crontab是linux系统下的任务服务，可以实现计划任务定时执行操作。这里以centos7系统为例演示，如何重启crontab服务。

[root@CentOS7 ~]# vi /etc/crontab 

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed

1.查询确认版本，这里是CentOS 7系统。

[root@CentOS7 ~]# cat /etc/redhat-release 
CentOS Linux release 7.7.1908 (Core)

2.关于crontab的使用方法，可以通过man crontab查看详细说明，crontab –help 可以查看使用参数

[root@CentOS7 ~]# crontab --help
crontab: invalid option -- '-'
crontab: usage error: unrecognized option
Usage:
 crontab [options] file
 crontab [options]
 crontab -n [hostname]

Options:
 -u <user>  define user
 -e         edit user's crontab
 -l         list user's crontab
 -r         delete user's crontab
 -i         prompt before deleting
 -n <host>  set host in cluster to run users' crontabs
 -c         get host in cluster to run users' crontabs
 -s         selinux context
 -x <mask>  enable debugging

Default operation is replace, per 1003.2

3. 编辑crontab设置计划任务，最常用的是crontab -e编辑

[root@CentOS7 ~]# crontab -e

30 * * * * /bin/sh /tmp/echo.sh

4. 关于crontab的任务格式，可以参考下图范例。 主要是 时间  执行人 命令

5.如何重启crontab
linux下 crontab服务对应的是crond服务。可以通过service crond restart重启

[root@CentOS7 ~]# service crond restart
Redirecting to /bin/systemctl restart crond.service

6.centos7系统下服务可以通过systemctl来重启，systemctl restart crond

[root@CentOS7 ~]# systemctl restart crond
[root@CentOS7 ~]# systemctl status crond 
● crond.service - Command Scheduler
   Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2020-08-26 09:06:42 CST; 30s ago
 Main PID: 1808 (crond)
   CGroup: /system.slice/crond.service
           └─1808 /usr/sbin/crond -n

Aug 26 09:06:42 CentOS7 systemd[1]: Started Command Scheduler.
Aug 26 09:06:42 CentOS7 crond[1808]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 89% if used.)
Aug 26 09:06:42 CentOS7 crond[1808]: (CRON) INFO (running with inotify support)
Aug 26 09:06:42 CentOS7 crond[1808]: (CRON) INFO (@reboot jobs will be run at computer's startup.)
[root@CentOS7 ~]# 

7.关于crontab的编辑方式，除了crontab -e之外。还可以通过直接编辑/etc/crontab 的配置文件来实现

[root@CentOS7 ~]# vi /etc/crontab

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed

30 * * * * /bin/sh /tmp/echo.sh

8.针对用户的可以编辑 /var/spool/cron/root（用户）

[root@CentOS7 ~]# vi /var/spool/cron/root

30 * * * * /bin/sh /tmp/echo.sh

9.当然还有一个系统的定时任务目录，/etc/下的 cron.daily

[root@CentOS7 ~]# cd /etc/cron.daily/
[root@CentOS7 cron.daily]# pwd
/etc/cron.daily
[root@CentOS7 cron.daily]# cat logrotate 
#!/bin/sh

/usr/sbin/logrotate -s /var/lib/logrotate/logrotate.status /etc/logrotate.conf
EXITVALUE=$?
if [ $EXITVALUE != 0 ]; then
    /usr/bin/logger -t logrotate "ALERT exited abnormally with [$EXITVALUE]"
fi
exit 0
[root@CentOS7 cron.daily]#