Platform x86 (16-bit) | ||
Developer(s) Seattle Computer Products, Microsoft, IBM, Novell Operating system Type Command line interpreter |
COMMAND.COM is the default operating system shell for DOS operating systems and the default command line interpreter on Windows 95, Windows 98 and Windows ME. It has an additional role as the first program run after boot, hence being responsible for setting up the system by running the AUTOEXEC.BAT configuration file, and being the ancestor of all processes. COMMAND.COM's successor on OS/2 and Windows NT systems is CMD.EXE. COMMAND.COM is also available on IA-32 versions of those systems to provide compatibility when running DOS applications within the NTVDM.
Contents
- Operating modes
- Batch file commands
- Variables
- Redirection piping and chaining
- Limitations
- In popular culture
- References
COMMAND.COM is a DOS program. Programs executed by COMMAND.COM are DOS programs that use the MS-DOS API to communicate with the operating system (DOS).
Operating modes
As a shell, COMMAND.COM has two distinct modes of work. First is the interactive mode, in which the user types commands which are then executed immediately. The second is the batch mode, which executes a predefined sequence of commands stored as a text file with the extension .BAT.
Internal Commands are commands stored directly inside the COMMAND.COM
binary. Thus, they can only be executed directly from the command interpreter.
All commands are run only after the Enter key is pressed at the end of the line. COMMAND.COM is not case-sensitive, meaning commands can be typed in any mixture of upper and lower case.
Batch file commands
Control structures are mostly used inside batch files, although they can also be used interactively.
Variables
Batch files for COMMAND.COM can be said to have four kinds of variables:
- ERRORLEVEL - contains the return code of the last program to run that sets a value (an integer between 0 and 255). Most programs have a certain convention for their return codes (for instance, 0 for a successful execution). Some programs do not establish a new value, and thus the older value persists after they execute. The value of ERRORLEVEL is tested for range with the IF statement.
- Environment variables - these have the form %VARIABLE% and are associated with values with the SET statement. Versions of COMMAND.COM before version 3 will only expand environment variables in batch mode (that is not interactively at the command prompt).
- Command-line parameters - these have the form %0, %1...%9, and initially contain the command name and the first nine command line parameters passed to the script (e.g., if the invoking command was "myscript.bat John Doe", then %0 is "myscript.bat", %1 is "John" and %2 is "Doe"). The parameters to the right of the ninth can be mapped into range by using the SHIFT statement.
- "For" variables - used by loops, have the format %%a when run in batch files. These variables are defined solely within a specific FOR statement, and iterate over a certain set of values defined in that FOR statement.
Redirection, piping, and chaining
Because DOS is a single-tasking operating system, piping is achieved by running commands sequentially, redirecting to and from a temporary file. COMMAND.COM makes no provision for redirecting the standard error channel.
command < filename
command > filename
command >> filename
command1 | command2
command1 & command2
Limitations
The command line length in interactive mode is limited to 126 characters.
In popular culture
“Loading COMMAND.COM” message can be seen on a HUD view of the Terminator and the internal viewport of RoboCop when he reboots.
In the CGI children's TV series ReBoot, which takes place inside computers, the leader of a system (the equivalent of a city) is called the COMMAND.COM.
For several years, the command.com domain hosted a website with an image of a command prompt with the "dir" command being run. It was apparently meant to trick users into thinking the contents of their "C:" drive were listed. However, the resulting command incorrectly displayed the . and .. directories, which are never shown in the root of a drive. Currently the domain is used by 3M to promote their line of Command adhesive products and wall hooks.