Retroms-doswindowsibmfile-paths
Windows Uses Backslashes in File Paths Because IBM's DOS 1.0 Utilities Had Already Taken the Forward Slash
Microsoft's Larry Osterman explained that when MS-DOS 2.0 added directories for the PC/XT's hard disk, the forward slash was already the switch character, so the team picked the backslash instead.
The story starts with a version of DOS that had nothing to separate. “For DOS 1.0, DOS only supported floppy disks,” Osterman wrote, and “DOS didn’t support directories, just files in a single root directory.” A path character wasn’t needed yet.
The switch character was. “Many of the DOS utilities (except for command.com) were written by IBM, and they used the “/” character as the “switch” character,” he wrote. So a command took its options as /x, where Unix users typed -x.
Then came the hard disk. “DOS 2.0 was tied to the PC/XT, whose major feature was a 10M hard disk. IBM asked the Microsoft to add support for hard disks,” Osterman wrote. Ten megabytes needed folders, and folders needed a separator. The Microsoft developers knew Unix well; Osterman notes “they all used Xenix machines for email and stuff.” But “they couldn’t use the *nix form of path separator of “/”, because the “/” was being used for the switch character.”
The period was out too, since “the “.” character was being used to differentiate between file and extension.” In his account, “they chose the next best thing - the “" character, which was visually similar to the “/” character.”
They also hedged. Osterman wrote that “they coded the OS to accept either “/” or “" character as the path character,” and he pointed out that this still worked in 2005: “try typing “notepad c:/boot.ini” on an XP machine.” The team “added an undocumented system call to change the switch character” and a config.sys option, SWITCHAR=, that “would let you set the switch character to “-”.” Set that way, it “flipped MS-DOS into a *nix style system where command lines used “-switch”, and paths were / delimited.”
That escape hatch didn’t last. “I don’t know the fate of the switchar API, it’s been long gone for many years now,” Osterman wrote. And the very first link in the chain is still uncertain in his own telling: “I’m grey on whether the “/” character came from IBM or from Microsoft,” he wrote, since “several of the original MS-DOS developers were old-hand DEC-20 developers.” This account rests on Osterman’s blog post; nobody in it says who typed the first /.
Small syntax choices made when a product has one user and one disk drive tend to outlive everything around them. File formats, separators and switch characters get baked into scripts, manuals and muscle memory, and every later version inherits the cost of changing them. The next long-lived quirk is probably being made the same way right now, in some version 1 command-line tool whose authors haven’t imagined its version 2.