system()

void system(string program, ...) 

The system() action causes the program specified by program to be executed as if it were given to the shell as input. The program string may contain any of the printf()/printa() format conversions. Arguments must be specified that match the format conversions. Refer to Chapter 12, Output Formatting for details on valid format conversions.

The following example runs the date(1) command once per second:

# dtrace -wqn tick-1sec'{system("date")}'
 Tue Jul 20 11:56:26 CDT 2004
 Tue Jul 20 11:56:27 CDT 2004
 Tue Jul 20 11:56:28 CDT 2004
 Tue Jul 20 11:56:29 CDT 2004
 Tue Jul 20 11:56:30 CDT 2004

The following example shows a more elaborate use of the action, using printf() conversions in the program string along with traditional filtering tools like pipes:

#pragma D option destructive
#pragma D option quiet

proc:::signal-send
/args[2] == SIGINT/
{
	printf("SIGINT sent to %s by ", args[1]->pr_fname);
	system("getent passwd %d | cut -d: -f5", uid);
}

Running the above script results in output similar to the following example:

# ./whosend.d
SIGINT sent to MozillaFirebird- by Bryan Cantrill
SIGINT sent to run-mozilla.sh by Bryan Cantrill
^C
SIGINT sent to dtrace by Bryan Cantrill

The execution of the specified command does not occur in the context of the firing probe – it occurs when the buffer containing the details of the system() action are processed at user-level. How and when this processing occurs depends on the buffering policy, described in Chapter 11, Buffers and Buffering. With the default buffering policy, the buffer processing rate is specified by the switchrate option. You can see the delay inherent in system() if you explicitly tune the switchrate higher than its one-second default, as shown in the following example:

#pragma D option quiet
#pragma D option destructive
#pragma D option switchrate=5sec

tick-1sec
/n++ < 5/
{
	printf("walltime  : %Y\n", walltimestamp);
	printf("date      : ");
	system("date");
	printf("\n");
}

tick-1sec
/n == 5/
{
	exit(0);
}

Running the above script results in output similar to the following example:

# dtrace -s ./time.d
 walltime  : 2004 Jul 20 13:26:30
date      : Tue Jul 20 13:26:35 CDT 2004

walltime  : 2004 Jul 20 13:26:31
date      : Tue Jul 20 13:26:35 CDT 2004

walltime  : 2004 Jul 20 13:26:32
date      : Tue Jul 20 13:26:35 CDT 2004

walltime  : 2004 Jul 20 13:26:33
date      : Tue Jul 20 13:26:35 CDT 2004

walltime  : 2004 Jul 20 13:26:34
date      : Tue Jul 20 13:26:35 CDT 2004

Notice that the walltime values differ, but the date values are identical. This result reflects the fact that the execution of the date(1) command occured only when the buffer was processed, not when the system() action was recorded.