Process and Service in Linux¶

Process¶

• A process in Linux is an instance of a running program. It is the basic unit of execution in Linux, and the Linux kernel manages multiple processes concurrently, giving the illusion of multitasking.

Key Characteristics:¶

• PID: Each process is identified by a Process ID (PID), a unique number.
• Parent-Child Relationship: Processes have a hierarchical structure, where a process can spawn child processes.
• States: A process can be in one of several states, such as running, sleeping, stopped, or zombie.
• Environment: Each process operates within a specific environment that includes environment variables, a current directory, and user permissions.

Working with Processes¶

1. Listing Processes: You can list processes using commands like ps and top.
   • ps : provides a snapshot of current processes.
   • top : displays an ongoing update of processes and system resources.
2. Managing Processes: You can manage processes using commands such as kill, nice, and renice.
   • kill sends a signal to a process, typically to terminate it.
   • nice and renice adjust the priority of a process.

Service¶

• A service in Linux is a special type of background process that is started at boot time or on demand and performs a specific function or provides a particular functionality to other processes or users

Key Characteristics:¶

• Daemon: Most services are implemented as daemons, background processes that are often started during the boot process.

Working with Services:¶

• service [service name] start | stop | restart | status
• systemctl start | stop | restart | status [service name]

1. check all available services
   • /usr/lib/systemd/system

runlevel in linux¶

Linux systems typically define the following runlevels:

Runlevel workflow¶

Power on $\to$ BIOS $\to$ /boot $\to$ initprocess $\to$ runlevel

• runlevel 1 have root power, do not need root password to signin, but need physical signin

Check runlevel¶

• vim /etc/inittab: this is the default runlevel

• systemctl get-default: get current runlevel
  • multi-user.target : runlevel 3
    • ctrl + alt + F3
  • graphical.target : runlevel 5
    • ctrl + alt + F1

Change the current runlevel¶

• systemctl set-default [Target runlevel].target

Set Boot Service and Firewall¶

1. Check Services status
   • systemctl status [Services]

2. Set to disable
   • systemctl disable [Services]

3. Check all Services status
   • systemctl list-unit-files

Set firewall¶

1. Check firewall status
   • systemctl status firewalld
   • systemctl stop firewalld : stop firewall service for this time

Power-off, Reboot and halt¶

1. sync : store data from memory to hard drive
2. halt : stop services and still power on
3. poweroff
4. reboot : equal to shutdown -r now

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