Ví dụ đại lý snmp python

10. 1. 1.  Main Features

In Red Hat Enterprise Linux 7, the systemd system and service manager provides the following main features

• Socket-based activation — At boot time, systemd creates listening sockets for all system services that support this type of activation, and passes the sockets to these services as soon as they are started. Điều này không chỉ cho phép systemd bắt đầu các dịch vụ song song mà còn cho phép khởi động lại một dịch vụ mà không làm mất bất kỳ tin nhắn nào được gửi tới nó khi nó không khả dụng. the corresponding socket remains accessible and all messages are queued.

  Systemd uses socket units for socket-based activation.

• Bus-based activation — System services that use D-Bus for inter-process communication can be started on-demand the first time a client application attempts to communicate with them. Systemd uses D-Bus service files for bus-based activation.
• Device-based activation — System services that support device-based activation can be started on-demand when a particular type of hardware is plugged in or becomes available. Systemd uses device units for device-based activation.
• Path-based activation — System services that support path-based activation can be started on-demand when a particular file or directory changes its state. Systemd uses path units for path-based activation.
• Mount and automount point management — Systemd monitors and manages mount and automount points. Systemd uses mount units for mount points and automount units for automount points.
• Aggressive parallelization — Because of the use of socket-based activation, systemd can start system services in parallel as soon as all listening sockets are in place. In combination with system services that support on-demand activation, parallel activation significantly reduces the time required to boot the system.
• Transactional unit activation logic — Before activating or deactivating a unit, systemd calculates its dependencies, creates a temporary transaction, and verifies that this transaction is consistent. If a transaction is inconsistent, systemd automatically attempts to correct it and remove non-essential jobs from it before reporting an error.
• Backwards compatibility with SysV init — Systemd supports SysV init scripts as described in the Linux Standard Base Core Specification , which eases the upgrade path to systemd service units.

10. 1. 2.  Compatibility Changes

The systemd system and service manager is designed to be mostly compatible with SysV init and Upstart. The following are the most notable compatibility changes with regards to the previous major release of the Red Hat Enterprise Linux system

• Systemd has only limited support for runlevels. It provides a number of target units that can be directly mapped to these runlevels and for compatibility reasons, it is also distributed with the earlier

  ~]# systemctl stop nfs-server.service

  26 command. Not all systemd targets can be directly mapped to runlevels, however, and as a consequence, this command might return

  ~]# systemctl stop nfs-server.service

  27 to indicate an unknown runlevel. It is recommended that you avoid using the

  ~]# systemctl stop nfs-server.service

  26 command if possible

  For more information about systemd targets and their comparison with runlevels, see Section 10. 3, “Working with systemd Targets”

• The

  ~]# systemctl stop nfs-server.service

  29 utility does not support custom commands. In addition to standard commands such as

  ~]# systemctl stop nfs-server.service

  30,

  ~]# systemctl stop nfs-server.service

  31, and

  ~]# systemctl stop nfs-server.service

  32, authors of SysV init scripts could implement support for any number of arbitrary commands in order to provide additional functionality. For example, the init script for

  ~]# systemctl stop nfs-server.service

  33 in Red Hat Enterprise Linux 6 could be executed with the

  ~]# systemctl stop nfs-server.service

  34 command, which immediately enabled panic mode and reconfigured the system to start dropping all incoming and outgoing packets. This is not supported in systemd and the

  ~]# systemctl stop nfs-server.service

  29 only accepts documented commands

  For more information about the

  ~]# systemctl stop nfs-server.service

  29 utility and its comparison with the earlier

  ~]# systemctl stop nfs-server.service

  37 utility, see Section 10. 2, “Managing System Services”

• The

  ~]# systemctl stop nfs-server.service

  29 utility does not communicate with services that have not been started by systemd. When systemd starts a system service, it stores the ID of its main process in order to keep track of it. The

  ~]# systemctl stop nfs-server.service

  29 utility then uses this PID to query and manage the service. Consequently, if a user starts a particular daemon directly on the command line,

  ~]# systemctl stop nfs-server.service

  29 is unable to determine its current status or stop it
• Systemd stops only running services. Previously, when the shutdown sequence was initiated, Red Hat Enterprise Linux 6 and earlier releases of the system used symbolic links located in the

  ~]# systemctl stop nfs-server.service

  41 directory to stop all available system services regardless of their status. With systemd, only running services are stopped on shutdown
• System services are unable to read from the standard input stream. When systemd starts a service, it connects its standard input to

  ~]# systemctl stop nfs-server.service

  42 to prevent any interaction with the user
• System services do not inherit any context [such as the

