Service Management in Linux Operations You Must Know!

Introduction

In Linux, whenever you install any package with services and daemons, the system will by default add the initialization and systemd scripts for these services, but they are not enabled at this point.

We need to manually start or stop those services.Linux has three well-known and widely used initialization systems.

In Linux/Unix-based operating systems, init (short for initialization) is the first process started during the kernel boot process.

init has a process id (pid) of 1, and it will run in the background until the system is shut down.

init first determines the run level of Linux based on the /etc/inittab file, and then starts all other processes and applications in the background according to the run level.

BIOS, MBR, GRUB, and the kernel program start working as part of the Linux boot process before init is executed.

Here are the run levels available in Linux (a total of seven run levels from 0 to 6):

• 0: Shutdown
• 1: Single-user mode
• 2: Multi-user mode (without NFS)
• 3: Full multi-user mode
• 4: Unused
• 5: Graphical interface mode
• 6: Reboot

Initialization Systems

Here are the three most commonly used initialization systems in Linux:

• System V (Sys V)
• Upstart
• systemd

System V (Sys V)

System V (Sys V) is the first and traditional initialization system for Unix-like systems.init is the first program started during the kernel boot process, and it is the parent process of all programs.

Most Linux distributions initially used the traditional initialization system called System V (Sys V). In recent years, several initialization systems have been released to address design limitations in the standard version, such as: launchd, Service Management Facility, systemd, and Upstart.

However, systemd has been adopted by several major Linux distributions to replace the traditional SysV initialization system.

Upstart

Upstart is an event-based replacement for the /sbin/init daemon that handles the starting of tasks and services during system boot, monitors them while the system is running, and shuts them down during system shutdown.

It was originally designed for Ubuntu, but it can also be perfectly deployed on all other Linux systems to replace the older System-V.

Upstart was used from Ubuntu 9.10 to Ubuntu 14.10 and on RHEL 6-based systems, after which it was replaced by systemd.

systemd

systemd is a new initialization system and system manager used in all major Linux distributions to replace the traditional SysV initialization system.

systemd is compatible with SysV and LSB initialization scripts. It can directly replace the SysV initialization system.systemd is the first program started by the kernel, and its PID is 1.

systemd is the parent process of all programs, and Fedora 15 was the first distribution to replace upstart with systemd.systemctl is used in the command line and is the main tool for managing systemd daemons/services, such as: (start, restart, stop, enable, disable, reload, and status)

systemd uses .service files instead of bash scripts (used by SysVinit).systemd adds all daemons to cgroups for sorting, and you can view the system levels by browsing the /cgroup/systemd file.

service

service command, as the name suggests, is used to manage services in the Linux operating system.

This command is not available in all linux distributions. It is mainly found in redhat, fedora, mandriva, and centos.

Check the current running status of all services
service --status-all

Check the running status of a specific service (vsftpd)
service vsftpd status

Stop a specific service (vsftpd)
service vsftpd stop

Restart the network service
service network restart

chkconfig

chkconfig is a command-line utility that allows you to start selected services at specified run levels and list all available services and their current settings.

Additionally, it allows us to enable or disable services from startup, provided you have superuser privileges (root or sudo) to run this command.

All service scripts are located in the /etc/init.d file.

Service Management in Linux Operations You Must Know — image-20210803160510022

Set mysqld to run at levels 3 and 5, --level 35 means the operation is only executed at levels 3 and 5, on means start, off means stop
chkconfig --level 35 mysqld on 

Set mysqld to on at all levels, "all levels" includes levels 2, 3, 4, 5
chkconfig mysqld on        

Modify the default startup level of the service
chkconfig --level 35 mysqld on

systemctl

Concept

systemd core concept unit types:unit represents different types of systemd objects, identified and configured through configuration files; the files mainly contain system services, listening sockets, saved system snapshots, and other init-related information.

Here are the unit types:

• service: file extension is .service, used to define system services
• target: file extension is .target, used to simulate run levels
• device: used to define devices recognized by the kernel
• mount: defines filesystem mount points
• socket: used to identify socket files for inter-process communication, and can also delay service startup at system boot, achieving on-demand startup
• snapshot: manages system snapshots
• swap: used to identify swap devices
• automount: filesystem auto-mount points
• path: used to define a file or directory in the filesystem, commonly used to delay service activation when the filesystem changes

How to view these types?

