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systemd 和 如何修改和创建一个 systemd service (Understanding and administering systemd)
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系统中经常会使用到 systemctl 去管理systemd程序，刚刚看了一篇关于 systemd 和 SysV 相关的文章，这里简要记录一下：
systemd定义: (英文来解释更为原汁原味)
systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts. systemd provides:
Aggressive parallelization capabilities
Uses socket and D-Bus activation for starting services
Offers on-demand starting of daemons, keeps track of processes using Linux cgroups
Supports snapshotting and restoring of the system state
Maintains mount and automount points
Implements an elaborate transactional dependency-based service control logic.
The systemctl command is the primary tool to manage systemd. It combines the functionality of SysVinit’s service and chkconfig commands into a single tool you can use to enable and disable services permanently or only for the current session.
systemd manages units, which are representations of system resources and services. This following list shows the unit types that systemd can manage:
A service on the system, including instructions for starting, restarting, and stopping the service.
A network socket associated with a service.
A device specifically managed with systemd.
A mountpoint managed with systemd.
A mountpoint automatically mounted on boot.
Swap space on the system.
A synchronization point for other units. Usually used to start enabled services on boot.
A path for path-based activation. For example, you can start services based on the state of a certain path, such as whether it exists or not.
A timer to schedule activation of another unit.
A snapshot of the current systemd state. Usually used to rollback after making temporary changes to systemd.
Restrivtion of resources through Linux Control Group nodes (cgroups).
Information from systemd bus interfaces. Usually used to manage external system processes.
systemctl 常用管理命令：
# systemctl start foo
# systemctl stop foo
# systemctl restart foo
# systemctl status foo
# systemctl enable foo
# systemctl disable foo
# systemctl mask foo
# systemctl is-enabled foo
systemctl service 创建：
systemctl service在两个目录中： /etc/systemd/system 和 /usr/lib/systemd/system 之中
我们一般自己创建的service 直接放在 /etc/systemd/system 之中即可：
vim xuyaowen.service 创建文件
输入如下内容：
[Unit]
Description=xuyaowen custom service
Requires=network.target[Service]
Type=simple
ExecStart=/usr/bin/sleep infinity[Install]
WantedBy=multi-user.target
保存退出后，使用 systemctl start xuyaowen 启动后，使用 systemctl status 查看service 状态：
➜ system systemctl status xuyaowen.service
● xuyaowen.service - xuyaowen custom service
Loaded: loaded (/etc/systemd/system/xuyaowen.service; disabled; vendor preset: disabled)
Active: active (running) since Thu -- :: CST; 38min ago
Main PID: (sleep)
Tasks: (limit: )
Memory: .0K
CGroup: /system.slice/xuyaowen.service
└─ /usr/bin/sleep infinityAug :: yaowenxu systemd[]: Started xuyaowen custom service.
service 创建成功。具体systemd 与 SysV 之间的关系可以在网络上较为容易查找到，我这里只是作为简要记录。关于systemd 更为详细的内容，请参考fedora doc .
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