COMMAND.COM
 | |
| Developer(s) | Seattle Computer Products, Microsoft, IBM, Novell |
|---|---|
| Operating system | |
| Platform | x86 (16-bit) |
| Type | Command line interpreter |
COMMAND.COM is the file-name of 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.
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
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.
- BREAK
- Controls the handling of program interruption with Ctrl+C or Ctrl+Break.
- CHCP
- Displays or changes the current system code page.
- CHDIR, CD
- Changes the current working directory or displays the current directory.
- CLS
- Clears the screen.
- COPY
- Copies one file to another (if the destination file already exists, MS-DOS asks whether to replace it). (See also XCOPY, an external command that could also copy directory trees).
- CTTY
- Defines the device to use for input and output.
- DATE
- Display and set the date of the system.
- DEL, ERASE
- Deletes a file. When used on a directory, deletes all files inside the directory only. In comparison, the external command DELTREE deletes all subdirectories and files inside a directory as well as the directory itself.
- DIR
- Lists the files in the specified directory.
- ECHO
- Toggles whether text is displayed (ECHO ON) or not (ECHO OFF). Also displays text on the screen (ECHO text).
- EXIT
- Exits from COMMAND.COM and returns to the program which launched it.
- LFNFOR
- Enables or disables the return of long filenames by the FOR command. (Windows 95/98/Me only).
- LOADHIGH, LH
- Loads a program into upper memory (HILOAD in DR DOS).
- LOCK
- Enables external programs to perform low-level disk access to a volume. (DOS 7.1, Windows 95, 98, Me only)
- MKDIR, MD
- Creates a new directory.
- PATH
- Displays or changes the value of the PATH environment variable which controls the places where COMMAND.COM will search for executable files.
- PROMPT
- Displays or change the value of the PROMPT environment variable which controls the appearance of the prompt.
- REN, RENAME
- Renames a file or directory.
- RMDIR, RD
- Removes an empty directory.
- SET
- Sets the value of an environment variable ; Without arguments, shows all defined environment variables.
- TIME
- Display and set the time of the system.
- TRUENAME
- Display the fully expanded physical name of a file, resolving ASSIGN, JOIN and SUBST logical filesystem mappings.
- TYPE
- Display the content of a file on the console.
- UNLOCK
- Disables low-level disk access. (DOS 7.1, Windows 95, 98, Me only)
- VER
- Displays the version of the operating system.
- VERIFY
- Enable or disable verification of writing for files.
- VOL
- Shows information about a volume.
Batch file commands
Control structures are mostly used inside batch files, although they can also be used interactively.
- :label
- Defines a target for GOTO.
- CALL
- Executes another batch file and returns to the old one and continues.
- FOR
- Iteration: repeats a command for each out of a specified set of files.
- GOTO
- Moves execution to a specified label. Labels are specified at the beginning of a line, with a colon (:likethis).
- IF
- Conditional statement, allows to branch the program execution.
- PAUSE
- Halts execution of the program and displays a message asking the user to press any key to continue.
- REM
- comment: any text following this command is ignored.
- SHIFT
- Replaces each of the command-line variables with the subsequent one (e.g. %0 with %1, %1 with %2 etc.).
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 - Redirect standard input from a file or device
-
command > filename - Redirect standard output, overwriting target file if it exists.
-
command >> filename - Redirect standard output, appending to target file if it exists.
-
command1 | command2 - Pipe standard output from command1 to standard input of command2
-
command1 & command2 - Commands separated by & are executed in sequence (=chaining of commands). In other words: first command1 is executed until termination, then command2.
Limitations
The command line length in interactive mode is limited to 126 characters.[1]
Cultural references
“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[2] 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.
See also
References
- ↑ Matthias Paul (1997-07-01) [1994], "MSDOSTIPs — Tips für den Umgang mit MS-DOS 5.0-7", MPDOSTIP (in German), retrieved 2013-10-25 MSDOSTIP.TXT is part of the author's yet larger MPDOSTIP.ZIP collection maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of the MSDOSTIP.TXT file.
- ↑ COMMAND.COM archived on February 2006 on Internet Archives Wayback Machine
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