Systemd and SysVinit in Linux

1. What are systemd and SysVinit?

systemdandSysVinitAllLinux initialization system (init system), used to manage system startup, services, processes and logs.

2. SysVinit Detailed explanation

SysVinit(System V init) was from early Unix systemsinitProcess, responsible for system guidance and service management.

(1) How SysVinit works

• Read/etc/inittabSureRunlevel
• exist/etc//Execute in the directoryStart the script
• according toSequence (serial)Start the service in turn (start slowly)

(2) SysVinit run level

(3) How to start the service in SysVinit

• SysVinituse/etc//scriptAs a service management method:

/etc//nginx start
/etc//nginx stop
/etc//nginx restart

• Can also be usedserviceOrder:

service nginx status
service nginx restart

• chkconfigUsed to manage power-on:

chkconfig nginx on  # Start up automaticallychkconfig --list nginx  # Check whether the power is turned on and on

(4) Problems with SysVinit

• Service startup is serial and slow
• Unable to track process status
• Log management dependency/var/log/messages, inconvenient query
• Service dependencies need to be processed manually

3. Systemd Detailed Explanation

systemdIt's from modern LinuxinitProcess, replaceSysVinit, providing faster startup speed and stronger service management capabilities.

(1) systemdFeatures

• ✅ Parallel startup: Multiple services are started at the same time to speed up the system boot.
• ✅ Automatic dependencies processing: no manual configuration requiredchkconfig。
• ✅UsageunitFile replacementinitScripts: More structured and easier to maintain.
• ✅ Built-injournalctlLog management: It is more convenient to query logs.
• ✅ SupportcgroupsProcess control: manages process lifecycle.

(2) systemdStart the service

• systemctlyessystemdService management tools provided:

systemctl start nginx   # Start the servicesystemctl stop nginx    # Stop servicesystemctl restart nginx # Restart the servicesystemctl status nginx  # Check service status

• Set up power-on and start:

systemctl enable nginx  # Start upsystemctl disable nginx # Cancel boot

• List all running services

systemctl list-units --type=service

(3) systemd unit file

• systemduse/etc/systemd/system/In the directory.serviceFile management services, such as:

[Unit]
Description=NGINX Web Server
After=

[Service]
ExecStart=/usr/sbin/nginx -g "daemon off;"
ExecReload=/bin/kill -s HUP $MAINPID
KillMode=process
Restart=always

[Install]
WantedBy=

• systemdReadunitAfter the file, you can use itsystemctlDirect management services:

systemctl daemon-reload  # Reload the configurationsystemctl restart nginx  # Restart the service

(4) systemd log management

• systemdusejournalctlRecord all system logs:

journalctl -u nginx  # View Nginx related logsjournalctl -f        # View logs in real time (similar to `tail -f`)journalctl --since "1 hour ago" # Check out the latest 1 Hourly logs

4. The difference between systemd and SysVinit

5. How to tell whether the system uses SysVinit or systemd?

Method 1: Check /sbin/init

ls -l /sbin/init

If output:

lrwxrwxrwx 1 root root 22 Mar 12  2025 /sbin/init -> /lib/systemd/systemd

• illustrateusesystemd。
• if/sbin/inityes/etc/Binary files in the directory, InstructionsSysVinit。

Method 2: Check the process

ps -p 1

If output:

PID TTY      STAT   TIME COMMAND
  1 ?        Ss     0:00 /lib/systemd/systemd

illustrateusesystemd。

If output:

PID TTY      STAT   TIME COMMAND
  1 ?        Ss     0:00 /sbin/init

illustrateuseSysVinit。

Method 3: Run systemctl directly

systemctl --version

• If returnsystemdVersion number (such assystemd 249),illustrateusesystemd。
• ifsystemctlThe command does not exist,useSysVinit。

6. Which Linux versions use systemd?

Last：

• SysVinityesTraditional Linux boot management system,useScripts, slow startup, rely on manual management.
• systemdyesModern Linux standardsinitsystem,supportParallel startup、Automatically manage dependencies, greatly improvedStartup speed。
• systemctlReplacedserviceandchkconfig, provide stronger management capabilities.
• journalctlReplaces tradition/var/log/messagesLog management.

If your system is CentOS 7+ or Ubuntu 16+, you should usesystemdInsteadSysVinit。 

Summarize

The above is personal experience. I hope you can give you a reference and I hope you can support me more.