  ~]# systemctl stop nfs-server.service

  43 and

  ~]# systemctl stop nfs-server.service

  44 environment variables] from the invoking user and their session. Each service runs in a clean execution context
• When loading a SysV init script, systemd reads dependency information encoded in the Linux Standard Base [LSB] header and interprets it at run time
• Tất cả các hoạt động trên các đơn vị dịch vụ phải tuân theo thời gian chờ mặc định là 5 phút để ngăn dịch vụ bị trục trặc đóng băng hệ thống. Giá trị này được mã hóa cứng cho các dịch vụ được tạo từ initscripts và không thể thay đổi. However, individual configuration files can be used to specify a longer timeout value per service, see Example 10. 21, “Changing the timeout limit”

For a detailed list of compatibility changes introduced with systemd, see the Migration Planning Guide for Red Hat Enterprise Linux 7

10. 2. 1.  Listing Services

To list all currently loaded service units, type the following at a shell prompt

~]# systemctl stop nfs-server.service

Đối với mỗi tệp đơn vị dịch vụ, lệnh này hiển thị tên đầy đủ của nó [

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

By default, the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

You can also list all available service units to see if they are enabled. To do so, type

~]# systemctl stop nfs-server.service

For each service unit, this command displays its full name [

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Example 10. 1.  Listing Services

To list all currently loaded service units, run the following command

~]# systemctl stop nfs-server.service

To list all installed service unit files to determine if they are enabled, type

~]# systemctl stop nfs-server.service

10. 2. 2.  Displaying Service Status

To display detailed information about a service unit that corresponds to a system service, type the following at a shell prompt

~]# systemctl stop nfs-server.service

Replace name with the name of the service unit you want to inspect [for example,

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Table 10. 5.  Available Service Unit Information

FieldDescription

~]# systemctl stop nfs-server.service

Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled

~]# systemctl stop nfs-server.service

Information whether the service unit is running followed by a time stamp

~]# systemctl stop nfs-server.service

The PID of the corresponding system service followed by its name

~]# systemctl stop nfs-server.service

Additional information about the corresponding system service

~]# systemctl stop nfs-server.service

Additional information about related processes

~]# systemctl stop nfs-server.service

Additional information about related Control Groups [cgroups]

To only verify that a particular service unit is running, run the following command

~]# systemctl stop nfs-server.service

Similarly, to determine whether a particular service unit is enabled, type

~]# systemctl stop nfs-server.service

Note that both

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Example 10. 2.  Displaying Service Status

The service unit for the GNOME Display Manager is named

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Example 10. 3.  Displaying Services Ordered to Start Before a Service

To determine what services are ordered to start before the specified service, type the following at a shell prompt

~]# systemctl stop nfs-server.service

Example 10. 4.  Displaying Services Ordered to Start After a Service

To determine what services are ordered to start after the specified service, type the following at a shell prompt

~]# systemctl stop nfs-server.service

10. 2. 3.  Starting a Service

To start a service unit that corresponds to a system service, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the service unit you want to start [for example,

~]# systemctl stop nfs-server.service

Example 10. 5.  Starting a Service

The service unit for the Apache HTTP Server is named

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 2. 4.  Stopping a Service

To stop a service unit that corresponds to a system service, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the service unit you want to stop [for example,

~]# systemctl stop nfs-server.service

Example 10. 6.  Stopping a Service

The service unit for the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 2. 5.  Restarting a Service

To restart a service unit that corresponds to a system service, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the service unit you want to restart [for example,

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Certain system services also allow you to reload their configuration without interrupting their execution. To do so, type as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Note that system services that do not support this feature ignore this command altogether. For convenience, the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Example 10. 7.  Restarting a Service

In order to prevent users from encountering unnecessary error messages or partially rendered web pages, the Apache HTTP Server allows you to edit and reload its configuration without the need to restart it and interrupt actively processed requests. To do so, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 2. 6.  Enabling a Service

To configure a service unit that corresponds to a system service to be automatically started at boot time, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the service unit you want to enable [for example,

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

This command disables the selected service unit and immediately enables it again. For information on how to determine whether a certain service unit is enabled to start at boot time, see Section 10. 2. 2, “Hiển thị trạng thái dịch vụ”. For information on how to start a service in the current session, see Section 10. 2. 3, “Starting a Service”

Example 10. 8.  Enabling a Service

To configure the Apache HTTP Server to start automatically at boot time, run the following command as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 2. 7.  Disabling a Service

To prevent a service unit that corresponds to a system service from being automatically started at boot time, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the service unit you want to disable [for example,

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

This command replaces the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

For information on how to determine whether a certain service unit is enabled to start at boot time, see Section 10. 2. 2, “Displaying Service Status”. For information on how to stop a service in the current session, see Section 10. 2. 4, “Stopping a Service”

Example 10. 9.  Disabling a Service

Example 10. 6, “Stopping a Service” illustrates how to stop the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 2. 8.  Starting a Conflicting Service

In systemd , positive and negative dependencies between services exist. Starting particular service may require starting one or more other services [positive dependency] or stopping one or more services [negative dependency].

