Windows Server 2008 Server Core : Starting the Command Interpreter (part 5)

5.7. Managing Environment Variables with the Set Command

The term environment variable is foreign to many users because environment variables work in the background to make life easier for both GUI users and those who work at the command line. Admittedly, someone working at the command line is more likely to see the effects of environment variables directly. For example, without a proper path environment variable, you can't execute many applications. The path points to the locations where the command processor should look for applications.

Windows supports two forms of environment variables: permanent and session. The session environment variables only affect the current command line session. As soon as you close the command window, these environment variables are gone. You create session environment variables using the Set command. Permanent environment variables exist for all Windows GUI applications as well as the command prompt. Unless you change them, permanent environment variables exist between reboots of your system as well. You set permanent environment variables using the WMIC Environment alias . For example, if you want to set the permanent path variable to include the C:\Temp directory, you would type WMIC Environment Where Name="Path" SET VariableValue="C:\Temp;%PATH%" and press Enter. Notice that you include the %PATH% expansion variable to preserve the existing path information.

When you work at the command line, you often need environment variables in addition to those Windows uses. For example, the COPYCMD environment variable affects the Copy, Move, and XCopy utilities. You can set the command line switches you want to use with these utilities by setting the COPYCMD environment variable. Of course, you can always override your selections by specifying a different set of command line switches at the command line. For example, if you set the /Y command line switch in the COPYCMD environment variable, you can override it by specifying the /-Y command line switch.

Many environment variables only affect one application. For example, you can set the MORE environment variable to set the command line switches for the More utility. Every time you use the More utility, it searches for this environment variable and sets itself up accordingly.

Command line environment variables need not impose on those used in Windows. Besides setting environment variables in the Environment Variables dialog box and at the command line using the Set command, you can add environment variables to the AutoExec.NT file. Windows executes the commands within AutoExec.NT every time it opens a command prompt for you. These environment variables appear every time you open a command prompt, but won't appear within Windows. 

For all of the tasks that it performs, the Set command is relatively simple. This command uses the following syntax:

SET [variable[=[string]]]
SET /A expression
SET /P variable=[promptString]

The following list describes each of the command line arguments.

variable

Specifies the environment variable name. The name can't contain spaces or most special characters. You can split words in a variable name using the underscore (_) or dash (-).

string

Defines the value of the environment variable. If you type the environment variable, followed by an equals sign (=), but without a value, the Set command deletes the environment variable. Type the Set command followed by the environment variable without an equals sign to determine the current environment variable value. In fact, you can type as little as a single letter to see a list of environment variables and their associated values that begin with that letter.

/A

expression

Creates an environment variable based on an expression, such as a math equation or the concatenation of multiple environment variables, instead of a standard string.

/P

variable=[promptString]

Prompts the user to assign a value to an environment variable, rather than assigning the value directly. You must supply a variable name. The optional prompt string lets you provide a specific prompt to the user. Otherwise, the display doesn't show any prompt at all.

Server Core comes with a number of standard environment variables. These hard-coded environment variables perform essential tasks, such as displaying the current directory for you. You can see a list of these environment variables on the Microsoft Web site at http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/ntcmds_shelloverview.mspx. Typing Set by itself and pressing Enter displays the list of environment variables set for your machine as shown in Figure 2. This list doesn't include the hard-coded environment variables such as CD.

You can use the /A command line switch to combine existing environmental variables or even perform math with them. For example, you could use the following Set command to create a new environment variable based on the existing %NUMBER_OF_PROCESSORS% environment variable.

SET /A TwiceTheProcessor=%NUMBER_OF_PROCESSORS% * 2

Figure 2. The Set command displays the list of environment variables defined for your machine.

You can use any of the operators shown in Table 4 when creating your expression. The operators appear in order of precedence.

Any expressions you create can also contain octal, decimal, or hexadecimal numbers. All octal values begin with a 0 (zero) and hexadecimal values begin with a 0x. 

5.8. Managing Environment Variables with the SetX Utility

The SetX utility performs essentially the same tasks as the Set command . However, it offers considerably more flexibility because you can use SetX over a network to interact with variables on other machines. In addition, it offers considerable flexibility in setting variables, such as letting you choose between the current user and the system as a whole. Rather than repeat all of the common information for using SetX, please refer to the Set command as a starting point. This command uses the following syntax:

SETX [/S system [/U [domain\]user [/P [password]]]] var value [/M]
SETX [/S system [/U [domain\]user [/P [password]]]] var /K regpath [/M]
SETX [/S system [/U [domain\]user [/P [password]]]] /F file {var
   {/A x,y | /R x,y string}[/M] | /X} [/D delimiters]

The following list describes each of the command line arguments.

