Definitions

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COMMAND.COM

COMMAND.COM is the filename of the default operating system shell for DOS operating systems and the default command line interpreter on 16/32-bit versions of Windows (9x/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-based operating systems is cmd.exe. COMMAND.COM is also available on those systems to provide compatibility when running DOS applications within the NT Virtual DOS machine.

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.

Notable internal commands

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 either case and are all equivalent (so dir, DIR and DiR will all work in the same way).

File system commands

In accordance with COMMAND.COM's main function as an operating system shell, it includes a number of built-in commands for working with files.

In order to run a program, simply type the name of its executable and then press "Enter" (it is not necessary to use the extension, e.g. nc.exe can be summoned simply as nc). In order to change the current working drive (see Drive letter assignment), type its letter followed by a colon (e.g. D:). Other file system commands include: DIR : Lists the files in the current directory CD, CHDIR : Changes the current working directory or displays the current directory. 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) REN, RENAME : Renames a file or directory DEL, ERASE : Deletes a file. When used on a directory, deletes all files in that directory, but does not recurse or delete the directory itself. MD, MKDIR : Creates a new directory RD, RMDIR : Removes an empty directory VOL : Shows information about a volume LABEL : Show or change the label of a volume VERIFY : Enable or disable verification of writing for files TYPE : Display the content of a file on the console BREAK : Controls the handling of program interruption with Ctrl+C. CLS : Clears the screen. CHCP : Displays or changes the current system code page. CTTY : Defines the device to use for input and output. DATE : Set the date of the system. ECHO : Toggles whether text is displayed (ECHO ON) or not (ECHO OFF). Also displays text on the screen (ECHO text). LH, LOADHIGH : Loads a program into upper memory (HILOAD in DR DOS). LOCK : Enables external programs to perform low-level disk access to a volume. (Windows 95/98/Me only) PATH : Displays or change the value of the PATH environment variable which controls the places where COMMAND.COM will search for executable files. PAUSE : Halts execution of the program and displays a message asking the user to press any key to continue. PROMPT : Displays or change the value of the PROMPT environment variable which controls the appearance of the prompt. SET : Sets the value of an environment variable ; Without arguments, shows all defined environment variables. TIME : Set the time of the system. UNLOCK : Disables low-level disk access. (Windows 95/98/Me only) VER : Displays the version of the operating system. LFNFOR : Enables or disables the return of long filenames by the FOR command. (Windows 95/98/Me only)

Undocumented commands

Some versions of MS-DOS COMMAND.COM recognises some internal commands which were not documented. TRUENAME : Display the "true name" of a file, by bypassing SUBST and ASSIGN filesystem mappings.

Control structures

Control structures are mostly used inside batch files, although they can also be used interactively. :label : Defines a target for GOTO. 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). REM : comment: any text following this command is ignored IF : Conditional statement, allows to branch the program execution CALL : Pauses execution of one batch file, runs another, and returns to the old one and continues. EXIT : Exits from Command.com and return to the program which launched it. SHIFT : Replaces each of the command-line variables with the consequent 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:

  1. 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.
  2. Environment variables - these have the form %VARIABLE% and are associated with values with the SET statement. Most versions of COMMAND.COM will only expand environment variables in batch mode.
  3. 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.
  4. "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 and piping

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

Limitations

The command line length in interactive mode is limited to 128 characters. It always returns a true value upon executing a command.

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.

The COMMAND.COM is a position of authority in the show ReBoot.

command.com was a website with an image of a command prompt with the "dir" command being run. It was 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.

See also

External links

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