When you attempt to start a new service, systemd resolves all dependencies automatically. Note that this is done without explicit notification to the user. If you are already running a service, and you attempt to start another service with a negative dependency, the first service is automatically stopped.

For example, if you are running the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 3. 1.  Viewing the Default Target

To determine which target unit is used by default, run the following command

~]# systemctl stop nfs-server.service

This command resolves the symbolic link located at

~]# systemctl stop nfs-server.service

Example 10. 10.  Viewing the Default Target

To display the default target unit, type

~]# systemctl stop nfs-server.service

10. 3. 2.  Viewing the Current Target

To list all currently loaded target units, type the following command at a shell prompt

~]# systemctl stop nfs-server.service

For each target unit, this commands displays its full name [

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

By default, the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

See Section 10. 3. 1, “Viewing the Default Target” for information on how to display the default target. For information on how to change the current target, see Section 10. 3. 4, “Changing the Current Target”

Ví dụ 10. 11.  Viewing the Current Target

To list all currently loaded target units, run the following command

~]# systemctl stop nfs-server.service

10. 3. 3.  Changing the Default Target

To configure the system to use a different target unit by default, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the target unit you want to use by default [for example,

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Example 10. 12.  Changing the Default Target

To configure the system to use the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 3. 4.  Changing the Current Target

To change to a different target unit in the current session, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Replace name with the name of the target unit you want to use [for example,

~]# systemctl stop nfs-server.service

Example 10. 13.  Changing the Current Target

To turn off the graphical user interface and change to the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 3. 5.  Changing to Rescue Mode

Rescue mode provides a convenient single-user environment and allows you to repair your system in situations when it is unable to complete a regular booting process. In rescue mode, the system attempts to mount all local file systems and start some important system services, but it does not activate network interfaces or allow more users to be logged into the system at the same time. In Red Hat Enterprise Linux 7, rescue mode is equivalent to single user mode and requires the root password.

To change the current target and enter rescue mode in the current session, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

This command is similar to

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

For information on how to enter emergency mode, see Section 10. 3. 6, “Changing to Emergency Mode”

Example 10. 14.  Changing to Rescue Mode

To enter rescue mode in the current session, run the following command as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 3. 6.  Changing to Emergency Mode

Emergency mode provides the most minimal environment possible and allows you to repair your system even in situations when the system is unable to enter rescue mode. Ở chế độ khẩn cấp, hệ thống chỉ gắn hệ thống tệp gốc để đọc, không cố gắn bất kỳ hệ thống tệp cục bộ nào khác, không kích hoạt giao diện mạng và chỉ bắt đầu một vài dịch vụ thiết yếu. In Red Hat Enterprise Linux 7, emergency mode requires the root password.

To change the current target and enter emergency mode, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

This command is similar to

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

For information on how to enter rescue mode, see Section 10. 3. 5, “Changing to Rescue Mode”

Ví dụ 10. 15.  Changing to Emergency Mode

To enter emergency mode without sending a message to all users that are currently logged into the system, run the following command as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 4. 1.  Shutting Down the System

The

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

To shut down the system and power off the machine, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Để tắt và tạm dừng hệ thống mà không cần tắt nguồn máy, hãy chạy lệnh sau là

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

By default, running either of these commands causes systemd to send an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run the selected command with the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

To shut down the system and power off the machine at a certain time, use a command in the following format as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Where hh. mm is the time in 24 hour clock format. The

~]# systemctl stop nfs-server.service

To shut down and halt the system after a delay, without powering off the machine, use a command in the following format as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Ở đâu +m là thời gian trễ tính bằng phút. The

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

A pending shutdown can be canceled by the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

See the

~]# systemctl stop nfs-server.service

10. 4. 2.  Restarting the System

To restart the system, run the following command as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

By default, this command causes systemd to send an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run this command with the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 4. 3.  Suspending the System

To suspend the system, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

This command saves the system state in RAM and with the exception of the RAM module, powers off most of the devices in the machine. When you turn the machine back on, the system then restores its state from RAM without having to boot again. Because the system state is saved in RAM and not on the hard disk, restoring the system from suspend mode is significantly faster than restoring it from hibernation, but as a consequence, a suspended system state is also vulnerable to power outages

For information on how to hibernate the system, see Section 10. 4. 4, “Hibernating the System”

10. 4. 4.  Hibernating the System

To hibernate the system, type the following at a shell prompt as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

This command saves the system state on the hard disk drive and powers off the machine. When you turn the machine back on, the system then restores its state from the saved data without having to boot again. Because the system state is saved on the hard disk and not in RAM, the machine does not have to maintain electrical power to the RAM module, but as a consequence, restoring the system from hibernation is significantly slower than restoring it from suspend mode

