symbolic links, aliases, cls clear

[Michael Paoli]

mp wrote:
> i have a python program which attempts to call 'cls' but fails:
> sh: line 1: cls: command not found
> i'm using os x.

[Note Followup-to: severely trimmed]

I'd guestimate (those more familiar with python can probably fill in
more relevant python specific details) that the python program is
probably doing something like:
system("cls")
... or whatever the python-specific way of writing something like that
is.  Most likely that was written, intended for some Microsoft
DOS/Windows/XP or similar type of operating system - where CLS is a
legitimate command (to clear the screen).  On UNIX (and probably also
OS X), there is no "cls" as a standard command, but there is the quite
standard command "clear", to clear the "terminal" screen.

In the land of UNIX, most languages implement the system() function or
equivalent, by typically fork(2)ing and exec(3)ing (at least one of the
exec(3) family of calls), generally invoking "the" or a default shell
(typically /bin/sh) with a first option argument of "-c", and then the
argument to system passed as one single string as the other argument to
the shell.  It would seem likely that python was "smart enough" (of
course) to know on UNIX to implement system() as sh -c ..., rather than
something like COMMAND /C ... or CMD /C ... as it would likely do on
DOS/Windows/XP or similar.  But "of course" python probably has no clue
what the cls is that's handed to it with system(), and likely just
blindly passes it on to the shell.  The diagnostic you got would also
seem to imply that's what happened (the shell (sh) couldn't find the
cls command). ... as a matter of fact, if I try that on Debian
GNU/Linux 3.1 (technically not "UNIX", but neither is OS X, but for
practical purposes they're both quite sufficiently close), I get
results that would appear exceedingly consistent with the hypothesis I
put forth:
$ sh -c cls
sh: line 1: cls: command not found

If it's desired to have the python program function as close to its
(apparent) original intent as feasible, it may be desirable to:
have it test the operating system, and if it is UNIX or similar, use
clear, instead of cls ... or if one wants to port/adapt it to UNIX
(and OS X, etc.), with no need or intention to move it back and forth
or among significantly different operating systems, then perhaps
consider simply replacing the system(cls) with system(clear), or
whatever the precise suitable change in the python code would be.
It would probably also be worth inspecting the code for other
occurrences of system() that may also need to be adjusted or changed.

Note also that some languages (e.g. Perl) will potentially take
shortcuts with the system() function.  For example, with Perl
(paraphrasing and perhaps over-simplifying a bit) if Perl sees no
need or reason to have to use the overhead of the shell to invoke the
system() function, it will just quite directly (after the fork(2))
exec(3) the command, setting the argument(s) suitably.  Python may
(or may not) try similar shortcuts.  For example, CLS, on DOS, etc.,
is internal to the "shell" (command interpreter), so, if python
didn't find an external CLS command, it would have to pass it to the
"shell", hoping the shell would know what to do with it.  That would
happen to work with DOS, but would generally fail on UNIX (where cls
would generally not exist as a command, and wouldn't be built-in to
the shell).