More Lego rules:
(7) and common signalling facility
(8) also nice if some files have magic properties like /dev/random or /proc or /dev/null
(9) every program starts with 3 streams, stdin/stdout for work and stderr for out of band errors
(7) and common signalling facility
(8) also nice if some files have magic properties like /dev/random or /proc or /dev/null
(9) every program starts with 3 streams, stdin/stdout for work and stderr for out of band errors
This is false - if a process closes stdin/out/err, its children won't have these files open.
That said, it's still a bit of a pedantic point, you can expect to always have those three fds in the same way you can expect `argv[0]` to represent the name of your executable (Which also isn't enforced by the kernel).
The privileged program opens a sensitive data file for writing, it gets assigned (say) file descriptor 2, because someone in a parent process took some advice to "close your standard I/O because you are a dæmon" to heart, other completely unrelated library code somewhere else blithely logs network-supplied input to standard error without sanitizing it, and (lo!) there's a remotely accessible file overwrite exploit of the sensitive data file.
Ironically, the correct thing to do as a dæmon is to not close standard I/O, but to leave setting it up to the service management subsystem, and use it as-is. That way, when the service management subsystem connects standard output and error to a pipe and arranges to log everything coming out the other end, daemontools-style, the idea actually works. (-:
The point of separate error streams echoes the main Unix philosophy of doing small things, on the happy path, that work quietly or break early. Your happy results can be chained on to the next thing and your errors go somewhere for autopsy if necessary. Eg,
p1 | p2 | p3 > result.txt 2> errors.txt
BUT speaking of logic, the concept is close to Railway Oriented Programming, where you can avoid all the failure conditionals and just pass a monad out of a function, which lets you chain happy path neatly. Eg, nice talk here https://fsharpforfunandprofit.com/posts/recipe-part2/