Server Admin Chapter 9

Process ID (PID)

A unique number to identify and manage processes.

Parent-Child ID (PPID)

Processes can create other processes, resulting in a hierarchical structure.

Process States

Running, Sleeping, Stopped, Zombie, Orphan

Running process

Actively executing on the CPU

Sleeping process

Waiting for an event

Stopped process

Paused, typically by a user command

Zombie process

A process that has finished executing, but its parent hasn't retrieved its exit status

Orphan process

A process whose parent has terminated, and it is re-parented to the init process.

Rogue process

A process that encounters an error and continuously uses up system resources.

ps command

Process status. Displays information about currently running processes.

top command

Provides a dynamic, real-time view of processes and system resource usage

htop command

An interactive and enhanced version of top

systemctl command

Manage system daemons

init daemon

First process started by Linux kernel (PPID of 0)

PID 0 refers to...

The kernel itself

ps -f command

Full option. Displays more complete info such as UID, PPID, start time, and CPU utilization

ps -e command

Entire option. Displays entire list of processes

PRI

Process priority. Measured between 0 (high) and 127 (low) priorities.

Nice value

Can be used to indirectly affect process priority. Measured between -20 (greater chance of higher priority) and +19 (greater chance of lower priority).

pstree command

Displays lineage of process by tracing its PPIDs back to init daemon

kill command

Sends a kill signal to a process to terminate it

kill -l command

Displays a list of kill signal names and associated numbers.

SIGHUP

1. Hang-up signal. It stops a process, then restarts it with the same PID.

SIGINT

2. Interrupt signal. Used when you do CTRL+C in the terminal.

SIGQUIT

3. Core dump. Terminates a process by taking the process info in memory and saving it to a file called 'core' on the disk

SIGTERM

15. Termination signal. The most common kill signal (and default for kill command)

SIGKILL

9. Absolute kill signal, forcing the kernel to stop executing the process.

killall command

Kills multiple processes of the same name in one command

pkill command

Kills processes by process name, PPID, UID, etc

Forking

Creating a new subshell

Subshell

Executes a program or script with the exec function

Foreground processes

Active in your terminal session

Background processes

Shell immediately returns the shell prompt to enter another command

jobs command

Indicates the two most recent background processes

kill -2 %1

Sends an interrupt signal to a background process

To move a background process to the foreground...

Use the fg command followed by the background job ID

Since the PRI can't be directly changed, what should you do?

Assign a certain nice value to a process.

Negative nice value

Increases the likelihood that a process will receive more time slices

Positive nice value

Decreases the likelihood that a process will receive more time slices.

renice command

Alters priority after a process has been started

at daemon

Executes once in the future

cron daemon

Executes repeatedly in the future

at -l command

Displays a list of at job IDs

at -c command

Displays contents of a specified at job

at -d command

Removes the specified at job

/etc/cron.allow file

Lists users allowed to use cron daemon

/etc/cron.deny file

Lists users not allowed to use cron daemon

What happens if both allow and deny files exist?

Only the allow file is processed.

What happens if neither the allow or deny file exists?

All users are allowed to schedule tasks.

crontab command

Creates or edits user cron tables

crontab -e command

To edit cron tables in an editor

crontab -l command

To list your user cron table

crontab -r command

Removes a cron table and all scheduled jobs

crontab -u command

Used by the root user to edit, list, or remove user cron table

Cron table format

minute (0 - 59)

hour (0 - 23)

day of month (1 - 31)

month (1 - 12 or jan - dec)

day of week (0 - 6 or sun - sat)

user

command