To hibernate and suspend the system, run the following command as

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

For information on how to suspend the system, see Section 10. 4. 3, “Suspending the System”

10. 6. 1.  Understanding the Unit File Structure

Unit files typically consist of three sections

• [Unit] — contains generic options that are not dependent on the type of the unit. These options provide unit description, specify the unit’s behavior, and set dependencies to other units. For a list of most frequently used [Unit] options, see Table 10. 9, “Important [Unit] Section Options”
• [ unit type ] — if a unit has type-specific directives, these are grouped under a section named after the unit type. For example, service unit files contain the [Service] section, see Table 10. 10, “Important [Service] Section Options” for most frequently used [Service] options.
• [Install] — contains information about unit installation used by

  ~]# systemctl stop nfs-server.service

  23 and

  ~]# systemctl stop nfs-server.service

  570 commands, see Table 10. 11, “Important [Install] Section Options” for a list of [Install] options

Table 10. 9.  Important [Unit] Section Options

Option[a] section, see the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

A meaningful description of the unit. This text is displayed for example in the output of the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Provides a list of URIs referencing documentation for the unit

~]# systemctl stop nfs-server.service

Defines the order in which units are started. The unit starts only after the units specified in

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Configures dependencies on other units. The units listed in

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Configures weaker dependencies than

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Configures negative dependencies, an opposite to

~]# systemctl stop nfs-server.service

[a] For a complete list of options configurable in the [Unit

[b] In most cases, it is sufficient to set only the ordering dependencies with

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Table 10. 10.  Important [Service] Section Options

Option[a] section, see the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Configures the unit process startup type that affects the functionality of

~]# systemctl stop nfs-server.service

*

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

*

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

*

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

*

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

*

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

*

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Specifies commands or scripts to be executed when the unit is started.

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Specifies commands or scripts to be executed when the unit is stopped

~]# systemctl stop nfs-server.service

Specifies commands or scripts to be executed when the unit is reloaded

~]# systemctl stop nfs-server.service

With this option enabled, the service is restarted after its process exits, with the exception of a clean stop by the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

If set to True, the service is considered active even when all its processes exited. Default value is False. This option is especially useful if

~]# systemctl stop nfs-server.service

[a] For a complete list of options configurable in the [Service

Table 10. 11.  Important [Install] Section Options

Option[a] section, see the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Provides a space-separated list of additional names for the unit. Most

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

A list of units that depend on the unit. When this unit is enabled, the units listed in

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

A list of units that weakly depend on the unit. Khi thiết bị này được bật, các thiết bị được liệt kê trong

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Specifies a list of units to be installed or uninstalled along with the unit

~]# systemctl stop nfs-server.service

Limited to instantiated units, this option specifies the default instance for which the unit is enabled. See Section 10. 6. 5, “Working with Instantiated Units”

[a] For a complete list of options configurable in the [Install

A whole range of options that can be used to fine tune the unit configuration, Example 10. 17, “postfix. service Unit File” shows an example of a service unit installed on the system. Moreover, unit file options can be defined in a way that enables dynamic creation of units as described in Section 10. 6. 5, “Working with Instantiated Units”

Example 10. 17.  postfix. service Unit File

What follows is the content of the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

The [Unit] section describes the service, specifies the ordering dependencies, as well as conflicting units. In [Service], a sequence of custom scripts is specified to be executed during unit activation, on stop, and on reload.

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 6. 2.  Creating Custom Unit Files

There are several use cases for creating unit files from scratch. you could run a custom daemon, create a second instance of some existing service [as in Example 10. 19, “Creating a second instance of the sshd service”], or import a SysV init script [more in Section 10. 6. 3, “Converting SysV Init Scripts to Unit Files”]. On the other hand, if you intend just to modify or extend the behavior of an existing unit, use the instructions from Section 10. 6. 4, “Modifying Existing Unit Files”. The following procedure describes the general process of creating a custom service

1. Prepare the executable file with the custom service. This can be a custom-created script, or an executable delivered by a software provider. If required, prepare a PID file to hold a constant PID for the main process of the custom service. It is also possible to include environment files to store shell variables for the service. Make sure the source script is executable [by executing the

   ~]# systemctl stop nfs-server.service

   732] and is not interactive

2. Create a unit file in the

   ~]# systemctl stop nfs-server.service

   22 directory and make sure it has correct file permissions. Execute as

   ~]# systemctl stop nfs-server.service

   402

   ~]# systemctl stop nfs-server.service

   71

   Replace name with a name of the service to be created. Note that file does not need to be executable.

