ugh.book

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.