1. Linux Crontab Format

MIN HOUR DOM MON DOW CMD

1. 各字段值允许的范围

MIN    0-59
HOUR   0-23
DOM    1-31
MON    1-12
DOW    0-6
CMD    command
#DOW: 0表示周日 6表示周六

2. *的含义

* * * * * CMD

The * means all the possible unit:

  • When you specify */5 in minute field means every 5 minutes.
  • When you specify 0-10/2 in minute field mean every 2 minutes in the first 10 minute.
  • Thus the above convention can be used for all the other 4 fields.

3. 特殊关键字的含义

@yearly     0 0 1 1 *
@monthly    0 0 1 * *
@daily      0 0 * * *
@hourly     0 * * * *
@reboot     run at startup

2. 相关命令

crontab -l #查看当前用户的任务列表
crontab -u username -l #查看指定用户的任务列表
crontab -e

3. 注意

1. 环境变量

crontab在运行时，会加载一个最简化的环境变量，如果希望保持用户的环境变量，可以通过. ~/.bash_profile文件，即：

* * * * * . ~/.bash_profile && node script.js

2. %转义

crontab中%有特殊含义

* * * * * echo $(date +"\%F \%H:\%M:\%S") >> /data/log/crontab.log 2>&1

# The "sixth" field (the rest of the line) specifies the command to be run. The
# entire command portion of the line, up to a newline or % character, will be
# executed by /bin/sh or by the shell specified in the SHELL variable of the
# crontab file. Percent-signs (%) in the command, unless escaped with backslash
# (\), will be changed into newline characters, and all data after the first %
# will be sent to  the  command as standard input. There is no way to split a
# single command line onto multiple lines, like the shell's trailing "\".

3. 标准输出/错误重定向

crontab在执行的时候，如果没有重定向，会将标准错误和标准输出都发送到mail，在重定向的时候，句柄绑定操作应该放在命令最后。

* * * * * echo $(date +"\%F \%H:\%M:\%S") >> /data/log/crontab.log 2>&1
# 2>&1 >> /data/log/crontab.log是错误的写法