3. Open the

   ~]# systemctl stop nfs-server.service

   735 file created in the previous step, and add the service configuration options. There is a variety of options that can be used depending on the type of service you wish to create, see Section 10. 6. 1, “Understanding the Unit File Structure”. The following is an example unit configuration for a network-related service

   ~]# systemctl stop nfs-server.service

   72

   Where

   • service_description is an informative description that is displayed in journal log files and in the output of the

     ~]# systemctl stop nfs-server.service

     573 command.
   • the

     ~]# systemctl stop nfs-server.service

     575 setting ensures that the service is started only after the network is running. Add a space-separated list of other relevant services or targets
   • path_to_executable stands for the path to the actual service executable.

   • ~]# systemctl stop nfs-server.service

     738 được sử dụng cho các daemon thực hiện lệnh gọi hệ thống fork. The main process of the service is created with the PID specified in path_to_pidfile . Find other startup types in Table 10. 10, “Important [Service] Section Options”.

   • ~]# systemctl stop nfs-server.service

     724 states the target or targets that the service should be started under. Think of these targets as of a replacement of the older concept of runlevels, see Section 10. 3, “Working with systemd Targets” for details

4. Notify systemd that a new

   ~]# systemctl stop nfs-server.service

   735 file exists by executing the following command as

   ~]# systemctl stop nfs-server.service

   402

   ~]# systemctl stop nfs-server.service

   73

   Warning

   Always run the

   ~]# systemctl stop nfs-server.service

   742 command after creating new unit files or modifying existing unit files. Otherwise, the

   ~]# systemctl stop nfs-server.service

   743 or

   ~]# systemctl stop nfs-server.service

   23 commands could fail due to a mismatch between states of systemd and actual service unit files on disk

   The name . service unit can now be managed as any other system service with commands described in Section 10. 2, “Managing System Services”.

Example 10. 18.  Creating the emacs. service File

When using the Emacs text editor, it is often faster and more convenient to have it running in the background instead of starting a new instance of the program whenever editing a file. The following steps show how to create a unit file for Emacs, so that it can be handled like a service.

1. Create a unit file in the

   ~]# systemctl stop nfs-server.service

   22 directory and make sure it has the correct file permissions. Execute as

   ~]# systemctl stop nfs-server.service

   402

   ~]# systemctl stop nfs-server.service

   74

2. Add the following content to the file

   ~]# systemctl stop nfs-server.service

   75

   With the above configuration, the

   ~]# systemctl stop nfs-server.service

   747 executable is started in daemon mode on service start. The SSH_AUTH_SOCK environment variable is set using the "%t" unit specifier that stands for the runtime directory. The service also restarts the emacs process if it exits unexpectedly

3. Execute the following commands to reload the configuration and start the custom service

   ~]# systemctl stop nfs-server.service

   76

As the editor is now registered as a systemd service, you can use all standard

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Example 10. 19.  Creating a second instance of the sshd service

System Administrators often need to configure and run multiple instances of a service. This is done by creating copies of the original service configuration files and modifying certain parameters to avoid conflicts with the primary instance of the service. The following procedure shows how to create a second instance of the

~]# systemctl stop nfs-server.service

1. Create a copy of the

   ~]# systemctl stop nfs-server.service

   752 file that will be used by the second daemon

   ~]# systemctl stop nfs-server.service

   77

2. Edit the

   ~]# systemctl stop nfs-server.service

   753 file created in the previous step to assign a different port number and PID file to the second daemon

   ~]# systemctl stop nfs-server.service

   78

   See the

   ~]# systemctl stop nfs-server.service

   752[5] manual page for more information on

   ~]# systemctl stop nfs-server.service

   755 and

   ~]# systemctl stop nfs-server.service

   756 options. Make sure the port you choose is not in use by any other service. The PID file does not have to exist before running the service, it is generated automatically on service start

3. Tạo một bản sao của tệp đơn vị systemd cho dịch vụ

   ~]# systemctl stop nfs-server.service

   551

   ~]# systemctl stop nfs-server.service

   79

4. Alter the

   ~]# systemctl stop nfs-server.service

   758 created in the previous step as follows

   1. Modify the

      ~]# systemctl stop nfs-server.service

      572 option

      ~]# systemctl stop nfs-server.service

      80

   2. Add sshd. service to services specified in the

      ~]# systemctl stop nfs-server.service

      575 option, so that the second instance starts only after the first one has already started

      ~]# systemctl stop nfs-server.service

      81
   3. The first instance of sshd includes key generation, therefore remove the ExecStartPre=/usr/sbin/sshd-keygen line.

