一、概述
Systemctl是一个systemd工具，主要负责控制systemd系统和服务管理器。
Systemd是一个系统管理守护进程、工具和库的集合，用于取代System V初始进程。Systemd的功能是用于集中管理和配置类UNIX系统。
在Linux生态系统中，Systemd被部署到了大多数的标准Linux发行版中，只有为数不多的几个发行版尚未部署。Systemd通常是所有其它守护进程的父进程，但并非总是如此。
使用Systemctl管理Linux服务
本文旨在阐明在运行systemd的系统上“如何控制系统和服务”。
二、Systemd初体验和Systemctl基础
1. 首先检查你的系统中是否安装有systemd并确定当前安装的版本
# systemctl --version systemd 219 +PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 -SECCOMP +BLKID +ELFUTILS +KMOD +IDN
上例中很清楚地表明，我们安装了219版本的systemd。
2. 检查systemd和systemctl的二进制文件和库文件的安装位置
# whereis systemd systemd: /usr/lib/systemd /etc/systemd /usr/share/systemd /usr/share/man/man1/systemd.1.gz
# whereis systemctl
systemctl: /usr/bin/systemctl /usr/share/man/man1/systemctl.1.gz
3. 检查systemd是否运行
# ps -elf |grep systemd 4 S root 1 0 0 80 0 - 11520 ep_pol 11:08 ? 00:00:05 /usr/lib/systemd/systemd --system --deserialize 20 4 S root 498 1 0 80 0 - 9204 ep_pol 11:08 ? 00:00:00 /usr/lib/systemd/systemd-journald 4 S root 824 1 0 80 0 - 6048 ep_pol 11:08 ? 00:00:00 /usr/lib/systemd/systemd-logind 4 S dbus 829 1 0 80 0 - 24597 ep_pol 11:08 ? 00:00:00 /bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation 4 S root 22256 1 0 80 0 - 11525 ep_pol 17:13 ? 00:00:00 /usr/lib/systemd/systemd-udevd 0 S root 22322 2104 0 80 0 - 28167 pipe_w 17:24 pts/0 00:00:00 grep --color=auto systemd
4. 分析systemd启动进程
# systemd-analyze Startup finished in 10.958s (kernel) + 3.538s (initrd) + 12.088s (userspace) = 26.585s
5. 分析启动时各个进程花费的时间
# systemd-analyze blame 4.969s NetworkManager-wait-online.service 2.035s lvm2-pvscan@8:2.service 1.546s lvm2-monitor.service 1.468s dev-mapper-clx2droot.device 889ms tuned.service 834ms network.service 770ms postfix.service 731ms sshd-keygen.service 536ms systemd-hwdb-update.service 516ms polkit.service 512ms firewalld.service 447ms abrt-ccpp.service 382ms systemd-logind.service 377ms chronyd.service 331ms systemd-user-sessions.service 203ms systemd-fsck-root.service 175ms NetworkManager.service 121ms systemd-tmpfiles-setup-dev.service 102ms dev-hugepages.mount
6. 分析启动时的关键链
# systemd-analyze critical-chain The time after the unit is active or started is printed after the "@" character. The time the unit takes to start is printed after the "+" character. multi-user.target @12.077s └─tuned.service @11.187s +889ms └─network.target @11.181s └─NetworkManager.service @5.375s +175ms └─dbus.service @3.946s └─basic.target @3.938s └─sockets.target @3.938s └─dbus.socket @3.938s └─sysinit.target @3.935s └─systemd-update-utmp.service @3.923s +11ms └─auditd.service @3.837s +84ms └─systemd-tmpfiles-setup.service @3.800s +36ms └─rhel-import-state.service @3.752s +46ms └─local-fs.target @3.750s └─boot.mount @3.653s +95ms └─local-fs-pre.target @3.652s └─lvm2-monitor.service @2.100s +1.546s └─lvm2-lvmetad.service @2.208s └─lvm2-lvmetad.socket @2.095s └─-.slice
重要：Systemctl接受服务（.service），挂载点（.mount），套接口（.socket）和设备（.device）作为单元。
7. 列出所有可用单元
# systemctl list-unit-files UNIT FILE STATE proc-sys-fs-binfmt_misc.automount static dev-hugepages.mount static dev-mqueue.mount static proc-sys-fs-binfmt_misc.mount static sys-fs-fuse-connections.mount static sys-kernel-config.mount static sys-kernel-debug.mount static tmp.mount disabled brandbot.path disabled systemd-ask-password-console.path static systemd-ask-password-plymouth.path static