/S

system

Specifies the remote system that you want to check. In most cases, you'll also need to supply the /U and the /P command line switches when using this switch.

/U

[domain\]user

Specifies the username on the remote system. This name may not match the username on the local system. You'll need to supply a domain name when working with a domain controller.

/P

[password]

Specifies the password for the given user. You can provide the command line switch without specifying the password on the command line in cleartext. The system prompts you for the password. Using this feature can help you maintain the security of passwords used on your system.

var

Contains the name of the variable you wish to change.

value

Sets the specified variable to the supplied value. You don't have to provide the equals sign (=) as you do when working with the Set command. Place values with spaces within double quotes.

/M

Sets the value as a system-wide (machine-level) variable that appears in HKEY_LOCAL_MACHINE. The default setting places the variable within the current user's environment.

/K

regpath

Sets the specified variable using the content of a registry key. You must provide the fully qualified registry value (including all of the keys), such as KEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TimeZoneInformation\StandardName.

/F

file

Specifies the name of a text file to use for settings. You must also provide the position of the setting within the file as an absolute or relative coordinate (line number and character position). In addition, if the setting is delimited, you must provide the delimiter used for it.

/A

x,y

Provides the absolute position of the setting within the file. You must provide the position as a line number and character number.

/R

x,y string

Provides a relative position of the setting within the file. SetX first searches for the string you specify within the file. It then moves relative to that string the number of lines and characters within the line that you specify.

/X

Displays the file contents using x and y coordinates. This argument doesn't actually set any variables. What it does is display the position of each potential setting within the file so that you can use the /A and /R arguments with greater ease.

/D

delimiters

Defines the delimiters used to begin and end settings entries within the file. The default settings include space, tab, carriage return, and linefeed. You can use any ASCII character as a delimiter. The maximum number of delimiters, including the default delimiters, is 15.

5.9. Executing Applications Using the Start Command

A common way to handle this problem, when the application supports it, is to ask it to return immediately and continue working in the background. The only problem with this approach is that you aren't sure that the command or utility completed successfully. The Start command helps you overcome this problem by creating a new window for the command or window to run in. The command or utility still controls the command prompt until it completes, but since it runs in another window, the user can continue working.

Another use for this utility is to start the command or utility in a window with a specific title. You can apply settings to a window that rely on the window title . Whenever you open a window with that title, it also has the special formatting that you specified. In short, you can create custom environments in which to run a utility. This utility uses the following syntax:

START ["title"] [/Dpath] [/I] [/MIN] [/MAX] [/SEPARATE | /SHARED] [/LOW
   | /NORMAL | /HIGH | /REALTIME | /ABOVENORMAL | /BELOWNORMAL] [/WAIT]
   [/B] [{command | program}] [parameters]

The following list describes each of the command line arguments.

"title"

Defines the title of the command window title bar.

/D

Path

Specifies the starting directory for the command window. This value doesn't have to be the same as the path for the command that you want to execute. For example, it might be the path to the data directory for the command.

/B

Starts the application without creating a new window. The application executes as a background task and doesn't display a user interface. 

/I

Passes the standard command environment to the new window, rather than using any changes made to the command environment by the current command window. Using this command line switch ensures that the application won't run in a contaminated environment where an environmental setting could adversely affect the way the application runs.

/MIN

Starts the application with the command window minimized.

/MAX

Starts the application with the command window maximized.

/SEPARATE

Starts 16-bit Windows applications in a separate memory space. Normally, Windows uses a separate memory space for all 32-bit applications, but a single memory space for all 16-bit applications. Older applications don't always use memory correctly, resulting in memory corruption that can cause other applications to fail. Using this command line switch eliminates shared memory corruption problems. However, this feature comes at the cost of performance and overall system resource usage. Unlike 32-bit memory spaces, 16-bit memory spaces remain fixed in memory, which uses the memory inefficiently and causes memory fragmentation. Memory fragmentation can cause Windows to perform inefficiently, leading to performance problems. In short, use this command line switch only when you actually experience problems.

/SHARED

Starts the 16-bit Windows application in a shared memory space. This is the default setting and you should normally use it unless you experience memory corruption problems while running the application.

/LOW

Starts the application in the IDLE priority class. Use this priority setting for applications that only run when other applications don't require the processor. This command line switch helps the system function more efficiently when you want to run the application as a background task and don't care when it completes its work.

/BELOWNORMAL

Starts the application in the BELOWNORMAL priority class. Use this priority setting for background applications that you want to complete in a timely manner, but not at the expense of foreground applications.

/NORMAL

Starts the application in the NORMAL priority class. This is the default setting. Foreground applications normally start at this priority. You should only use this setting for applications that you want to complete at a normal pace. This setting does affect all foreground application responsiveness.

