Shell Scripting 101, Lesson 5

Occasionally, you might need to perform math in a shell script. In Lesson 3, I showed you how to use the Set command to create custom environment variables. You can also use the Set command to perform simple math calculations, such as addition, by including the /a switch. This switch tells the scripting engine to evaluate the integers that follow as numerical expressions.

For example, if you run the code

Set /a Fred=2+3Echo %Fred%

you receive the sum of these two numbers (i.e., 5). If you run the code without the /a switch

Set Fred=2+3Echo %Fred%

you won't receive a result of 5 but rather the string 2+3. Without the /a switch, you're merely making the Fred environment variable contain the string 2+3.

The Set /a command performs math calculations with integers only. The calculations you can perform with this command are

The Set /a calculations are precise as long as the integers you’re calculating are 10 or fewer digits.

Combining the Set and If Commands
Systems administrators often use the Set /a command in conjunction with the If command when they perform math calculations. The If command executes specific code if a certain condition exists (i.e., the conditional code evaluates to a true statement). The If command has many syntax versions, but the basic syntax is

If Condition CodeToExecute

where Condition is the conditional code to evaluate and CodeToExecute is the code you want to run if the conditional code evaluates to a true statement. When you combine a Set /a command with an If command, you can use these abbreviations:

For example, if you run the code

Set /a Fred=2+3Set /a Wilma=2*3If %Fred% LSS %Wilma% Echo Yes, Fred is less than Wilma

you receive the result Fred is less than Wilma. However, if you run the code

Set /a Fred=2+3Set /a Wilma=2*3If %Fred% GTR %Wilma% Echo Yes, Fred is less than Wilma

you don't receive any result because the conditional code evaluates to a false statement.

In Lesson 6, I’ll cover the If statement in greater detail. For now, let's look at a more practical example of how you can use the Set /a and If commands together.

A Practical Example
Suppose you want a script to loop a certain number of times. CountLoops.bat in Listing 1 illustrates how you can use the Set /a and If commands to prompt that script to run a certain number of times.

CountLoops.bat uses several new concepts: counters, the GoTo command, and labels. You can use counters to track the number of times a script completes an operation. For example, you can use counters to capture how many user accounts a script adds or how many files are in a folder. An important point to remember when you use counters is that you need to initialize the counter before you use it by including the command

Set /a counter=0

at the beginning of the script. This command sets an initial value (in this case, 0). If you use the counter without setting an initial value, you can receive some unusual errors.

By default, the scripting engine reads code from top to bottom. However, you can use the GoTo command and labels to change a script’s flow of execution—you can jump to any location in the script. You place the GoTo command at the point at which you want to jump to a different part of the script. The label marks the spot to which you want to jump. The GoTo command’s syntax is simply

GoTo :Label

You can use any term for the label as long as you preface it with a colon (:). Although you can use spaces in the label, you can’t use commas, semicolons, or other separators. The label can be any length. However, the scripting engine reads only the first eight characters, so it reads the labels :MyLabel01 and :MyLabel02 both as :MyLabel0, which can cause errors in a script.

In Listing 1, the code’s flow needs to return to the top of the script whenever the value of the counter variable is less than 10. So, I used the GoTo command with the label :Top to move the flow to that point, where the execution again proceeds downward.

Homework Assignments

1. Run the commands

Set /?|More

and

If /?|More

to see all the options that the Set and If commands provide.

2. Download CountLoops.bat by clicking the Download the code link in the Article Information box. Comment out the counter initialization line

Set /a counter=0

by prefacing it with a double colon (::) or the word Rem. Run the script several times in the same command-shell window to see the errors that result.