   4. Add the

      ~]# systemctl stop nfs-server.service

      761 parameter to the

      ~]# systemctl stop nfs-server.service

      551 command, so that the alternative configuration file is used

      ~]# systemctl stop nfs-server.service

      82

   5. After the above modifications, the sshd-second. service should look as follows

      ~]# systemctl stop nfs-server.service

      83

5. If using SELinux, add the port for the second instance of sshd to SSH ports, otherwise the second instance of sshd will be rejected to bind to the port

   ~]# systemctl stop nfs-server.service

   84

6. Enable sshd-second. service, so that it starts automatically upon boot

   ~]# systemctl stop nfs-server.service

   85

   Verify if the sshd-second. service is running by using the

   ~]# systemctl stop nfs-server.service

   573 command. Also, verify if the port is enabled correctly by connecting to the service

   ~]# systemctl stop nfs-server.service

   86

   If the firewall is in use, make sure that it is configured appropriately in order to allow connections to the second instance of sshd

To learn how to properly choose a target for ordering and dependencies of your custom unit files, see the following articles

• How to write a service unit file which enforces that particular services have to be started
• How to decide what dependencies a systemd service unit definition should have

Additional information with some real-world examples of cases triggered by the ordering and dependencies in a unit file is available in the following article. Is there any useful information about writing unit files?

If you want to set limits for services started by

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 6. 3.  Converting SysV Init Scripts to Unit Files

Before taking time to convert a SysV init script to a unit file, make sure that the conversion was not already done elsewhere. All core services installed on Red Hat Enterprise Linux 7 come with default unit files, and the same applies for many third-party software packages

Converting an init script to a unit file requires analyzing the script and extracting the necessary information from it. Based on this data you can create a unit file as described in Section 10. 6. 2, “Creating Custom Unit Files”. As init scripts can vary greatly depending on the type of the service, you might need to employ more configuration options for translation than outlined in this chapter. Note that some levels of customization that were available with init scripts are no longer supported by systemd units, see Section 10. 1. 2, “Compatibility Changes”

The majority of information needed for conversion is provided in the script’s header. The following example shows the opening section of the init script used to start the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

In the above example, only lines starting with # chkconfig and # description are mandatory, so you might not find the rest in different init files. The text enclosed between the # BEGIN INIT INFO and # END INIT INFO lines is called Linux Standard Base [LSB] header . If specified, LSB headers contain directives defining the service description, dependencies, and default runlevels. What follows is an overview of analytic tasks aiming to collect the data needed for a new unit file. The postfix init script is used as an example, see the resulting postfix unit file in Example 10. 17, “postfix. service Unit File”.

Find descriptive information about the script on the line starting with #description . Use this description together with the service name in the

~]# systemctl stop nfs-server.service

The LSB header might contain several directives that form dependencies between services. Most of them are translatable to systemd unit options, see Table 10. 12, “Dependency Options from the LSB Header”

Table 10. 12.  Dependency Options from the LSB Header

LSB OptionDescriptionUnit File Equivalent

~]# systemctl stop nfs-server.service

Specifies the boot facility name of the service, that can be referenced in other init scripts [with the "$" prefix]. This is no longer needed as unit files refer to other units by their file names

–

~]# systemctl stop nfs-server.service

Contains boot facility names of required services. This is translated as an ordering dependency, boot facility names are replaced with unit file names of corresponding services or targets they belong to. For example, in case of

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Constitutes weaker dependencies than Required-Start. Failed Should-Start dependencies do not affect the service startup

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Constitute negative dependencies

~]# systemctl stop nfs-server.service

The line starting with #chkconfig contains three numerical values. Điều quan trọng nhất là số đầu tiên đại diện cho mức chạy mặc định trong đó dịch vụ được bắt đầu. Use Table 10. 6, “Comparison of SysV Runlevels with systemd Targets” to map these runlevels to equivalent systemd targets. Then list these targets in the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

The other two values specified on the #chkconfig line represent startup and shutdown priorities of the init script. These values are interpreted by systemd if it loads the init script, but there is no unit file equivalent

Init scripts require loading a function library from a dedicated directory and allow importing configuration, environment, and PID files. Environment variables are specified on the line starting with #config in the init script header, which translates to the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

The key information that is not included in the init script header is the path to the service executable, and potentially some other files required by the service. In previous versions of Red Hat Enterprise Linux, init scripts used a Bash case statement to define the behavior of the service on default actions, such as start , stop , or restart , as well as custom-defined actions. The following excerpt from the

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

The extensibility of the init script allowed specifying two custom functions,

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Systemd supports only the predefined actions, but enables executing custom executables with

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Chuyển đổi các tập lệnh init phức tạp đòi hỏi phải hiểu mục đích của mọi câu lệnh trong tập lệnh. Một số câu lệnh dành riêng cho phiên bản hệ điều hành, do đó bạn không cần phải dịch chúng. Mặt khác, một số điều chỉnh có thể cần thiết trong môi trường mới, cả trong tệp đơn vị cũng như trong các tệp hỗ trợ và thực thi dịch vụ

