3.5. Вивчення тексту за фахом.“Операційна система ms-dos” Частина 1 …………………………………………………….

3.6. Вивчення тексту за фахом.“Операційна система ms-dos” Частина 2 ……………………………………………………. 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).

File system command 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 file system 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 XCOPY, an external command that could also copy directory trees)

MOVE 

The same as COPY, but removes the original afterwards. (MS-DOS 6 and higher).

DEL, ERASE, DELETE 

Deletes a file. DEL is best since it is the shortest. (See also DELTREE, an external command which can delete entire directory trees)

MD, MKDIR 

Create a new directory

RD, RMDIR 

Removes an empty directory

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.