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Files in Unix are organized by listing them in directories. Directories are themselves files, and so may appear within other directories. The result is a tree-like hierarchy. The bin directory contains many of the programs for performing common Unix commands. The usr directory contains many of the data files that are required by those and other commands.
Of particular interest, however, is the home directory, which contains all of the files associated with individual users like you and me. Each individual user gets a directory within home bearing their own login name. My login name is zeil. So these files are arranged as:. For example, the full names paths of the four programs in the above diagram are. Now, when I say that a path is a step-by-step set of directions, understand that we seldom have to follow those directions step by step.
Almost any time and any place I need to name a file in Unix, I can simply give a path to it and let the operating system follow those step-by-step directions.
If the file you want is within that directory or within other directories contained in the working directory , the name of the working directory may be omitted from the start of the file name.
When you first log in, your home directory is your working directory. Unix file names can be almost any length and may contain almost any characters. As a practical matter, however, you should avoid using punctuation characters other than the hyphen, the underscore, and the period. Also, avoid blanks, and non-printable characters within file names. All of these have special meanings when you are typing commands and so would be very hard to enter within a file name. Some things to keep in mind about Unix file names that may be different from other file systems you have used:.
Unix file names are often very long so that they describe their contents. Upper and lower case letters are distinct in Unix file names. By convention, files containing executable programs such as clpr and psnup in the above examples generally do not receive such an extension. Broadly speaking, we can divide files into two categories: Text files are files that consist entirely of human-readable more or less text, while binary files are files that encode data in a fashion intended only for interpretation by a machine.
A lot of what we store in files is just text. Text is represented in files, much like it is stored in memory, by placing one character in each successive byte of the file.
Of course, bytes actually hold numbers in the range ASCII encodes different characters. Technically, you can say that it wastes one bit of every 8-bit byte. The characters encoded are. Here is a dump of the opening bytes of the text file from which this particular document was generated. Certainly we will be able to do everything to a text file that we can do with generic binary files: In addition, we will learn that Linux has quite a few commands for working with text, including commands for viewing, changing, editing, and measuring properties of text.
Now, the characters defined in ASCII is good enough for basic purposes, particularly if you speak and write in English. But before long, pressure built to expand the available characters. Some of this pressure came from specialized applications. For a while, developers tried to stem the tide by defining character set that used all possible values of a byte, but even that was little more than a temporary respite. Even more pressure came from different languages. Greeks and Russians have their own entire alphabets.
And once we get past Europe, there are entire families of alphabets for Asian, Middle Eastern, and African languages. Complicating matters, Unicode allows for a variety of different ways to arrange the numbers within a stream of bytes. For example one encoding, UTF, stores a single 26 bit Unicode character in a block of 4 bytes bits. This is fairly simply, but if Some will work with Unicode files encoded in UTF-8 but not in other encodings.
Presumably, more and more of the text processing commands will support Unicode in the future. A binary file is a sequence of bytes that can contain almost anything. In practice, some software developer working on a program defined a file format for holding the data needed by that program. The file format was probably designed to be compact and easily processed by that program.
Here is a dump of the opening bytes of the file containing the first picture in section 1 of this document. Any binary data file is bound to contain some bytes that just happen to match an ASCII character code. Typically the contents of that binary file can only be processed by that one program or by other programs written later with the specific goal of processing that same file format. For example, every operating system defines a file format for executable programs.
For the most part, a program is just a block of machine code instructions encoded in binary. Load that block into memory, point the CPU at the address where it was loaded, and it runs.
Because these headers are operating-system specific, trying to execute a program designed for one operating system will usually result in a quick error message if you try to run it on a different operating system, because that operating system will quickly realize that the header is not in the proper format. It does, however, account for the fact that if you try this, you will usually get stopped before doing any real damage. Originally, the only programs that could interpret the GIF format were conversion programs provided by Compuserve for converting between GIF and older exiting graphics file formats.
Eventually, web browsers added code designed to interpret and render GIF, and now almost every program that deals in graphics includes code designed to handle GIF. You cannot hand a GIF file to the operating system to be executed like a program, nor can you ask a web browser or graphics viewer to render a program as if it were an image.
Doing so will result in an error message a best, garbage output if you are not so lucky, a hung system if you are still less likely, or a corrupted file system if you are really having a bad day.
You can use them as input to a program specifically designed to handle their file format. When you write a program by typing in source code, you are working with text. Program source code files e. On the other hand, when you run that source code through a compiler and get an executable program, that executable is binary. When you type in a word processor, you are certainly working with text.
But the wide range of formatting options, e. Something more elaborate than plain text is needed to handle all of that. And every word processor defines its own distinct format for storing that information.
So when you save the output of your favorite word processor e. On the other hand, you may occasionally wotk with a simpler text editor that provides none of those fancy formatting options e.
Such programs provide text files as output. Web pages, like the one you are reading now, offer nearly as many formatting options as a typical word processor. So you might expect that they are working from a binary file. In fact, however, web pages are text files, but use special commands embedded into the text via the Hyper Text Markup language HTML to indicate what formatting is needed.
Directories in Linux and folders in Windows are actually files. But they are in a binary format that is understood by the various navigation and file manipulation commands in Linux. That binary format tracks information such as the file name and, most importantly, the location of the file on the disk. As we will see, one almost never needs to type an entire file name in a Unix command, so long file names are no harder to work with than short ones.
Aug 18, Contents: We can expand our view of the Unix files then as: So these files are arranged as: Look in that directory. In that directory, you should see a directory named zeil. In that directory, you should see a directory named bin. In that directory, you should see a file named psnup. Where do I find it? Some things to keep in mind about Unix file names that may be different from other file systems you have used: These include Blank number 32 Numeric digits