创建crontab任务
参考：https://www.cnblogs.com/Icanflyssj/p/5138851.html
3. crontab常用的几个命令格式
crontab -l //显示用户的crontab文件的内容
crontab -e //编辑用户的crontab文件的内容
crontab -r //删除用户的crontab文件
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在第一次使用crontab -e，会要求选择编辑器，我选择是vim.basic
编辑文件，内容如下
root@ubuntu:/var/log# crontab -e
# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
* * * * * /home/xwdreamer/eclipse-workspace/RELEASED_V2_10_20180331/iotkit-embedded/output/release/bin/linkkit-example 其他间隔执行时间设定
每分钟执行 * * * * *
每五分钟执行 */5 * * * *
每小时执行 0 * * * *
每天执行 0 0 * * *
每周执行 0 0 * * 0
每月执行 0 0 1 * *
每年执行 0 0 1 1 * 日志服务
参考：https://blog.csdn.net/ggz631047367/article/details/50185425
ubuntu默认没有开启cron日志记录
1. 修改rsyslog
sudo vim /etc/rsyslog.d/50-default.conf
cron.* /var/log/cron.log #将cron前面的注释符去掉
2.重启rsyslog
sudo service rsyslog restart
3.查看crontab日志
less /var/log/cron.log
注意：这里必须重启日志服务，重启以后使用less查看可能还没有日志，需要等任务执行以后才能看到日志。
查看cron日志
vim /var/log/cron.log 在日志中找到
No MTA installed, discarding output 解决方案：将日志输出到指定文件
*/5 * * * * /home/xwdreamer/iotkit/linkkit-example_INV/linkkit-example_INV > linkkit-example_INV_temp.log 完整如下
root@ubuntu:~# crontab -e
# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
*/5 * * * * /home/xwdreamer/iotkit/linkkit-example_INV/linkkit-example_INV > linkkit-example_INV_temp.log
*/5 * * * * /home/xwdreamer/iotkit/linkkit-example_penguan/linkkit-example_penguan > linkkit-example_penguan_temp.log
最后日志会被输出到当前账户下的目录，比如当前这个文件被输出到/root目录下
root@ubuntu:/home/xwdreamer/iotkit# find / -name linkkit-example_INV_temp.log
find: ‘/run/user/1000/gvfs’: Permission denied
/root/linkkit-example_INV_temp.log
find: ‘/mnt/hgfs’: Protocol error 作者：xwdreamer
出处：http://www.cnblogs.com/xwdreamer
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