This article discusses the following:
- How COMMAND.COM finds executable files
- The order of precedence involved in this search
- How the order can be overridden in MS-DOS versions 4.x and later
- The difference in filename recognition between COMMAND.COM and the MS-DOS EXEC system call
COMMAND.COMCOMMAND.COM can execute a file typed on the command line or as a line in a batch file.
MS-DOS Versions Earlier Than Version 4.0When a filename is given, whether or not an extension is supplied, COMMAND.COM keeps the base name only. (COMMAND.COM removes the filename extension if one exists [for example, "FILE" remains if the filename is "FILE.EXE"]). After it has the base name, COMMAND.COM looks in the following order for an executable file that has this name:
If COMMAND.COM cannot find this file in the current directory of the current drive, it begins searching the directories specified in the PATH environment variable in the AUTOEXEC.BAT file. If such a file is not found, the following error message is displayed:
.COM .EXE .BAT
Bad command or file nameFor example, if there are three files in the current directory: FILE.COM, FILE.EXE, and FILE.BAT, and you try to execute any one of them by typing
FILE.COM, FILE.EXE, FILE.BAT, or FILE, COMMAND.COM executes only FILE.COM, because this is the first file it finds.
MS-DOS Versions 4.0 and LaterIn MS-DOS versions 4.0 and later, you can override the COM-EXE-BAT precedence by typing the command with the extension.
For example, if there are three files in the current directory: FILE.COM, FILE.EXE, and FILE.BAT, and you try to execute any one of them by typing
FILE.COM, FILE.EXE, or FILE.BAT, COMMAND.COM executes the one you specify. However, if no filename extension is given, the usual order of precedence is used (.COM, .EXE, and .BAT).
One side effect of this behavior is that some incorrect or expedient ways of executing files no longer work. For example, in versions of MS-DOS earlier than 4.0, you can execute FILE.BAT by typing the filename with a period following it FILE.. However, in MS-DOS 4.0 and later, if you type FILE., MS-DOS searches for a file by this name, cannot find one, and returns a "bad command or file name" error message.
MS-DOS Versions 5.0 and LaterMS-DOS versions 5.0 and later search for files in the following order:
For example, if there are three files in the current directory: FILE.COM, FILE.EXE, and FILE.BAT, and there is also a DOSKEY macro named FILE, the macro is given precedence and executed when FILE is run from the command line of the current directory. This is not true in earlier versions of MS-DOS, as there was no DOSKEY macro feature.
DOSKEY Macro .COM .EXE .BAT
Also, in MS-DOS versions 5.0 and later, DOSKEY macros have precedence over internal commands. To override a DOSKEY macro, type one or more spaces before the command.
EXEC FunctionThe MS-DOS EXEC function (interrupt 21h service 4Bh) does not understand filename extensions; therefore, it simply takes the name of the file specified when invoked and tries to execute it.
This lack of filename information is an advantage to programs that have files that are executable. Such files are device drivers (which normally have the extension .SYS) or overlay files (which normally have the extension .OVL). COMMAND.COM cannot execute these files; only programs that explicitly call the EXEC function can run such programs.
The EXEC function understands only the files that in most cases have .COM and .EXE extensions. It is not able to execute .BAT files; these files are specifically "executed", one line at a time, by COMMAND.COM.
Номер статьи: 35284 — последний просмотр: 9 may 2003 — редакция: 1