cron 执行的命令有特殊限制吗? [英] Is there a special restriction on commands executed by cron?

问题描述

我有一个 crontab 看起来像

0 0 * * * pg_dump DB_NAME > /path/to/dumps/`date +%Y%m%d`.dmp

当我手动运行它时效果很好，但当 cron 运行它时就不行了.翻阅日志后，我看到

which works fine when I run it manually, but not when cron runs it. After digging through the logs, I see

Dec 12 00:00:01 localhost crond[17638]: (postgres) CMD (pg_dump DB_NAME > /path/to/dumps/`date +)

看起来是百分号的问题，但是man页面甚至根本不包含百分号，所以我认为它们没问题.

It looks like a problem with percent signs, but the man page doesn't even contain the percent character at all, so I thought they were alright.

推荐答案

你必须用反斜杠转义百分号:

You have to escape percent signs with a backslash:

0 0 * * * pg_dump DB_NAME > /path/to/dumps/`date +\%Y\%m\%d`.dmp

来自man 5 crontab:

''sixth'' 字段(该行的其余部分)指定要执行的命令是跑.该行的整个命令部分，最多一个换行符或%字符，将由/bin/sh 或由指定的 shell 执行这crontab 文件的 SHELL 变量.百分比符号 (%)命令，除非用反斜杠 () 转义，否则将更改为换行符字符，第一个 % 之后的所有数据都将发送到命令作为标准输入.无法拆分单个命令行上多行，就像 shell 的尾随"一样.

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 "".

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