Comment by derefr

> which avoided all that by starting out with some configuration that supported one terminal and limited storage devices and would recompile the OS for the exact hardware configuration of the machine and print it to a paper tape or magnetic tape and they'd boot off that.

Not even. The OEM-shipped machine-specific bootstrap tape (i.e. the one that "supported one terminal and limited storage devices") was still used for initial system bringup, even after you recompiled your userland software distribution of choice for your computer's weird ISA and wrote it out to a tape. The OEM-shipped machine-specific bootstrap tape got mounted on /; brought up the system just enough to mount other devices; and then the userland software distribution got mounted to /usr.

(Back then, you wouldn't want to keep running things from the machine-specific bootstrap tape — the binaries were mostly very cut-down versions, of the kind that you could punch in from panel toggle switches in a pinch. You couldn't unspool the tape, because the kernel and some early daemons were still executing from the tape; but you wouldn't want anything new to execute from there. Thus $PATH. In /usr/bin you'd find a better shell; a better ls(1); and even what you'd think of today as rather "low-level" subsystems — things like init(8). $PATH was /usr/bin:/bin because "once you have /usr/bin, you don't want to be invoking any of those hyperminimal /bin versions of anything any more; you want the nice convenient /usr/bin ones.")