10. 6. 4. Sửa đổi tập tin đơn vị hiện có

Các dịch vụ được cài đặt trên hệ thống đi kèm với các tệp đơn vị mặc định được lưu trữ trong thư mục

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

• Tạo một thư mục cho các tệp cấu hình bổ sung tại

  ~]# systemctl stop nfs-server.service

  804. Phương pháp này được khuyến nghị cho hầu hết các trường hợp sử dụng. Nó cho phép mở rộng cấu hình mặc định với chức năng bổ sung, trong khi vẫn tham chiếu đến tệp đơn vị gốc. Do đó, các thay đổi đối với đơn vị mặc định được giới thiệu khi nâng cấp gói được áp dụng tự động. Xem phần có tên “Mở rộng Cấu hình Đơn vị Mặc định” để biết thêm thông tin
• Create a copy of the original unit file

  ~]# systemctl stop nfs-server.service

  20 in

  ~]# systemctl stop nfs-server.service

  22 and make changes there. Bản sao sẽ ghi đè tệp gốc, do đó, những thay đổi được đưa ra với bản cập nhật gói không được áp dụng. Phương pháp này hữu ích để thực hiện các thay đổi đơn vị quan trọng sẽ tồn tại bất kể các bản cập nhật gói. Xem phần có tên “Ghi đè Cấu hình Đơn vị Mặc định” để biết chi tiết

Để quay lại cấu hình mặc định của thiết bị, chỉ cần xóa các tệp cấu hình được tạo tùy chỉnh trong

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Tùy chọn

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Ngoài ra, nếu tệp đơn vị đã sửa đổi thuộc về một dịch vụ đang chạy, dịch vụ này phải được khởi động lại để chấp nhận cài đặt mới

~]# systemctl stop nfs-server.service

Quan trọng

Để sửa đổi các thuộc tính, chẳng hạn như phần phụ thuộc hoặc thời gian chờ, của một dịch vụ được xử lý bởi bản initscript SysV, không sửa đổi chính bản initscript đó. Thay vào đó, hãy tạo tệp cấu hình thả xuống

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Ví dụ: để mở rộng cấu hình của dịch vụ

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Để mở rộng tệp đơn vị mặc định với các tùy chọn cấu hình bổ sung, trước tiên hãy tạo một thư mục cấu hình trong

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Thay thế tên bằng tên của dịch vụ bạn muốn gia hạn. Cú pháp trên áp dụng cho tất cả các loại đơn vị.

Tạo tệp cấu hình trong thư mục đã tạo ở bước trước. Lưu ý rằng tên tệp phải kết thúc bằng . conf hậu tố. Loại hình.

~]# systemctl stop nfs-server.service

Thay thế config_name bằng tên của tệp cấu hình. Tệp này tuân thủ cấu trúc tệp đơn vị bình thường, do đó, tất cả các lệnh phải được chỉ định trong các phần thích hợp, hãy xem Phần 10. 6. 1, “Tìm hiểu cấu trúc tệp đơn vị”.

Ví dụ: để thêm phụ thuộc tùy chỉnh, hãy tạo tệp cấu hình với nội dung sau

~]# systemctl stop nfs-server.service

Ở đâu phần phụ thuộc mới là viết tắt của đơn vị được đánh dấu là phần phụ thuộc. Một ví dụ khác là tệp cấu hình khởi động lại dịch vụ sau khi quá trình chính của nó đã thoát, với độ trễ là 30 giây.

~]# systemctl stop nfs-server.service

Nên tạo các tệp cấu hình nhỏ chỉ tập trung vào một tác vụ. Các tệp như vậy có thể dễ dàng di chuyển hoặc liên kết với các thư mục cấu hình của các dịch vụ khác

Để áp dụng các thay đổi được thực hiện cho đơn vị, hãy thực hiện như

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Ví dụ 10. 20. Mở rộng httpd. cấu hình dịch vụ

Để sửa đổi httpd. đơn vị dịch vụ để tập lệnh shell tùy chỉnh được thực thi tự động khi khởi động dịch vụ Apache, hãy thực hiện các bước sau. Đầu tiên, tạo một thư mục và tệp cấu hình tùy chỉnh

~]# systemctl stop nfs-server.service

Với điều kiện là tập lệnh bạn muốn bắt đầu tự động với Apache được đặt tại

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Để áp dụng các thay đổi đơn vị, hãy thực hiện

~]# systemctl stop nfs-server.service

Note

Các tệp cấu hình từ các thư mục cấu hình trong

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Để thực hiện các thay đổi sẽ tồn tại sau khi cập nhật gói cung cấp tệp đơn vị, trước tiên hãy sao chép tệp vào thư mục

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Ở đâu tên là tên của đơn vị dịch vụ mà bạn muốn sửa đổi. Cú pháp trên áp dụng cho tất cả các loại đơn vị.

