Java™ Application Development on Linux - Dator

22Chapter 1An Embarrassment of Riches: The Linux EnvironmentEntire books can be—and have been—written on shell programming.(It’s one of our favorite subjects to teach.) Programs written in the shell languageare often called shell scripts. Such scripts can be powerful yet easy to write(once you are familiar with the syntax) and can make you very productive indealing with all those little housekeeping tasks that accompany programdevelopment. All you need to do (dangerous words, no?) is to put commandsin a text file and give the file execute permissions. But that’s a subject foranother day.Some elements of shell scripting, however, are useful even if you nevercreate a single shell script. Of these, perhaps the most important to know(especially for Java programmers) is how to deal with shell variables.NOTEWe’ll be describing the syntax for bash, the default shell on most Linuxdistributions. The syntax will differ for other shells, but the concepts are largelythe same.Any string of alphanumeric or underscore characters can be used as thename of a variable. By convention shell variables typically use uppercasenames—but that is only convention (although it will hold true for most ifnot all of our examples, too). Since commands in Linux are almost alwayslowercase, the use of uppercase for shell variables helps them to stand out.Set the value of a shell variable with the familiar method—the equal sign:$ FILE=/tmp/abc.out$This has assigned the variable FILE the value /tmp/abc.out. But to makeuse of the value that is now in FILE, the shell uses syntax that might not befamiliar to you: The name must be preceded with a “$”.Shell variables can be passed on to other environments if they are exported,but they can never be passed back up. To set a shell variable for use by yourcurrent shell and every subsequent subshell, export the variable:$ export FILE$