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For troubleshooting purpose, I'd like to record PID and its command every 15s and write them to a text file.

The desired output: TimeStamp, PID, Command

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This will get you you the three columns you want:

$ ps -hopid,comm | perl -anle'print time, ", $F[0], $F[1]"'
1541626566, 6496, gedit
1541626566, 7513, bash

You can limit or extend the scope of ps, ie. what processes it lists. Then you put this in a loop with redirection.

while true; do echo x; sleep 15; done > out

Replace echo x with the command proper and out with the file name you choose.

As for the command producing the information, here's a run-down.

  • ps -hopid,comm - ps is obvious, -h turns off the header line and -o stands for the output (PID and command).
  • This goes to the Perl command, namely perl -anle'print time, ", $F[0], $F[1]"'. Here perl is Perl. The -anle flags reprent correspondingly: a - load input into an array, n - take care of new lines, l - process each single line, and finally e - execute the code that follows.
  • And now the code follows: 'print time, ", $F[0], $F[1]"'. First print the time stamp, then the first column of the array that holds the input material, and then the second.

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