Some useful commands for us macOS users, although some apply to UNIX systems generally.

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Source: http://dtrace.org/blogs/brendan/2011/10/10/top-10-dtrace-scripts-for-mac-os-x/

sudo iosnoop: gives live I/O. Figure out what is using the disk. 

sudo execsnoop: prints live process creation. Figure out what new processes are being spawned. 

sudo opensnoop: prints live file accessing. 

sudo truss: print all the syscalls made by an application. That is, requests it’s making to the kernel to do something. 

nettop: live network activity. Press ‘d’ in the view to toggle to deltas. 

lsof -p [PID] to find the directory of an application