新發布的CentOS 7 中使用systemd服務代替了之前版本的SysV服務，對比下兩種啓動方式的不同。
修改系統啓動級別
舊版
編輯配置文件/etc/inittab，設置啓動級別爲3 (多用戶文字界面)，修改initdefault前面的數字爲3，保存重啓
新版
修改默認啓動級別爲3
systemctl enable multi-user.target
這個命令實際則是在目錄 /etc/systemd/system 下創建了一個軟鏈接
ln -s '/usr/lib/systemd/system/multi-user.target' '/etc/systemd/system/default.target'
若修改默認啓動級別爲5，需要將之前的啓動級別disable
systemctl disable multi-user.target
該命令實際刪除了軟鏈接default.target
rm '/etc/systemd/system/default.target'
然後再啓用啓動級別5
systemctl enable graphical.target
實際創建了新的軟鏈接
ln -s '/usr/lib/systemd/system/graphical.target' '/etc/systemd/system/default.target'
當然你也可以跳過命令直接以創建軟鏈接的方式來改變啓動級別
應用程序的自啓動項
當通過yum安裝了httpd服務後，準備將其添加到自啓動項裏，
舊版
chkconfig httpd on
新版
systemctl enable httpd
實際是創建了軟鏈接
ln -s '/usr/lib/systemd/system/httpd.service' '/etc/systemd/system/multi-user.target.wants/httpd.service'
關掉httpd的自啓動
systemctl disable httpd
實際軟鏈接被刪除
rm '/etc/systemd/system/multi-user.target.wants/httpd.service'
關於啓動級別3下面的服務啓動項都在/etc/systemd/system/multi-user.target.wants目錄下，而級別5的則在目錄graphical.target.wants下面
我們打開這個鏈接文件 httpd.service 可以看到內容，就是有關httpd的啓動腳本
[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target
[Service]
Type=notify
EnvironmentFile=/etc/sysconfig/httpd
ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND
ExecReload=/usr/sbin/httpd $OPTIONS -k graceful
ExecStop=/bin/kill -WINCH ${MAINPID}
# We want systemd to give httpd some time to finish gracefully, but still want
# it to kill httpd after TimeoutStopSec if something went wrong during the
# graceful stop. Normally, Systemd sends SIGTERM signal right after the
# ExecStop, which would kill httpd. We are sending useless SIGCONT here to give
# httpd time to finish.
KillSignal=SIGCONT
PrivateTmp=true
[Install]
WantedBy=multi-user.target關於服務的啓動/關閉/重啓
舊版
service httpd {start|stop|restart}新版
systemctl {start|stop|restart} httpd查看當前httpd的運行狀態
systemctl status httpd
輸出結果
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: active (running) since Thu 2014-07-17 15:12:50 CST; 4s ago
Process: 2762 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
Main PID: 2769 (httpd)
Status: "Processing requests..."
CGroup: /system.slice/httpd.service
?..2769 /usr/sbin/httpd -DFOREGROUND
?..2770 /usr/sbin/httpd -DFOREGROUND
?..2771 /usr/sbin/httpd -DFOREGROUND
?..2772 /usr/sbin/httpd -DFOREGROUND
?..2773 /usr/sbin/httpd -DFOREGROUND
?..2774 /usr/sbin/httpd -DFOREGROUND
Jul 17 15:12:50 localhost.localdomain systemd[1]: Starting The Apache HTTP Server...
Jul 17 15:12:50 localhost.localdomain httpd[2769]: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using localhost.localdomain. Set the ...his message
Jul 17 15:12:50 localhost.localdomain systemd[1]: Started The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full查看所有服務自啓動狀態
舊版
chkconfig --list
新版
systemctl list-unit-files
以上是我對systemd的一個初步印象，需要一個逐步適應的過程，希望大家多多討論交流。