/ABOVENORMAL

Starts the application in the ABOVENORMAL priority class. This setting places the application at a slightly higher priority than standard applications. The other foreground tasks continue to run, but at a noticeably slower pace. Use this setting for priority applications that must complete tasks quickly.

/HIGH

Starts the application in the HIGH priority class. You won't normally have a good reason to use this priority for any application. Using this priority level can affect system functionality and definitely slows other foreground applications to a crawl. Always use this setting with extreme caution.

/REALTIME

Starts the application in the REALTIME priority class. Using this priority level stops execution of other foreground tasks and some system tasks as well. In addition, system functionality degrades noticeably. In some circumstances, the system could actually freeze and require a reboot. Generally, you don't want to use this priority for any reason.

/WAIT

Starts the application and waits for it to terminate. This setting ensures that you know when an application completes its task.

{command | program}

Specifies the internal command/batch file or the external utility/ application to run. The command window remains visible after you run an internal command or batch file because the system executes the command processor (CMD.EXE) with the /K switch.  The system runs external utilities and applications in a window or in a full-screen console. Unless you change the default behavior using a command line switch, the window or full-screen console closes when the application completes execution.

parameters

Specifies the parameters (arguments) passed to the command or program.

5.10. Determining the Operating System Version with the Ver Command

The Ver command is one of the easier commands to use. It doesn't require any arguments and provides only one type of output. Whenever you use the Ver command, you receive the operating system version information. The version information contains the operating system name in human-readable form, the major version number, the minor version number, and the build number. For example, the current Windows XP version at the time of this writing is Microsoft Windows XP [Version 5.1.2600]. Server Core displays something like Microsoft Windows [Version 6.0.6001].

5.11. Getting Volume Information with the Vol Command

The Vol command displays the volume information for the current or selected drive. The information includes the drive letter, the drive volume name, and the serial number. This command uses the following syntax:

VOL [drive:]

The following describes the command line argument.

drive Specifies the letter of the drive for which you want to obtain volume information. The default is the current drive.

6. Using Debugging Utilities

The command line is an admittedly simple place to work from a programming perspective. In fact, using batch files is more along the lines of working with a very simple kind of script, rather than a complex application. Even so, you'll eventually need to perform some kind of debugging. The debugging may involve checking inside a file to ensure you aren't going to execute a virus. You may also have to perform some low-level tasks with very old applications to get them to run in the Server Core environment. The following sections describe the debugging utilities that Server Core makes available for your use.

6.1. Examining, Modifying, and Debugging Files with the Debug Utility

Microsoft originally intended the Debug utility to help developers debug assembly language applications. You can still use the utility for that purpose, but the number of assembly language programs still in use is very small. In fact, it's likely that you'll never see an application originally written in assembler.

Fortunately, the Debug utility does have other purposes. For example, you can use it to view hidden information in executable files. Application developers leave many notes inside applications and it's often helpful to view these messages when deciding on the alternative uses of a utility. You can also locate copyright information in files when you want to know more about the file. Sometimes, you can also use the Debug utility to locate information in data files or make quick fixes to damaged data files. Most people refer to this other use for Debug as hex (short for hexadecimal) editing. However, the Debug utility interface is archaic and many people find it hard to use. If you need low-level access to files regularly, you might want to invest in a third-party utility for the task such as XVI32 . This utility uses the following syntax:

DEBUG [[drive:][path]filename [testfile-parameters]]

The following list describes each of the command line arguments.

[drive:][path]filename

Specifies the name of the file that you want to view, edit, or debug.

testfile-parameters

Specifies the command line arguments required by the application you want to test. You never use this second argument to simply view or edit a file.

6.2. Converting Executables with the Exe2Bin Utility

The Exe2Bin utility converts an application from the Portable Executable (PE) format to the binary format. The main difference is in how the executable loads. At some time in the past, you needed to use this utility quite often to convert executable files because the linkers of the time didn't provide the required functionality, but now it's a relic that you might never use. The resulting binary file is smaller than the executable and may execute slightly faster. At a time when developers counted individual bits, this utility served an important purpose. This utility uses the following syntax:

EXE2BIN [drive1:][path1]input-file [[drive2:][path2]output-file]

The following list describes each of the command line arguments.

[drive:][path1]input-file

Specifies the location and name of the EXE file you want to convert. If you don't provide drive and path information, the utility assumes that you want to use the current drive and path.

[drive1:][path1]output-file

Specifies the location and name of the binary file (usually with a COM extension) that you want to output. If you don't provide drive and path information, the utility assumes that you want to use the current drive and path.