COMMAND.COM

COMMAND.COM
Developer(s) Seattle Computer Products, Microsoft, IBM, Novell
Operating system
Platform x86 (16-bit)
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.

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:

  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. Versions of COMMAND.COM before version 3 will only expand environment variables in batch mode (that is not interactively at the command prompt).
  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, 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]

“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 that has finished running the "dir" command on the root of an empty C drive. However, the resulting command incorrectly displayed the . and .. directories, which are never shown in the root of a drive.[2]

See also

References

  1. ↑ 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.
  2. ↑ COMMAND.COM archived on February 2006 on Internet Archives Wayback Machine
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.