……
9. 列出所有失败单元
# systemctl --failed 0 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use 'systemctl list-unit-files'.
10. 检查某个单元（如 cron.service）是否启用
# systemctl is-enabled crond.service
enabled
11. 检查某个单元或服务是否运行
# systemctl status firewalld.service ● firewalld.service - firewalld - dynamic firewall daemon Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled) Active: active (running) since 日 2020-07-19 11:14:32 CST; 6h ago Docs: man:firewalld(1) Main PID: 1552 (firewalld) CGroup: /system.slice/firewalld.service └─1552 /usr/bin/python -Es /usr/sbin/firewalld --nofork --nopid 7月 19 11:14:31 localhost.localdomain systemd[1]: Starting firewalld - dynamic firewall daemon... 7月 19 11:14:32 localhost.localdomain systemd[1]: Started firewalld - dynamic firewall daemon.
二、使用Systemctl控制并管理服务
12. 列出所有服务（包括启用的和禁用的）
# systemctl list-unit-files --type=service UNIT FILE STATE abrt-ccpp.service enabled abrt-oops.service enabled abrt-pstoreoops.service disabled abrt-vmcore.service enabled abrt-xorg.service enabled abrtd.service enabled arp-ethers.service disabled auditd.service enabled autovt@.service enabled blk-availability.service disab
13. Linux中如何启动、重启、停止、重载服务以及检查服务（如 httpd.service）状态
13. Linux中如何启动、重启、停止、重载服务以及检查服务（如 httpd.service）状态 # systemctl start httpd.service # systemctl restart httpd.service # systemctl stop httpd.service # systemctl reload httpd.service # systemctl status httpd.service httpd.service -TheApache HTTP Server Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled) Active: active (running) since Tue2015-04-2817:21:30 IST;6s ago Process:2876ExecStop=/bin/kill -WINCH ${MAINPID}(code=exited, status=0/SUCCESS) Main PID:2881(httpd) Status:"Processing requests..." CGroup:/system.slice/httpd.service ├─2881/usr/sbin/httpd -DFOREGROUND ├─2884/usr/sbin/httpd -DFOREGROUND ├─2885/usr/sbin/httpd -DFOREGROUND ├─2886/usr/sbin/httpd -DFOREGROUND ├─2887/usr/sbin/httpd -DFOREGROUND └─2888/usr/sbin/httpd -DFOREGROUND Apr2817:21:30 tecmint systemd[1]:StartingTheApache HTTP Server... Apr2817:21:30 tecmint httpd[2881]: AH00558: httpd:Couldnot reliably determine the server's fully q...ssage Apr 28 17:21:30 tecmint systemd[1]: Started The Apache HTTP Server. Hint: Some lines were ellipsized, use -l to show in full. 注意：当我们使用systemctl的start，restart，stop和reload命令时，我们不会从终端获取到任何输出内容，只有status命令可以打印输出。
14. 如何激活服务并在启动时启用或禁用服务（即系统启动时自动启动服务）
# systemctl enable httpd.service
# systemctl disable httpd.service
15. 使用systemctl命令杀死服务
# systemctl kill 服务名
三、使用Systemctl控制并管理挂载点
16. 列出所有系统挂载点
# systemctl list-unit-files --type=mount UNIT FILE STATE dev-hugepages.mount static dev-mqueue.mount static proc-sys-fs-binfmt_misc.mount static sys-fs-fuse-connections.mount static sys-kernel-config.mount static sys-kernel-debug.mount static tmp.mount disabled
17. 挂载、卸载、重新挂载、重载系统挂载点并检查系统中挂载点状态
# systemctl start tmp.mount # systemctl stop tmp.mount # systemctl restart tmp.mount # systemctl reload tmp.mount # systemctl status tmp.mount
18. 在启动时激活、启用或禁用挂载点（系统启动时自动挂载）
# systemctl enable tmp.mount # systemctl disable tmp.mount
四、其他
19、检查某个服务的所有配置细节
# systemctl show 服务名