Sometimes, you need to run a command (capture, etc …) for a given amount of time. Here’s an example of a script capturing mysql traffic for half an hour :

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    #!/bin/sh
    DATE=`date +%Y%m%d%H%M%S`
    tcpdump -i eth0 port 3306  -s 65535 -x -n -q -tttt > capture-$DATE.out &
    sleep 1800
    kill $!