Common Statements and Commands for Scripts

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So far in this book, I’ve discussed commands but haven’t really introduced what a statement is. While these terms are often used interchangeably, the term statement technically refers to the keyword for a command, such as the rem statement, but it can also refer to a line of code that includes all the command text on that line. In some programming languages, such as Java, each statement must be terminated with a specific character. With Java, the terminator is a semicolon. The command line doesn’t look for a specific terminator, other than the end of the line, which is assumed when the command interpreter reads any of the following:

• Line break (such as when you press Shift+Enter)

• Carriage return and line break (such as when you press Enter)

• End-of-file marker

Now that we’ve discussed how to create scripts, let’s look at common statements and commands you’ll use in scripts, including

• Cls Clears the console window and resets the screen buffer

• Rem Creates comments in scripts

• Echo Displays messages at the command line and turns command echoing on or off

• @ Controls command echo on a line-by-line basis

• Title Sets the title for the command shell window

• Color Sets the text and background colors used in the command shell window

Clearing the Command-Shell Window

Clearing the command-shell window before writing script output is usually a good idea. You clear the command-shell window using the CLS command. Why not try it? At the command line, type cls and press Enter. The console window clears and the cursor is positioned in the top left corner of the window. All the text in the screen buffer is cleared as well.

You could add the CLS command to the sample script listed previously, as shown in this example:

cls
hostname
ver
ipconfig -all

Adding Comments to Scripts

You use the rem statement to add comments to your scripts. Every script you create should have comments detailing

• When the script was created and last modified

• Who created the script

• What the script is used for

• How to contact the script creator

• Whether and where script output is stored

Not only are the answers to these Who, What, How, Where, and When questions important for ensuring that the scripts you create can be used by other administrators, they can also help you remember what a particular script does, especially if weeks or months have passed since you last worked with the script. An example of a script that uses comments to answer these questions is shown as Listing 3-2.

Listing 3-2: Updated Sample Script with Comments

rem ************************
rem Script: SystemInfo.bat
rem Creation Date: 2/2/2004
rem Last Modified: 3/15/2004
rem Author: William R. Stanek
rem E-mail: [e-mail]
rem ************************
rem Description: Displays system configuration information
rem              including system name, IP configuration
rem              and Windows version.
rem ************************
rem Files: Stores output in c:\current-sys.txt.
rem ************************
 
hostname > c:\current-sys.txt
ver >> c:\current-sys.txt
ipconfig -all >> c:\current-sys.txt

Later in this chapter, in the section titled “Passing Arguments to Scripts,” I’ll show you how to use your comments as automated help documentation. Before we get to that, however, keep in mind that rem statements can also be used to

• Insert explanatory text within scripts, such as documentation on how a procedure works.

• Prevent a command from executing. On the command line, add rem before the command to comment it out.

• Hide part of a line from interpretation. Add rem within a line to block interpretation of everything that follows the rem statement.

Managing Text Display and Command Echoing

The echo command has two purposes. You use the ECHO command to write text to the output, which can be the command shell or a text file. You also use the ECHO command to turn command echoing on or off. Normally, when you execute commands in a script, the commands as well as the resulting output of the command are displayed in the console window. This is called command echoing.

To use the ECHO command to display text, enter echo followed by the text to display, such as

echo The system host name is:
hostname 

To use ECHO to control command echoing, type echo off or echo on as appropriate, such as

echo off
echo The system host name is:
hostname

Use output redirection to send output to a file rather than the command shell, as follows:

echo off
echo The system host name is: > current.txt
hostname >> current.txt

To experiment with suppressing command echoing, start a command shell, type echo off, and then enter other commands. You’ll find that the command prompt is no longer displayed. Instead, you see only what you type into the console window and the resulting output from the commands you’ve entered. In scripts, the ECHO OFF command turns off command echoing as well as the command prompt. By adding the command ECHO OFF to your scripts, you keep the command-shell window or the output file from getting cluttered with commands when all you care about is the output from those commands.

Tip

By the way, if you want to determine whether command echoing is enabled or disabled, type the ECHO command by itself. Give it a try. If command echoing is on, you’ll see the message Echo Is On. Otherwise, you’ll see the message Echo Is Off. Experiment with the ECHO OFF command in your scripts and you may detect a bit of a problem here. If the ECHO OFF command turns off command echoing, how do you prevent the ECHO OFF command itself from echoing? Don’t worry; that’s discussed in the next section.

Real World

One question that other command-line programmers frequently ask me is, How do you get a blank line to echo in the command shell? You might think that putting the ECHO command on a line by itself would do the job, but it doesn’t. Typing echo on a line by itself displays the status of command echoing as mentioned in the previous tip. Typing echo followed by a space doesn’t work either, because the Windows command line treats spaces (in this situation) as meaningless and you get the same results as typing echo followed by nothing at all. To get ECHO to display a blank line, you must enter echo. (Note that there is no space between the period and the ECHO command.)

Fine-Tuning Command Echo with @

The @ command prevents commands from echoing to the output on a line-by- line basis and you can think of it as a line-specific echo off statement. You could use @ to turn off command echoing like this:

@echo The system host name is:
@hostname

Using @, the output that shows the command prompt and commands like this:

C:\>echo The system host name is: 
The system host name is:
 
C:\>hostname
mailer1

becomes

The system host name is:
mailer1 

But the real value of @ is that it allows you to tell the command shell not to display the command prompt or ECHO OFF command, and thereby ensures that the only output of your scripts is the output of the commands you enter. Here is an example of a script that uses @ to hide the ECHO OFF command so that it isn’t displayed in the output:

@echo off
echo The system host name is:
hostname

The output from this script is

The system host name is:
mailer1

Tip

I recommend using @echo off at the beginning of all your command-line scripts. By the way, if you start a command shell and type @echo off, you can turn off the display of the command prompt as well.

Setting the Console Window Title and Colors

If you’re going to take the time to write a script, you might as well add a few special features to jazz it up. Some of the basic techniques that I’ve already discussed are using the ECHO OFF command and clearing the console window before you write output. You may also want to set a title for the window or change the colors the window uses.

The title bar for the command shell is located at the top of the console window. Normally, this title bar displays “Command Prompt” or the file path to the command shell. You can customize the title using the TITLE command. This command works much like the ECHO command in that it displays whatever text follows it on the console’s title bar. For example, if you wanted to set the title of the current console to “System Information,” you could do this by entering the following at the command line:

title System Information

You can not only use the TITLE command to show the name of the script that is running; you can also use TITLE to show the progress of the script as it executes, such as

rem add blocks of work commands
title Gathering Information
 
rem add blocks of logging commands
title Logging System Information 

By default, the console window displays white text on a black background. As you learned in Chapter 1, “Overview of the Windows Command Line,” you can modify this behavior using the Colors tab of the Command Prompt Properties dialog box. You can also set console colors by using the COLOR command. You do this by passing the command a 2-digit hexadecimal code. The first digit corresponds to the background color and the second digit corresponds to the text color, such as

color 21

which sets the text to blue and the background color to green.

The color codes you can use with the COLOR command are shown in Table 3-1. Keep in mind that you can’t set the text and background colors to the same value. If you try to do this, the color doesn’t change. Additionally, you can restore the default colors at any time by using the COLOR command without any arguments, such as

color

Table 3-1: Color Codes for the Command Shell Window

Code	Color	Code	Color
0	Black	8	Gray
1	Blue	9	Bright Blue
2	Green	A	Bright Green
3	Aqua	B	Bright Aqua
4	Red	C	Bright Red
5	Purple	D	Bright Purple
6	Yellow	E	Bright Yellow
7	White	F	Bright White

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