Christopher Negus - Linux Bible

BIGNUM=1024 let RESULT=$BIGNUM/16 RESULT=`expr $BIGNUM / 16` RESULT=`echo "$BIGNUM / 16" | bc` let foo=$RANDOM; echo $foo

Another way to grow a variable incrementally is to use $(())notation with ++Iadded to increment the value of I. Try typing the following:

$ I=0 $ echo "The value of I after increment is $((++I))" The value of I after increment is 1 $ echo "The value of I before and after increment is $((I++)) and $I" The value of I before and after increment is 1 and 2

Repeat either of those commands to continue to increment the value of $I.

Although most elements of shell scripts are relatively freeform (where white space, such as spaces or tabs, is insignificant), both letand exprare particular about spacing. The letcommand insists on no spaces between each operand and the mathematical operator, whereas the syntax of the exprcommand requires white space between each operand and its operator. In contrast to those, bcisn't picky about spaces, but it can be trickier to use because it does floating-point arithmetic.

To see a complete list of the kinds of arithmetic that you can perform using the letcommand, type help letat the bash prompt.

One of the features that makes shell scripts so powerful is that their implementation of looping and conditional execution constructs is similar to those found in more complex scripting and programming languages. You can use several different types of loops, depending on your needs.

The most commonly used programming construct is conditional execution, or the ifstatement. It is used to perform actions only under certain conditions. There are several variations of ifstatements for testing various types of conditions.

The first if…thenexample tests if VARIABLEis set to the number 1. If it is, then the echocommand is used to say that it is set to 1. The fistatement then indicates that the ifstatement is complete, and processing can continue.

VARIABLE=1 if [ $VARIABLE -eq 1 ] ; then echo "The variable is 1" fi

Instead of using -eq, you can use the equal sign ( =), as shown in the following example. The =works best for comparing string values, while -eqis often better for comparing numbers. Using the elsestatement, different words can be echoed if the criterion of the ifstatement isn't met ( $STRING = ″Friday″). Keep in mind that it's good practice to put strings in double quotes.

STRING="Friday" if [ $STRING = "Friday" ] ; then echo "WhooHoo. Friday." else echo "Will Friday ever get here?" fi

You can also reverse tests with an exclamation mark ( !). In the following example, if STRINGis not Monday, then ″At least it's not Monday″is echoed.

STRING="FRIDAY" if [ "$STRING" != "Monday" ] ; then echo "At least it's not Monday" fi

In the following example, elif(which stands for “else if”) is used to test for an additional condition (for example, whether filenameis a file or a directory).

filename="$HOME" if [ -f "$filename" ] ; then echo "$filename is a regular file" elif [ -d "$filename" ] ; then echo "$filename is a directory" else echo "I have no idea what $filename is" fi

As you can see from the preceding examples, the condition you are testing is placed between square brackets [ ]. When a test expression is evaluated, it returns either a value of 0, meaning that it is true, or a 1, meaning that it is false. Notice that the echolines are indented. The indentation is optional and done only to make the script more readable.

Table 7.1lists the conditions that are testable and is quite a handy reference. (If you're in a hurry, you can type help teston the command line to get the same information.)

TABLE 7.1 Operators for Test Expressions

There is also a special shorthand method of performing tests that can be useful for simple one-command actions . In the following example, the two pipes ( ||) indicate that if the directory being tested for doesn't exist ( -d dirname), then make the directory ( mkdir $dirname):