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Tcpdump will, if not run with the -c flag, continue capturing packets 
+until it is interrupted by a SIGINT signal (generated, for example, b
+y typing your interrupt character, typically control-C) or a SIGTERM 
+signal (typically generated with the kill(1) command); if run with th
+e -c flag, it will capture packets until it is interrupted by a SIGIN
+T or SIGTERM signal or the specified number of packets have been proc
+essed.

 $SIG{'FOO'} = sub { do_foo; };