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Since crontab entries must be a single line long, it's hard to include any standard input with them. Sure, you can use commands like:
0 22 * * * echo "It's 10PM; do you know where your children are?" | wall
but you can't use "here documents" and other methods of generating multiline input; they intrinsically take several lines.
To solve this problem, cron allows you to include standard input directly on the command line. If the command contains a percent sign (%), cron uses any text following the sign as standard input for cmd. Additional percent signs can be used to subdivide this text into lines. For example, the following crontab entry:
30 11 31 12 * /etc/wall%Happy New Year!%Let's make next year great!
runs the wall command at 11:30 a.m. on December 31, using the text:
Happy New Year! Let's make next year great!
as standard input. [If you need a literal percent sign in your entry, for a command like date +%a, escape the percent sign with a backslash: \%. -- JP]
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| 25.3. Adding crontab Entries |  | 25.5. The at Command |
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