`strace` - trace system calls and signals

strace [-p PID] [-f] [-e trace=SET] [command]

Options you will use

-p PID: Attach to a running process. Needs root here.
-f: Follow forks.
-e trace=SET: Only these syscalls, e.g. -e trace=openat,read.
-c: Summarise counts and time per syscall.

Gotchas

A process stuck in read() with nothing coming back is waiting on the other side - a network peer that will never answer, usually because no timeout was configured.

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