Shell Variables

There are several characteristics of your environment that you may want to customize but that cannot be expressed as an on/off choice. Characteristics of this type are specified in shell variables. Shell variables can specify everything from your prompt string to how often the shell checks for new mail.

Like an alias, a shell variable is a name that has a value associated with it. bash keeps track of several built-in shell variables; shell programmers can add their own. By convention, built-in variables should have names in all capital letters. bash does, however, have two exceptions.[7]The syntax for defining variables is somewhat similar to the syntax for aliases:

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            varname=value

There must be no space on either side of the equal sign, and if the value is more than one word, it must be surrounded by quotes. To use the value of a variable in a command, precede its name by a dollar sign ($).

You can delete a variable with the command unset varname. Normally this isn't useful, since all variables that don't exist are assumed to be null, i.e., equal to the empty string "". But if you use the set option nounset, which causes the shell to indicate an error when it encounters an undefined variable, then you may be interested in unset.

The easiest way to check a variable's value is to use the echo built-in command. All echo does is print its arguments, but not until the shell has evaluated them. This includes—among other things that will be discussed later—taking the values of variables and expanding filename wildcards. So, if the variable wonderland has the value alice, typing:

$ echo "$wonderland"

will cause the shell to simply print alice. If the variable is undefined, the shell will print a blank line. A more verbose way to do this is:

$ echo "The value of \$
            varname 
            is \"$
            varname
            \"."

The first dollar sign and the inner double quotes are backslash-escaped (i.e., preceded with \ so the shell doesn't try to interpret them—see Chapter 1) so they appear literally in the output, which for the above example would be:

The value of $wonderland is "alice".