• Choose a category
• All
• Classic-Fiction

By Root 1123 0

files beginning with a dot? They are called hidden files. Putting a dot in front of their names keeps them from being shown during a normal ls command. Many programs employ hidden files for user options—things about their default behavior that you want to change. For instance, you can put commands in the file .Xdefaults to alter how programs using the X Window System operate. Most of the time you can forget these files exist, but when you're configuring your system you'll find them very important. We list some of them later.

Another useful ls option is -l for "long." It shows extra information about the files. Figure 4-1 shows typical output and what each field means. Adding the -h ("human" option) shows the file sizes rounded to something more easily readable.

Figure 4-1. Output of ls -l

We discuss the permissions, owner, and group fields in a later chapter, Chapter 11. The ls command also shows the size of each file and when it was last modified.

Viewing Files, More or Less

One way to look at a file is to invoke an editor, such as:

$ xemacs .bashrc

But if you just want to scan a file quickly, rather than edit it, other commands are quicker. The simplest is the strangely named cat command (named after the verb concatenate because you can also use it to concatenate several files into one):

$ cat .bashrc

But a long file will scroll by too fast for you to see it, so most people use the more command instead:

$ more .bashrc

This prints a screenful at a time and waits for you to press the spacebar before printing more. more has a lot of powerful options. For instance, you can search for a string in the file: press the slash key (/), type the string, and press Return.

A popular variation on the more command is called less . It has even more powerful features; for instance, you can mark a particular place in a file and return there later.

Symbolic Links

Sometimes you want to keep a file in one place and pretend it is in another. This is done most often by a system administrator, not a user. For instance, you might keep several versions of a program around, called prog.0.9, prog.1.1, and so on, but use the name prog to refer to the version you're currently using. Or you may have a file installed in one partition because you have disk space for it there, but the program that uses the file needs it to be in a different partition because the pathname is hard-coded into the program.

Unix provides links to handle these situations. In this section, we'll examine the symbolic link, which is the most flexible and popular type. A symbolic link is a kind of dummy file that just points to another file. If you edit or read or execute the symbolic link, the system is smart enough to give you the real file instead. Symbolic links work a lot like shortcuts under MS-Windows, but are much more powerful.

Let's take the prog example. You want to create a link named prog that points to the actual file, which is named prog.1.1. Enter the following command:

$ ln -s prog.1.1 prog

Now you've created a new file named prog that is kind of a dummy file; if you run it, you're really running prog.1.1. Let's look at what ls -l has to say about the file:

$ ls -l prog

lrwxrwxrwx 2 mdw users 8 Nov 17 14:35 prog -> prog.1.1

The l at the beginning of the output line shows that the file is a link, and the little -> indicates the real file to which the link points.

Symbolic links are really simple, once you get used to the idea of one file pointing to another. You'll encounter links all the time when installing software packages.

Shells

As we said before, logging into the system in console mode puts you into a shell. If your system is configured with a graphical login, logging in brings you to the graphical interface where you can open an xterm (or similar) window in order to get a shell. The shell interprets and executes all your commands. Let's look a bit at different shells before we keep going, because they're going to affect some of the material coming up.

If it seems confusing that Unix offers many