The meaning of crontab and 2% gtternative1

1. Format writing

Common command

Crontab-e # opens editing scheduled tasks, just like using the vim tool

Crontab-l # View scheduled tasks

two。 Example

Execute / bin/ls once a month at the 0th minute of every hour

0 * / bin/ls

/ usr/bin/backup is performed every 3 hours and 0 minutes from 6 a.m. to 12:00 every December

0 6-12 beat 3 * 12 * / usr/bin/backup

Send a letter to alex@domain.name at 5:00 every afternoon from Monday to Friday

0 17 * 1-5 mail-s "hi" alex@domain.name

< /tmp/maildata 每月每天的午夜 0 点 20 分, 2 点 20 分, 4 点 20 分....执行 echo "haha" 20 0-23/2 * * * echo "haha" 下面再看看几个具体的例子： 0 */2 * * * /sbin/service httpd restart 意思是每两个小时重启一次apache 50 7 * * * /sbin/service sshd start 意思是每天7：50开启ssh服务 50 22 * * * /sbin/service sshd stop 意思是每天22：50关闭ssh服务 0 0 1,15 * * fsck /home 每月1号和15号检查/home 磁盘 1 * * * * /home/bruce/backup 每小时的第一分执行 /home/bruce/backup这个文件 00 03 * * 1-5 find /home "*.xxx" -mtime +4 -exec rm {} \; 每周一至周五3点钟，在目录/home中，查找文件名为*.xxx的文件，并删除4天前的文件。30 6 */10 * * ls 意思是每月的1、11、21、31日是的6：30执行一次ls命令 运行脚本实例

# means to run the script every 1 minute. Note that the script path is written to determine the path and the script X should be given permission to run.

/ dev/null means to send all the output generated by the script, whether standard or incorrect, to the null device.

& finally, a & indicates running in the background. It is recommended to add & when writing a boot script.

3. View the log

Tail-f / var/log/cron # Monitoring log "/ var/log/cron" is the log path of crontab

As you can see from the log, the script is executed once in the first second of every minute.

4. 2 > & 1 several basic symbols and their meanings

/ dev/null indicates an empty device file

0 indicates stdin standard input

1 represents stdout standard output

2 indicates stderr standard error

For the understanding of 2 > & 1, 2 is standard error and 1 is standard output, so this command is equivalent to redirecting standard error to standard output.

In case 1, the redirect error is output to a file

Case 2 the standard error output is redirected to the file

Case 3 error output redirected to standard output

# nginx-V is error output, but grep does not handle error output, so it is more troublesome for us to find modules

# I changed the statement to output all the error output to the standard so that grep can handle it