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You can also append the standard error to a file by using the number 2 and the redirection append operator, >>. In the next example, the user appends the standard error to the myerrors file, which then functions as a log of errors:
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You define variables within a shell, and such variables are known-logically enough- as shell variables. Many different shells exist. Some utilities, such as the mailx utility, have their own shells with their own shell variables. You can also create your own shell using what are called shell scripts. You have a user shell that becomes active as soon as you log in. This is often referred to as the login shell. Special system variables are defined within this login shell. Shell variables can also be used to define a shell's environment, as described in 13. Note Shell variables exist as long as your shell is active-that is, until you exit the shell. For example, logging out will exit the login shell. When you log in again, any variables you may need in your login shell must be defined once again.
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Definition and Evaluation of Variables: =, $, set, unset
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You define a variable in a shell when you first use the variable's name. A variable's name may be any set of alphabetic characters, including the underscore. The name may also include a number, but the number cannot be the first character in the name. A name may not have any
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other type of character, such as an exclamation point, an ampersand, or even a space. Such symbols are reserved by the shell for its own use. Also, a name may not include more than one word. The shell uses spaces on the command line to distinguish different components of a command such as options, arguments, and the name of the command. You assign a value to a variable with the assignment operator, =. You type in the variable name, the assignment operator, and then the value assigned. Do not place any spaces around the assignment operator. The assignment operation poet = Virgil, for example, will fail. (The C shell has a slightly different type of assignment operation that is described in the section on C shell variables later in this chapter.) You can assign any set of characters to a variable. In the next example, the variable poet is assigned the string Virgil:
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$ poet=Virgil
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Once you have assigned a value to a variable, you can then use the variable name to reference the value. Often you use the values of variables as arguments for a command. You can reference the value of a variable using the variable name preceded by the $ operator. The dollar sign is a special operator that uses the variable name to reference a variable's value, in effect evaluating the variable. Evaluation retrieves a variable's value, usually a set of characters. This set of characters then replaces the variable name on the command line. Wherever a $ is placed before the variable name, the variable name is replaced with the value of the variable. In the next example, the shell variable poet is evaluated and its contents, Virgil, are then used as the argument for an echo command. The echo command simply echoes or prints a set of characters to the screen.
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$ echo $poet Virgil
You must be careful to distinguish between the evaluation of a variable and its name alone. If you leave out the $ operator before the variable name, all you have is the variable name itself. In the next example, the $ operator is absent from the variable name. In this case, the echo command has as its argument the word "poet", and so prints out "poet":
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