Java™ Application Development on Linux - Dator

1.3 Basic Linux Concepts and Commands23You can combine the assignment of a value with the exporting into onestep. Since repeating the export doesn’t hurt, you will often see shell scripts usethe export command every time they do an assignment, as if it were part ofthe assignment syntax—but you know better.$ export FILE="/tmp/way.out"$NOTEThe shell uses the dollar sign to distinguish between the variable name and justtext of the same letters. Consider the following example:$ echo first > FILE$ echo second > TEXT$ FILE=TEXT$ cat FILEfirst$The cat command will dump the contents of the file named FILE to thescreen—and you should see first. But how would you tell the shell that youwant to see the contents of the file whose name you have put in the shellvariable FILE? For that you need the “$”:$ cat $FILEsecond$This is a contrived example, but the point is that shell syntax supports arbitrarystrings of characters in the command line—some of them are filenames,others are just characters that you want to pass to a program. It needs a wayto distinguish those from shell variables. It doesn’t have that problem on theassignment because the “=” provides the needed clue. To say it in computerscience terms, the “$” syntax provides the R-value of the variable. (Not theinsulation R-value, but what you expect when a variable is used on the Righthand-sideof an assignment operator, as opposed to the L-value used on theLeft-hand-side of an assignment operator.)There are several shell variables that are already exported because they areused by the shell and other programs. You may need or want to set them tocustomize your environment. Since they are already exported, you won’t needto use the export command and can just assign a value, but it doesn’t hurt.