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 31st, using the text:
Happy New Year! Let's make next year great!
- from O'Reilly & Associates' Essential System Administration