274 The File System ber stored in a structure), and obtains a nonsensical value. What to do? Unix could abort the operation (returning an error to the user). Unix could declare the device “bad” and unmount it. Unix could even try to “fix” the value (such as doing something that makes sense). Unix takes the fourth, easiest way out: it gives up the ghost and forces you to put things back together later. (After all, what are sysadmins paid for, anyway?) In recent years, the Unix file system has appeared slightly more tolerant of disk woes simply because modern disk drives contain controllers that present the illusion of a perfect hard disk. (Indeed, when a modern SCSI hard disk controller detects a block going bad, it copies the data to another block elsewhere on the disk and then rewrites a mapping table. Unix never knows what happened.) But, as Seymour Cray used to say, “You can’t fake what you don’t have.” Sooner or later, the disk goes bad, and then the beauty of UFS shows through. Don’t Touch That Slash! UFS allows any character in a filename except for the slash (/) and the ASCII NUL character. (Some versions of Unix allow ASCII characters with the high-bit, bit 8, set. Others don’t.) This feature is great—especially in versions of Unix based on Berkeley’s Fast File System, which allows filenames longer than 14 characters. It means that you are free to construct informative, easy-to-understand filena- mes like these: 1992 Sales Report Personnel File: Verne, Jules rt005mfkbgkw0.cp Unfortunately, the rest of Unix isn’t as tolerant. Of the filenames shown above, only rt005mfkbgkw0.cp will work with the majority of Unix utili- ties (which generally can’t tolerate spaces in filenames). However, don’t fret: Unix will let you construct filenames that have control characters or graphics symbols in them. (Some versions will even let you build files that have no name at all.) This can be a great security feature— especially if you have control keys on your keyboard that other people don’t have on theirs. That’s right: you can literally create files with names that other people can’t access. It sort of makes up for the lack of serious security access controls in the rest of Unix.

UFS: The Root of All Evil 275 Recall that Unix does place one hard-and-fast restriction on filenames: they may never, ever contain the magic slash character (/), since the Unix kernel uses the slash to denote subdirectories. To enforce this requirement, the Unix kernel simply will never let you create a filename that has a slash in it. (However, you can have a filename with the 0200 bit set, which does list on some versions of Unix as a slash character.) Never? Well, hardly ever. Date: Mon, 8 Jan 90 18:41:57 PST From: sun!wrs!yuba!steve@decwrl.dec.com (Steve Sekiguchi) Subject: Info-Mac Digest V8 #35 I’ve got a rather difficult problem here. We've got a Gator Box run- ning the NFS/AFP conversion. We use this to hook up Macs and Suns. With the Sun as a AppleShare File server. All of this works great! Now here is the problem, Macs are allowed to create files on the Sun/ Unix fileserver with a “/” in the filename. This is great until you try to restore one of these files from your “dump” tapes. “restore” core dumps when it runs into a file with a “/” in the filename. As far as I can tell the “dump” tape is fine. Does anyone have a suggestion for getting the files off the backup tape? Thanks in Advance, Steven Sekiguchi Wind River Systems sun!wrs!steve, steve@wrs.com Emeryville CA, 94608 Apparently Sun’s circa 1990 NFS server (which runs inside the kernel) assumed that an NFS client would never, ever send a filename that had a slash inside it and thus didn’t bother to check for the illegal character. We’re surprised that the files got written to the dump tape at all. (Then again, perhaps they didn’t. There’s really no way to tell for sure, is there now?) 5Forwarded to UNIX-HATERS by Steve Strassmann.