Centos7 systemd 基礎學習
系統開機啟動流程：
POST--> boot sequence --> bootloader --> kernel + initramfs(initrd)--> rootfs --> /sbin/init
# 又想起了加載kernel時，kernel是在文件系統中，所以還是需要先掛載文件系統，所以還是需要驅動，掛載文件系統的驅動在哪里呢？
init:
centos 5: sysv init
centos 6: upstart
centos 7: systemd 向后不完全兼容sysv init,upstart
systemd新特性：
> 系統引導時，實現服務并行啟動
> 按需激活進程
> 系統狀態快照
> 基于依賴關系定義服務控制邏輯
核心概念： unit(單元)
> 配置文件進行標識和配置，文件中主要包含了系統服務，監聽socket,保存系統快照以及其他與init相關信息。
> 保存位置：(常用位置)
> /usr/lib/systemd/system
> /run/systemd/system
> /etc/systemd/system
Unit類型：
> Service unit: 文件擴展名為.service 用于定義系統服務
> Target unit: 文件擴展名為.target,用于模擬實現“運行級別”，但并不完全是。
> Device unit: .device 用于定義內核識別的設備
> Mount unit: .mount,定義文件系統掛載點
> socket unit: .socket,用于標識進程間通信用的socket文件
> snapshot unit: .snapshot,管理系統快照
> Swap unit: .swap,用于標識swap設備
> Automount unit: .automount,文件系統的自動掛載點
> Path unit: .path,用于定義文件系統中的一個文件或目錄
關鍵特性：
基于socket的激活機制： socket與服務程序分離，當有進程需要訪問此socket才真正啟動
基于bus的激活機制
基于device的激活機制：類似插入u盤，需要激活mount服務來掛載設備
基于path的激活機制：監控文件路徑或目錄發生變化，立即激活某服務
系統快照：將unit當前狀態數據保存在持久設備中
向后兼容sysv init腳本
不兼容：
systemctl命令固定不變，不像sysv init腳本可以自由定義。
非由systemd啟動的服務，systemctl無法與之通信。
5分鐘超時時間。
在Centos 5,6當中運行，級別切換過程(運行級別2切換到運行級別3)：將級別3的k服務全部kill掉，s服務全部start
管理系統服務命令幫助：
Centos 7 : service unit,能兼容早期的服務腳本
命令： systemctl COMMAND NAME.service
Unit Commands:
list-units [PATTERN...] List loaded units (列出系統已經load的unit,-a查看所有的unit ,-t service查看service類型)
list-sockets [PATTERN...] List loaded sockets ordered by address (列出系統已經加載socket類型unit)
list-timers [PATTERN...] List loaded timers ordered by next elapse
start NAME... Start (activate) one or more units (啟動服務)
stop NAME... Stop (deactivate) one or more units (停止服務)
reload NAME... Reload one or more units (重載服務)
restart NAME... Start or restart one or more units (重啟服務)
try-restart NAME... Restart one or more units if active
reload-or-restart NAME... Reload one or more units if possible,
otherwise start or restart
reload-or-try-restart NAME... Reload one or more units if possible,
otherwise restart if active
isolate NAME Start one unit and stop all others (切換運行級別)
kill NAME... Send signal to processes of a unit (向某unit進程發送信號)
is-active PATTERN... Check whether units are active
is-failed PATTERN... Check whether units are failed
status [PATTERN...|PID...] Show runtime status of one or more units
show [PATTERN...|JOB...] Show properties of one or more
units/jobs or the manager
cat PATTERN... Show files and drop-ins of one or more units
set-property NAME ASSIGNMENT... Sets one or more properties of a unit
help PATTERN...|PID... Show manual for one or more units
reset-failed [PATTERN...] Reset failed state for all, one, or more
units
list-dependencies [NAME] Recursively show units which are required
or wanted by this unit or by which this
unit is required or wanted
Unit File Commands:
list-unit-files [PATTERN...] List installed unit files (列出unit服務開機啟動狀態，-t service 僅查看service類型的服務)
enable NAME... Enable one or more unit files (開機啟動)
disable NAME... Disable one or more unit files (禁止開機啟動)
reenable NAME... Reenable one or more unit files
preset NAME... Enable/disable one or more unit files
based on preset configuration
preset-all Enable/disable all unit files based on
preset configuration
is-enabled NAME... Check whether unit files are enabled (查看服務是否開機自啟動)
mask NAME... Mask one or more units
unmask NAME... Unmask one or more units
link PATH... Link one or more units files into
the search path
