Q59400: CHDIR & SHELL "CHDIR" in OS/2 Protected Mode Differs from DOS

Article: Q59400
Product(s): See article
Version(s): 6.00 6.00b 7.00
Operating System(s): OS/2
Keyword(s): ENDUSER | SR# S900226-124 | mspl13_basic
Last Modified: 9-MAR-1990

In OS/2 protected mode, if a BASIC program issues a CHDIR, the command
will affect only the current BASIC process; other processes in the
system will not be affected. However, this behavior changes when a
SHELL statement is issued because a SHELL statement executes a copy of
CMD.EXE. The SHELLed program is considered to be a "child process" of
the program that issued the SHELL. If the SHELLed program executes
OS/2's CD or CHDIR command, only the SHELLed process will be affected,
not the currently running BASIC program that issued the SHELL. This is
not a problem with BASIC under OS/2; it is correct behavior that is
dictated by the design of OS/2.

This information applies to Microsoft BASIC Compiler Versions 6.00 and
6.00b for OS/2 and to Microsoft BASIC Professional Development System
(PDS) Version 7.00 for OS/2.

The OS/2 API function, DosChDir(), has the same effect as the BASIC
CHDIR statement. For both of these, the currently executing BASIC
process is changed, but the parent process (which started the BASIC
process) is unchanged. Similar to CHDIR, the ENVIRON statement (and
other statements that modify the OS/2 protected-mode environment) has
no effect on the parent process.

This is a feature of the protected mode of OS/2; child processes
cannot affect the parent process's environment. This represents a
notable change from DOS. In DOS, there are "programs," but not
"processes" as in OS/2. In DOS, you cannot have multiple programs
running simultaneously. There is only one program running and
therefore it is reasonable to assume that any change directory command
will change the current DOS directory for both the parent and child in
a program. Changing the "current directory" under DOS changes it for
the entire operating system -- the change is global. This means the
change also takes effect for any programs run later unless the user or
program specifically changes the current directory. In OS/2 protected
mode where there can be many programs running at once, having one
global working directory that could be changed by any process at any
moment would not be reasonable.

Code Example
------------

In OS/2 protected mode, the following code example has no effect on
the screen group that starts the EXE file, but the FILES statement
illustrates that the directory has been changed for the current
process:

a$ = "\bc7"
CHDIR a$            'This only affects the current BASIC process
                    'API Function DosChDir has same effect as CHDIR
'SHELL "cd "+a$     'This only affects the SHELLed process. After
FILES "*.*"         'returning from the SHELL, it has no effect.

In DOS, either the CHDIR statement or the commented SHELL statement in
the above program will change the directory for the DOS command line.

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