Like /etc/crontab, the files in the /etc/cron.d directory are monitored for changes. In general, the system administrator should not use /etc/cron.d/, but use the standard system crontab /etc/crontab.
/etc/crontab and the files in /etc/cron.d must be owned by root, and must not be group- or other-writable. In contrast to the spool area, the files under /etc/cron.d or the files under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly may also be symlinks, provided that both the symlink and the file it points to are owned by root. The files under /etc/cron.d do not need to be executable, while the files under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly do, as they are run by run-parts (see run-parts(8) for more information).
從文中我們可以獲取這麼一些信息:
我們只需要使用 crontab 文件, 其餘的事情由系統管理
cron.d 和 其他的 cron* 文件夾都是放腳本用的, 準確說來, 是放符號連接
cron.d 和 cron* 的區別是 放在 cron.d 目錄下的腳本必須可執行, 後者不需要
init.d
$ cat /etc/init.d/README
You are looking for the traditional init scripts in /etc/rc.d/init.d,
and they are gone?
Here's an explanation on what's going on:
You are running a systemd-based OS where traditional init scripts have
been replaced by native systemd services files. Service files provide
very similar functionality to init scripts. To make use of service
files simply invoke "systemctl", which will output a list of all
currently running services (and other units). Use "systemctl
list-unit-files" to get a listing of all known unit files, including
stopped, disabled and masked ones. Use "systemctl start
foobar.service" and "systemctl stop foobar.service" to start or stop a
service, respectively. For further details, please refer to
systemctl(1).
Note that traditional init scripts continue to function on a systemd
system. An init script /etc/rc.d/init.d/foobar is implicitly mapped
into a service unit foobar.service during system initialization.
Thank you!
Further reading:
man:systemctl(1)
man:systemd(1)
http://0pointer.de/blog/projects/systemd-for-admins-3.html
https://www.freedesktop.org/wiki/Software/systemd/Incompatibilities
上述陳述基本已經講清楚了情況:
傳統的啟動腳本功能已經整合進了 systemd, 由 systemctl 程序提供給用戶操作
現在的 /etc/init.d/ 其實是 /etc/rc.d/init.d/ 的符號鏈接
/etc/init.d/ 仍然可以藉由 systemd 提供的兼容正常工作
講清楚了這個, 下一個就是看 rc.d 文件夾掉內容了
rc 組合
rc 的意思是 “run command”, d 意味 “sub-directory”, 就是 /etc 下的字目錄的意思
$ cat /etc/rc.d/rc.local
#!/bin/bash
# THIS FILE IS ADDED FOR COMPATIBILITY PURPOSES
#
# It is highly advisable to create own systemd services or udev rules
# to run scripts during boot instead of using this file.
#
# In contrast to previous versions due to parallel execution during boot
# this script will NOT be run after all other services.
#
# Please note that you must run 'chmod +x /etc/rc.d/rc.local' to ensure
# that this script will be executed during boot.
touch /var/lock/subsys/local
$ cat inittab
# inittab is no longer used.
#
# ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.
#
# Ctrl-Alt-Delete is handled by /usr/lib/systemd/system/ctrl-alt-del.target
#
# systemd uses 'targets' instead of runlevels. By default, there are two main targets:
#
# multi-user.target: analogous to runlevel 3
# graphical.target: analogous to runlevel 5
#
# To view current default target, run:
# systemctl get-default
#
# To set a default target, run:
# systemctl set-default TARGET.target