Mở tệp đã sao chép bằng trình soạn thảo văn bản và thực hiện các thay đổi mong muốn. Để áp dụng các thay đổi đơn vị, hãy thực hiện như

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Ví dụ 10. 21. Thay đổi giới hạn thời gian chờ

Bạn có thể chỉ định giá trị thời gian chờ cho mỗi dịch vụ để ngăn dịch vụ bị trục trặc đóng băng hệ thống. Mặt khác, thời gian chờ được đặt theo mặc định là 90 giây đối với các dịch vụ thông thường và 300 giây đối với các dịch vụ tương thích với SysV

Ví dụ: để mở rộng giới hạn thời gian chờ cho dịch vụ

~]# systemctl stop nfs-server.service

1. Sao chép tệp đơn vị

   ~]# systemctl stop nfs-server.service

   89 vào thư mục

   ~]# systemctl stop nfs-server.service

   22

   ~]# systemctl stop nfs-server.service

   022

2. Mở tệp

   ~]# systemctl stop nfs-server.service

   836 và chỉ định giá trị

   ~]# systemctl stop nfs-server.service

   837 trong phần

   ~]# systemctl stop nfs-server.service

   838

   ~]# systemctl stop nfs-server.service

   023

3. Tải lại trình nền

   ~]# systemctl stop nfs-server.service

   82

   ~]# systemctl stop nfs-server.service

   89

4. Tùy chọn. Xác minh giá trị thời gian chờ mới.

   ~]# systemctl stop nfs-server.service

   025

Note

Để thay đổi giới hạn thời gian chờ trên toàn cầu, hãy nhập

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

To display an overview of overridden or modified unit files, use the following command

~]# systemctl stop nfs-server.service

Ví dụ: đầu ra của lệnh trên có thể trông như sau

~]# systemctl stop nfs-server.service

Bảng 10. 13, “Các loại khác biệt systemd-delta” liệt kê các loại ghi đè có thể xuất hiện trong đầu ra của

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Bảng 10. 13. Các loại khác biệt systemd-delta

TypeDescription

[CHE MẶT]

Masked unit files, see Section 10. 2. 7, “Disabling a Service” for description of unit masking

[TƯƠNG ĐƯƠNG]

Các bản sao chưa sửa đổi ghi đè lên các tệp gốc nhưng không khác về nội dung, thường là các liên kết tượng trưng

[REDIRECTED]

Files that are redirected to another file

[OVERRIDEN]

Overridden and changed files

[EXTENDED]

Files that are extended with . conf files in the

~]# systemctl stop nfs-server.service

[UNCHANGED]

Unmodified files are displayed only when the

~]# systemctl stop nfs-server.service

It is good practice to run

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

10. 6. 5.  Working with Instantiated Units

It is possible to instantiate multiple units from a single template configuration file at runtime. The "@" character is used to mark the template and to associate units with it. Instantiated units can be started from another unit file [using

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Where template_name stands for the name of the template configuration file. Replace instance_name with the name for the unit instance. Một số phiên bản có thể trỏ đến cùng một tệp mẫu với các tùy chọn cấu hình phổ biến cho tất cả các phiên bản của đơn vị. Template unit name has the form of.

~]# systemctl stop nfs-server.service

For example, the following

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

first makes systemd search for given service units. Nếu không tìm thấy đơn vị nào như vậy, phần giữa "@" và hậu tố loại sẽ bị bỏ qua và systemd tìm kiếm tệp

~]# systemctl stop nfs-server.service

Wildcard characters, called unit specifiers , can be used in any unit configuration file. Bộ xác định đơn vị thay thế các tham số đơn vị nhất định và được giải thích trong thời gian chạy. Table 10. 14, “Important Unit Specifiers” lists unit specifiers that are particularly useful for template units.

Table 10. 14.  Important Unit Specifiers

Unit SpecifierMeaningDescription

~]# systemctl stop nfs-server.service

Full unit name

Stands for the full unit name including the type suffix.

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Prefix name

Stands for a unit name with type suffix removed. Đối với các đơn vị được khởi tạo %p là viết tắt của phần tên đơn vị trước ký tự "@"

~]# systemctl stop nfs-server.service

Tên trường hợp

Is the part of the instantiated unit name between the "@" character and the type suffix.

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Host name

Viết tắt của tên máy chủ của hệ thống đang chạy tại thời điểm cấu hình đơn vị được tải

~]# systemctl stop nfs-server.service

thư mục thời gian chạy

Represents the runtime directory, which is either

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

For a complete list of unit specifiers, see the

~]# systemctl stop nfs-server.service

Ví dụ: mẫu

~]# systemctl stop nfs-server.service

~]# systemctl stop nfs-server.service

Khi getty@ttyA. service and getty@ttyB. service are instantiated form the above template,

~]# systemctl stop nfs-server.service