Command.com

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The title given to this article is incorrect due to technical limitations. The correct title is command.com.

command.com is the name for the default operating system shell (or command line interpreter) for DOS and some versions of Windows. It also has an additional role, as the first program run after boot, hence being responsible for setting up the system as specified in the config.sys and autoexec.bat configuration files, and being the ancestor of all processes.

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. Its function as the default command interpreter is analogous to that of the Unix shells, although command.com's functionality is considerably more limited than that of its Unix counterparts.

Syntax

That this section does not try to give a full overview to the syntax of command.com, but rather serve as an overview and a mnemonic for the most common and interesting features. All commands are run only after the Enter key is pressed at the end of the line. command.com is case-insensitive, meaning commands can be typed in either case and are all equivalent (so dir, DIR and DiR will all work in the same way).

Filesystem 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 (D:). Other filesystem commands include:

DIR 

Lists the files in the current directory

CD, CHDIR 

Changes the current working directory or displays the currect directory. CD is best since it is shorter.

COPY 

Copies one file to another (if the destination file already exists, MS-DOS asks whether to replace it). (See also menu (MS-DOS 6 and higher). Defaults to Y/N.

CALL 

Pauses execution of one batch file, runs another, and returns to the old one and continues.

COMMAND 

The COMMAND command starts a new copy of command.com.

EXIT 

Exits from a new copy of Command.com, or exits the dos prompt in windows.

VER 

This command is built into command.com and displays the current version.

Control structures

Control structures are mostly used inside batch files, although they can also be used interactively.

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

Variables

Batch files for command.com can be said to have 4 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.
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. These variables are defined solely within a specific FOR statement, and iterate over a certain set of values defined in that FOR statement.

See also: List of DOS commands