## 1.11. Filenames

Like all operating systems, Unix files have names. (Unix directories, devices, and so on also have filenames -- and are treated like files (Section 1.19).) The names are words (sequences of characters) that let you identify a file. Older versions of Unix had some restrictions on the length of a filename (14 characters), but modern versions have removed these restrictions for all practical purposes. Sooner or later you will run into a limit, but if so, you are probably being unnecessarily verbose.

Technically, a filename can be made from almost any group of characters (including nonprinting characters and numbers) except a slash (`/`). However, you should avoid filenames containing most punctuation marks and all nonprinting characters. To be safe, limit your filenames to the following characters:

Upper- and lowercase characters
  Unix filenames are always case sensitive. That is, upper- and lowercase letters are always different (unlike Microsoft Windows and others that consider upper- and lowercase letters the same). Therefore, myfile and Myfile are different files. It is usually a bad idea to have files whose names differ only in their capitalization, but that's your decision.
Underscores (_)
  Underscores are handy for separating "words" in a filename to make them more readable. For example, my_long_filename is easier to read than mylongfilename.
Periods (.)
  Periods are used by some programs (such as the C compiler) to separate filenames from filename extensions (Section 1.12). Extensions are used by these programs to recognize the type of file to be processed, but they are not treated specially by the shell, the kernel, or other Unix programs.

  Filenames that begin with a period are treated specially by the shell: wildcards won't match (Section 1.13) them unless you include the period (like `.*`). The ls command, which lists your files, ignores files whose names begin with a period unless you give it a special option (ls -a (Section 8.9)). Special configuration files are often "hidden" in directories by beginning their names with a period.

Certain other punctuation
  About the only other punctuation mark that is always safe is the comma (,), although it isn't part of the POSIX-portable character set.

I'm so dead-set against using weird, nonprinting characters in filenames that I won't even tell you how to do it. I will give you some special techniques for deleting files with weird names (Section 14.11), though, in case you create some by accident.

Some things to be aware of:

- Unix does not have any concept of a file version. There are some revision control programs (Section 39.4) that implement their own notion of a version, but there is nothing built into the operating system that handles this for you. If you are editing a file, don't count on Unix to save your previous versions -- you can program this (Section 35.16, Section 18.14) though, if you want to; the GNU Emacs editor also makes backups (Section 19.4).

- Once you delete a file in Unix, it is gone forever (Section 14.3). You can't get it back without restoring it from a backup. So be careful when you delete files. Later, we'll show you programs that will give you a "grace period" between the time you delete a file and the time it actually disappears.

-- ML

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