished executing, but whose parent has not yet released its PID; the zombie retains a spot in the kernel's process table.

parent process

A process that has started other processes (child processes).

child process

A process that was started by another process (parent process).

process

A program currently loaded into physical memory and running on the system.

program

A structured set of commands stored in an executable file on a filesystem. They can be executed to create a process.

process ID (PID)

A unique identifier assigned to every process as it begins.

ps -f

Displays a full list of information about each process, including the UID, PID, PPID, CPU utilization, start time, terminal, processor time, and command name.

ps -l

Displays a long list of information about each process, including the flag, state, UID, PID, PPID, CPU utilization, priority, nice value, address, size, WCHAN, terminal, and command name.

ps a

Displays all processes associated with your terminal.

ps -e

Displays all processes running on terminals as well as processes that do not run on a termnal (daemons).

ps x

Displays all processes that do not run on terminals.

parent process ID (PPID)

The PID of the parent process that created the current process.

forking

The act of creating a new BASH shell child process from a parent BASH shell process.

time slice

The amount of time a process is given on a CPU in a multiprocessing operating system.

killall command

The command that kills all instances of a process by command name.

renice command

The command used to alter the nice value of a process currently running on the system.

nice command

The command used to change the priority of a process as it is started.

top command

The command used to give real-time information about the most active processes on the system; it can also be used to renice or kill processes.

ps command

The command used to obtain information about processes currently running on the system.

foreground (fg) command

The command used to run a background process in the foreground.

background (bg) command

The command used to run a foreground process in the background.

at command

The command used to schedule commands and tasks to run at a preset time in the future.

jobs command

The command used to see the list of background processes running in the current shell.

crontab command

The command used to view and edit user cron tables.

trapping

The process of ignoring a kill signal.

at daemon (atd)

The system daemon that executes tasks at a future time; it is configured with the at command.

cron daemon (crond)

The system daemon that executes tasks repetitively in the future and that is configured using cron tables.

kill signal

The type of signal sent to a process by the kill command; different kill signals affect processes in different ways. There are 64 kill signals.

SIGKILL

This kill signal forces the Linux kernel to stop executing the process by sending the process's resources to a special device file called /dev/null. Should only be used as a last resort. It uses the number 9.

SIGINT

This signal is one of the weakest, it works most of the time. It uses the number 2.

SIGHUP

This signal stops a process, then restarts it with the same PID. It uses the number 1.

Binary programs, shell scripts, shell functions

You can execute three main types of Linux commands.

process state

The current state of the process on the processor, used to determine how many time slices on the processor that process will receive; the higher the number, the lower the priority.

nice value

The value that indirectly represents the priority of a process; the higher the value, the lower the priority.

SIGTERM

This software signal is the most common kill signal used by programs to kill other processes. It is the default kill signal used by the kill command. It uses the number 15.