add-wants TARGET NAME... Add 'Wants' dependency for the target
on specified one or more units
add-requires TARGET NAME... Add 'Requires' dependency for the target
on specified one or more units
edit NAME... Edit one or more unit files
get-default Get the name of the default target (查看默認允許級別)
set-default NAME Set the default target (設置默認運行級別)
Machine Commands:
list-machines [PATTERN...] List local containers and host
Job Commands:
list-jobs [PATTERN...] List jobs
cancel [JOB...] Cancel all, one, or more jobs
Snapshot Commands:
snapshot [NAME] Create a snapshot (創建unit快照)
delete NAME... Remove one or more snapshots (刪除unit快照)
Environment Commands:
show-environment Dump environment (查看運行環境，如語言，搜索路徑)
set-environment NAME=VALUE... Set one or more environment variables
unset-environment NAME... Unset one or more environment variables
import-environment [NAME...] Import all or some environment variables
Manager Lifecycle Commands:
daemon-reload Reload systemd manager configuration
daemon-reexec Reexecute systemd manager
System Commands:
is-system-running Check whether system is fully running
default Enter system default mode
rescue Enter system rescue mode (緊急救援模式)
emergency Enter system emergency mode (emergency模式，比rescue模式更徹底)
halt Shut down and halt the system (關機)
poweroff Shut down and power-off the system (關機)
reboot [ARG] Shut down and reboot the system (重啟)
kexec Shut down and reboot the system with kexec
exit Request user instance exit
switch-root ROOT [INIT] Change to a different root file system (切換根文件系統)
suspend Suspend the system (掛起系統)
hibernate Hibernate the system （保存系統快照)
hybrid-sleep Hibernate and suspend the system (保存系統快照并掛起)
運行級別：
runlevel0.target -> poweroff.target
runlevel1.target -> rescue.target
runlevel2.target -> multi-user.target
runlevel3.target -> multi-user.target
runlevel4.target -> multi-user.target
runlevel5.target -> graphical.target
runlevel6.target -> reboot.target
運行級別切換：systemctl isolate graphical.target
不能使用runlevel切換運行級別，可以使用runlevel查看
切換至緊急求援模式：
systemctl rescue
切換至emergency模式：
systemctl emergency
設置服務開機自啟動：
systemctl enbale nginx.service
Created symlink from /etc/systemd/system/multi-user.target.wants/nginx.service to /usr/lib/systemd/system/nginx.service.
# 只是在/etc/systemd/system/multi-user.target.wants/目錄下做了一個軟鏈接
# 禁止開機自啟動也只是將此目錄下的服務名稱軟鏈接刪除
類似的還有設置默認運行級別：
# systemctl set-default graphical.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target.
思考:這樣的設計思想與git在分支管理，版本管理上是否有異曲同工之妙？
內容按需快照一份，設計一個head指針，將指針指向我們需要的內容位置即可，高效，靈活，易理解。
我們人的記憶不也類似這樣的一個工作方式，將記憶碎片重現。
unit編寫：
[Unit]
Description=The nginx HTTP and reverse proxy server 描述
After=network.target remote-fs.target nss-lookup.target 服務先后次序與依賴關系
[Service]
Type=forking
PIDFile=/run/nginx.pid
# Nginx will fail to start if /run/nginx.pid already exists but has the wrong
# SELinux context. This might happen when running `nginx -t` from the cmdline.
# https://bugzilla.redhat.com/show_bug.cgi?id=1268621
ExecStartPre=/usr/bin/rm -f /run/nginx.pid start前，先刪除是否有pid文件
ExecStartPre=/usr/sbin/nginx -t start前，先測試配置文件準確性
ExecStart=/usr/sbin/nginx 啟動nginx
ExecReload=/bin/kill -s HUP $MAINPID 重啟，即是向nginx進程發送HUP信號
KillSignal=SIGQUIT
TimeoutStopSec=5
KillMode=process
PrivateTmp=true
[Install]
WantedBy=multi-user.target 多用戶模式啟動
# unit 單元文件編寫：
# https://www.freedesktop.org/software/systemd/man/systemd.unit.html、
# http://blog.csdn.net/yuesichiu/article/details/51331136#t0