Getting Acquainted with Variables in CMD

Getting Acquainted with Variables

In command-line scripting, what we commonly call variables are more properly called environment variables. Environment variables can come from many sources. Some variables are built into the operating system or derived from the system hardware during startup. These variables, called built-in system variables, are available to all Windows processes regardless of whether anyone is logged on interactively. System variables can also come from the Windows Registry. Other variables are set during logon and are called built-in user variables. The built-in user variables available are the same, no matter who is logged on to the computer. As you might expect, they are valid only during an actual logon session, that is, when a user is logged on.

You can see a listing of all the variables known in the current instance of the command shell by typing set at the prompt. In addition to the normal system and user variables, you can create variables whenever Windows is running, which is exactly what you’ll do when you program in the command shell. You define variables for the current instance of the command shell using the SET command and the following syntax:

set variable_name=variable_value

such as

set working=C:\Work\Data

set value=5

set string="Hello World"

Some variables, including system and user environment variables, have special meaning in the command shell. These variables include path, computername, homedrive, and many other important environment variables. One environment variable that you should learn more about is errorlevel, which tracks the exit code of the most recently used command. If the command executes normally, the error level is zero (0). If an error occurs while executing the command, the error level is set to an appropriate nonzero value. Error values include

• 1  Indicates a general error
• 2  Indicates an execution error, meaning the command failed to execute properly
• –2  Indicates a math error, such as when you create a number that is too large for the command shell to handle

You can work with the errorlevel variable in several ways. You can check for a specific error condition, such as

if "%ERRORLEVEL%"=="2" echo "An error occurred!"

Or, you can use the following special syntax and check for a condition equal to or greater than the specified exit code:

if errorlevel 2 echo "An error occurred!"

Note

You’ll see more on errorlevel and if statements later in the chapter in the section titled “Using Conditional Statements.”

When you are finished working with variables, it’s good form to dispose of them. You do this to free memory used by the variable and prevent problems or unexpected results if you accidentally refer to the variable in the future. To clear out a variable, you simply set the variable equal to nothing, such as

set working=

Now the variable is cleared out of memory and is no longer available.

Passing Arguments to Scripts in Windows

Passing Arguments to Scripts

As with most command-line utilities, arguments can be passed to scripts when they are started. You use arguments to set special parameters in a script or to pass along information needed by the script. Each argument should follow the script name and be separated by a space (and enclosed in quotation marks if necessary). In the following example, a script named Check-sys is passed the parameters Mailer1 and Full:

check-sys mailer1 full

Each value passed along to a script can be examined using formal parameters. The script name itself is represented by the parameter %0. The parameter %1 represents the first argument passed in to the script, %2 the second, and so on until %9 for the ninth argument. For example, if you create a script called Check-sys and then use the following command to call the script:

check-sys mailer1 full actual

you would find that the related parameter values are

• %0 — check-sys
• %1 — mailer1
• %2 — full
• %3 — actual

You access arguments in scripts using the parameter name: %0 for the script name, %1 for the first script parameter, and so on. For example, if you wanted to display the script name and the first argument passed to the script, you could enter

echo %0

echo %1

If you pass in more than nine parameters, the additional parameters are not lost. Instead, they are stored in a special parameter: %* (percent + asterisk). The %* parameter represents all arguments passed to the script and you can use the SHIFT command to examine additional parameters. If you call SHIFT without arguments, the script parameters are shifted by 1. This means the related value for %0 is discarded and replaced by the related value for %1, and the related value for %2 becomes the related value for %1, and so on. You can also specify where shifting begins so you can retain previous parameters if necessary. For example, if you use

shift /3

%4 becomes %3, %5 becomes %4, and so on. But %0, %1, and %2 are unaffected.

Creating Command Line Scripts in Windows

Creating Command Line Scripts

Command line scripts are text files containing the commands you want to execute. These are the same commands you would normally type into the Windows command shell. However, rather than enter the commands each time you want to use them, you create a script to store the commands for easy execution.

Because scripts contain standard text characters, you can create and edit scripts using a standard text editor, such as Notepad. When you enter commands, be sure to place each command or group of commands that should be executed together on a new line. This ensures proper execution of the commands. When you have finished creating a command-line script, save the script file using the .bat or .cmd extension. Both extensions work with command-line scripts in the same way. For example, if you wanted to create a script to display the system name, Windows version and IP configuration, you could enter these three commands into a file called SysInfo.bat or SysInfo.cmd:

hostname

ver

ipconfig -all

Once you save the script, you can execute it as if it were a Windows utility; simply type the name of the script in a command shell and press Enter. When you do this, the command shell reads the script file and executes its commands one by one. It stops executing the script when it reaches the end of the file or reads an EXIT command. For the example script, the command line would display output similar to Listing 3-1.

Listing 3-1: Output of Sample Script

C:\>hostname

mailer1






C:\>ver

Microsoft Windows [Version 5.2.3790]






C:\>ipconfig -all 

Windows IP Configuration

Host Name . . . . . . . . . . . . : mailer1

Primary Dns Suffix  . . . . . . . : adatum.com

Node Type . . . . . . . . . . . . : Unknown

IP Routing Enabled. . . . . . . . : No

WINS Proxy Enabled. . . . . . . . : No

DNS Suffix Search List. . . . . . : adatum.com






Ethernet adapter Local Area Connection:






Connection-specific DNS Suffix  . : 

Description . . . . . . . . . . . : Intel(R) PRO/100 VE Network

Connection

Physical Address. . . . . . . . . : X0-EF-D7-AB-E2-1E

DHCP Enabled. . . . . . . . . . . : No

IP Address. . . . . . . . . . . . : 192.168.10.50

Subnet Mask . . . . . . . . . . . : 255.255.255.0

Default Gateway . . . . . . . . . : 192.168.10.1

DNS Servers . . . . . . . . . . . : 192.168.10.155

If you examine the listing, you’ll see that the command prompt and the actual commands are displayed as well as the output of the commands themselves. The reason for this is that the command shell does some extra work behind the scenes while executing scripts in the default processing mode. First, the command shell displays the command prompt. Next, it reads a line from the script, displays it, and then interprets it. If the command shell reaches the end of the file or reads an EXIT command, execution stops. Otherwise, the command shell starts this process all over again by displaying the prompt and preparing to read the next line in the script.

Although the default processing mode with command echoing on can be useful for troubleshooting problems in scripts, you probably don’t want to use this display mode with scripts you’ll use regularly. Fortunately, you can change the default behavior by turning command echo off, as I’ll show you later in the chapter in the section titled “Managing Text Display and Command Echoing.”