6. Use the Setlocal and Endlocal Commands
The Setlocal command copies all environment variables, and Endlocal restores
all variables to the values they had before the script started. Endlocal also
deletes any variables the script created. Using both these commands makes a
script more self-contained and ensures that a script "cleans up after itself"
by restoring environment variables to their original values and deleting variables
the script created.
Also, you can use the Setlocal Enableextensions command to make sure that command
extensions are enabled. Command extensions are enhancements to a group of Cmd.exe
internal commands (e.g., If, For, Call) that provide expanded capabilities beyond
the Command.com commands that have the same names. Command extensions are enabled
by default, but in the rare cases in which command extensions are disabled,
the Setlocal Enableextensions command ensures that command extensions are enabled.
To see more information about command extensions, type Cmd /? at a command prompt.
7. Use the Escape Character When Needed
Cmd.exe uses the caret (^) as an escape character that bypasses the normal meanings
of reserved shell characters. For example, the ampersand (&) is the command
separator—it lets you put multiple commands on one line. If you intend
to use the & literally, you must "escape" its normal meaning by prefacing
it with the ^. So in the line
Echo The ^& character is the command separator
the ^ causes the shell to bypass the normal interpretation of the character
that follows. You need to escape the characters ( ) < > ^ & and |.
You don't need to escape these characters if they occur inside a quoted string.
8. Don't Use the Exit Command Without the /b Option
Without the /b option, the Exit command closes the current shell. If someone
starts a Cmd.exe session and executes a script that contains the Exit command,
the current shell will abruptly close. The Exit /b command closes the current
script without terminating the current shell. For more information about the
Exit command, type the command Exit /? at a command prompt.
9. Watch Out for If Errorlevel
The If Errorlevel command tests the exit code of the last command that was executed.
Scripts can test a program's exit code and behave accordingly. For example,
consider the following script code:
Myprogram
If Errorlevel 1 Goto :ERROR
In these lines, the Goto command will execute if Myprogram.exe returns an exit
code greater than or equal to 1. In other words, "If Errorlevel n" doesn't
mean "if the last exit code is exactly n;" it really means, "if the last
exit code is at least n." Because of this behavior, make sure to test
exit codes in descending order (highest to lowest). To test for a specific exit
code, use the ERRORLEVEL dynamic variable instead of If Errorlevel. Note that
the If Errorlevel command is different from the ERRORLEVEL variable listed in
Table 1. The If Errorlevel command is backward compatible with Command.com's
If Errorlevel command; the ERRORLEVEL variable is only available in Cmd.exe.
10. Be Aware of Start's Quirks
The Start command starts a program or a command in a new console window. However,
if you try to start a program from a directory that contains spaces (or if the
program's name contains spaces), the Start command won't behave as you might
expect. For example, the following command
Start "C:\Program Files\Microsoft
Office\Office11\Winword.exe"
doesn't start Microsoft Word, as you might expect. Instead, the Start command
will open a new Cmd.exe session with the quoted string as the title of the console
window. This behavior occurs because the Start command uses the first quoted
string on its command line as a console window title.
To work around this quirk, use a pair of quotes to specify a blank title, then
follow the title with the program you want to run. The corrected Start command
will look like this:
Start "" "C:\Program Files\Microsoft
Office\Office11\Winword.exe"
Avoid Potential Problems
Cmd.exe shell scripts are widely used. With these guidelines under your belt,
you can write more robust shell scripts and avoid common problems.