You can use -t followed by the type to view, for example, service:

ststemctl -t service

systemd Configuration File Directories

• /usr/lib/systemd/system/: the main configuration for each service’s startup script is placed here, similar to the old /etc/init.d;

• /run/systemd/system/: the directory where service scripts generated during system execution are located, these scripts have a higher priority than /usr/lib/systemd/system/;

• /etc/systemd/system/: the directory where execution scripts created by the administrator based on the host system’s needs are located, with a higher execution priority than /run/systemd/system/;

From the above functions and priority order, we can see that the relevant configurations in the /etc/systemd/system/ directory determine whether the system will execute certain services, so this directory generally contains a lot of link files. The actual executable systemd startup script configuration files are placed under /usr/lib/systemd/system/<code>. Therefore, if you want to modify the startup settings of a service, you should go to <code>/usr/lib/systemd/system/<code> to make changes. The <code>/etc/systemd/system/ only links to the correct executable script configuration file.

Common Commands

Task	Old Command	New Command
Enable a service to start automatically	chkconfig –level 3 httpd on	systemctl enable httpd.service
Disable a service from starting automatically	chkconfig –level 3 httpd off	systemctl disable httpd.service
Check service status	service httpd status	systemctl status httpd.service (detailed service information) systemctl is-active httpd.service (only shows if Active)
Show all started services	chkconfig –list	systemctl list-units –type=service
Start a service	service httpd start	systemctl start httpd.service
Stop a service	service httpd stop	systemctl stop httpd.service
Restart a service	service httpd restart	systemctl restart httpd.service

Check service status

systemctl status httpd

Running Status:

• active (running): means one or more programs are currently executing in the system;
• active (exited): a service that has executed once and ended normally, currently no programs are executing in the system;
• active (waiting): currently executing, but needs to wait for other events to continue processing;
• inactive: this service is currently not running;
• dead: the program has been cleared;

Startup Status:

• enabled: this daemon will be executed at boot;
• disabled: this daemon will not be executed at boot;
• static: this daemon cannot start by itself (enable is not possible), but may be awakened by other enabled services (associated properties services);
• mask: this daemon cannot be started under any circumstances, as it has been forcibly disabled (not deleted), and can be restored to its original state using systemctl unmask;

Check all configuration details of the service

systemctl show httpd

Check the auto-start status of each service

systemctl list-unit-files --type=service

Get the list of dependencies for a service

systemctl list-dependencies httpd.service

List control groups hierarchically

systemd-cgls

List control groups by CPU, memory, input, and output

systemd-cgtop

Analyze the time each process takes during boot

systemd-analyze blame

List all available system sockets

systemctl list-unit-files --type=socket

Socket operations

systemctl start cups.socket # Start the socket
systemctl restart cups.socket # Restart the socket
systemctl stop cups.socket # Stop the socket
systemctl reload cups.socket # Reload the socket
systemctl status cups.socket # Check the socket status

systemctl is-active cups.socket
systemctl enable cups.socket
systemctl disable cups.socket

Other commands

systemctl mask httpd.service   # Disable a service
systemctl unmask httpd.service   # Unmask a service

systemctl isolate multi-user.target  # Change the current operating environment to text mode, turn off the graphical interface
systemctl isolate graphical.target  # Change the current operating environment to graphical interface

systemctl poweroff # Shutdown the system
systemctl reboot  # Reboot the system
systemctl suspend  # Enter suspend mode
systemctl hibernate # Enter hibernate mode
systemctl rescue  # Force enter rescue mode
systemctl emergency # Force enter emergency mode

-END-

One-stop Solution Provider for Industrial Internet

Weak Current Intelligentization/ Smart Security / Data Center / Intelligent Manufacturing Software / Industrial Internet Platform/ Smart Factory

Pragmatic Innovative Striving Enterprising

Free Consultation Hotline: 400-8327-811 ext. 804

Website: www.0512rh.com

Address: 9th Floor, Industrial Capital Center, No. 160 Longxi Road, Wuzhong High-tech Zone, Suzhou

— Jiangsu Ronghui Technology —