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And specifically when you tell the shell to redirect stdout to a file, the shell is opening a file descriptor to handle the request. A file descriptor also sits in the middle for piping stdout for one command to stdin of another. It seems like it's doing magic, but it's not.

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Are we continuously told in our courses to flush() the file so that it gets written to disk?

If you're talking about fflush() from the C standard libray, fflush() doesn't write to disk. It pushes writes from the FILE's buffer to the kernel and drops any read data that has been cached. The C standard library doesn't really provide you with all the tools needed to commit writes to disk fully.

Hopefully not, because that murders performance. The caching exists for a reason, let it do its thing.

If you care about data integrity, you don’t trust the disk anyway, so flushing to the disk isn’t enough.

There's fsync. Unfortunately, if you want atomicity but not durability you have ~zero options on common filesystems.

(On ZFS and bcachefs you can get that by disabling syncs completely; they never reorder writes in a visible manner.)

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