Command shell overview

The command shell is a separate software program that provides direct communication between the user and the operating system. The non-graphical command shell user interface provides the environment in which you run character-based applications and utilities. The command shell executes programs and displays their output on the screen by using individual characters similar to the MS-DOS command interpreter Command.com. The Windows XP command shell uses the command interpreter Cmd.exe, which loads applications and directs the flow of information between applications, to translate user input into a form that the operating system understands.

You can use the command shell to create and edit batch files (also called scripts) to automate routine tasks. For example, you can use scripts to automate the management of user accounts or nightly backups. You can also use the Windows Script Host, CScript.exe, to run more sophisticated scripts in the command shell. You can perform operations more efficiently by using batch files than you can by using the user interface. Batch files accept all commands that are available at the command line. For more information about batch files and scripting, see Using batch files

You can customize the command prompt window for easier viewing and to increase control over how you run programs. For more information about customizing the command prompt window, see To configure the command prompt

Using command syntax

Syntax appears in the order in which you must type a command and any parameters that follow it. The following example of the xcopy command illustrates a variety of syntax text formats:

xcopy Source [Destination] [/w] [/p] [/c] [/v] [/q] [/f] [/l] [/g] [/d[:mm-dd-yyyy]] [/u] [/i] [/s [/e]] [/t] [/k] [/r] [/h] [{/a|/m}] [/n] [/o] [/x] [/exclude:file1[+[file2]][+[file3]] [{/y|/-y}] [/z]

The following table explains how to interpret the different text formats.

Formatting legend

Using multiple commands and conditional processing symbols

You can run multiple commands from a single command line or script using conditional processing symbols. When you run multiple commands with conditional processing symbols, the commands to the right of the conditional processing symbol act based upon the results of the command to the left of the conditional processing symbol. For example, you might want to run a command only if the previous command fails. Or, you might want to run a command only if the previous command is successful.

You can use the special characters listed in the following table to pass multiple commands.

Note

• The ampersand (&), pipe (|), and parentheses ( ) are special characters that must be preceded by the escape character (^) or quotation marks when you pass them as arguments.

• If a command completes an operation successfully, it returns an exit code of zero (0) or no exit code. For more information about exit codes, see Microsoft Windows Resource Kits 

Nesting command shells

You can nest command shells within Cmd.exe by opening a new instance of Cmd.exe at the command prompt. By default, each instance of Cmd.exe inherits the environment of its parent Cmd.exe application. By nesting instances of Cmd.exe, you can make changes to the local environment without affecting the parent application of Cmd.exe. This allows you to preserve the original environment of Cmd.exe and return to it after you terminate the nested command shell. The changes you make in the nested command shell are not saved.

To nest a command shell, at the command prompt, type:

cmd

A message similar to the following appears:

Microsoft (R) Windows XP (TM)
(C) Copyright 1985-2001 Microsoft Corp.

To close the nested command shell, type exit.

You can localize changes even further in an instance of Cmd.exe (or in a script) by using the setlocal and endlocal commands. Setlocal creates a local scope and endlocal terminates the local scope. Any changes made within the setlocal and endlocal scope are discarded, thereby leaving the original environment unchanged. You can nest these two commands to a maximum of 32 levels. For more information about the setlocal and endlocal commands, see Setlocal and Endlocal

Using environment variables with Cmd.exe

The Cmd.exe command-shell environment is defined by variables that determine the behavior of the command shell and the operating system. You can define the behavior of the command-shell environment or the entire operating system environment by using two types of environment variables, system and local. System environment variables define the behavior of the global operating system environment. Local environment variables define the behavior of the environment of the current instance of Cmd.exe.

System environment variables are preset in the operating system and available to all Windows XP processes. Only users with administrative privileges can change system variables. These variables are most commonly used in logon scripts.

Local environment variables are only available when the user for whom they were created is logged on to the computer. Local variables set in the HKEY_CURRENT_USER hive are valid only for the current user, but define the behavior of the global operating system environment.

The following list describes the various types of variables in descending order of precedence:

1. Built-in system variables

2. System variables found in the HKEY_LOCAL_MACHINE hive

3. Local variables found in the HKEY_CURRENT_USER hive

4. All environment variables and paths set in the Autoexec.bat file

5. All environment variables and paths set in a logon script (if present)

6. Variables used interactively in a script or batch file

In the command shell, each instance of Cmd.exe inherits the environment of its parent application. Therefore, you can change the variables in the new Cmd.exe environment without affecting the environment of the parent application.

The following table lists the system and local environment variables for Windows XP.

Setting environment variables

Use the set command to create, change, delete, or display environment variables. The set command alters variables in the current shell environment only.

To view a variable, at a command prompt, type:

set VariableName

To add a variable, at a command prompt, type:

set variablename= value

To delete a variable, at a command prompt, type:

set VariableName**=**

You can use most characters as variable values, including white space. If you use the special characters <, >, |, &, or ^, you must precede them with the escape character (^) or quotation marks. If you use quotation marks, they are included as part of the value because everything following the equal sign is taken as the value. Consider the following examples:

• To create the variable value new&name, type:

  set varname=new^&name

• To create the variable value "new&name", type:

  set varname="new&name"

• If you type set varname=new&name at the command prompt, an error message similar to the following appears:

  "'name' is not recognized as an internal or external command, operable program or batch file."

Variable names are not case-sensitive. However, set displays the variable exactly as you typed it. You can combine uppercase and lowercase letters in your variable names to make your code more readable (for example, UserName).

Note

• The maximum individual environment variable size is 8192bytes.

• The maximum total environment variable size for all variables, which includes variable names and the equal sign, is 65,536KB.

Substituting environment variable values

To enable the substitution of variable values at the command line or in scripts, enclose the variable name in percent signs (that is, %variablename%). By using percent signs, you ensure that Cmd.exe references the variable values instead of making a literal comparison. After you define variable values for a variable name, enclose the variable name in percent signs. Cmd.exe searches for all instances of the variable name and replaces it with the defined variable value. For example, if you create a script that contains different values (for example, user names) and you want to define the USERNAME environment variable for each user with these values, you can write one script using the variable USERNAME enclosed in percent signs. When you run this script, Cmd.exe replaces %USERNAME% with the variable values, which eliminates the need to perform this task manually for each user. Variable substitution is not recursive. Cmd.exe checks variables once. For more information about variable substitution, see For and Call