Summary
Along with conditional execution, variables are one of the features required of a language to make it genuinely useful. The ability to read data in from the environment and execute different code as a result means that the shell is a genuinely useful programming language, not just a batch processor. The shell is slightly different from most languages, in that the syntax to assign and to read variables is not the same, and also because it has almost no concept of different types of variables.
Some variables are pre-defined by the system, and some of those (such as UID) can only be read, not written to. Others (like RANDOM) lose their special meaning if they are assigned a value.
Chapter 7 goes much more in-depth with variables, including a lot of bash-specific features. Chapter 9 deals with arrays, a special type of variable which is available in bash, ksh, and zsh.
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