symbolic links, aliases, cls clear

[Floyd L. Davidson]

"mp" <mailpitches at email.com> wrote:
>i have a python program which attempts to call 'cls' but fails:
>
>sh: line 1: cls: command not found

Hmmmmm...  (I don't program in Python, so precisely what is
happening isn't something I'm sure about).

But, note how that line starts with "sh:"!  That indicates it is
/bin/sh which is reporting an inability to find a "cls"
command.  It suggests that Python (like most other programming
languages) calls shell scripts using /bin/sh as the default
shell.

The problem is that unix has no command named "cls".

>i tried creating an alias from cls to clear in .profile, .cshrc, and
>/etc/profile, but none of these options seem to work.

In /etc/profile and ~/.profile you will cause an alias to be
defined *for* *interactive* *login* *shells*.  But not for
non-interactive non-login shells, which is what you are invoking
with Python.  The reason is because aliases are not inherited by
sub-shells, and only interactive login shells read those two
files.

Hence your alias is never defined in the subshell your program
executes.

>my conclusion is that a python program that is executing does not use
>the shell (because it does not recognize shell aliases). is this
>correct?

No.

>should i use a symbolic link? if so, where should i place it?

No.

>what is the difference between aliases and symbolic links?

Aliases are a mechanism used by a shell to define a command name
that executes a series of commands.  A symbolic link is a
directory entry that de-references another directory entry, so
that either entry points to the same actual file.

>if i execute a command like 'clear' to clear the screen, where does the
>shell look to find the command 'clear'?

It looks in locations specified by the PATH variable.  By
default that will be a minimal list defined by the login
program, but it might be significantly added to in the
/etc/profile or other shell init scripts.

The question you need to answer first is what happens if your
Python program tries to execute /clear/ rather than /cls/.  If
that works, then your PATH variable is set correctly.  If it
doesn't work, verify that there is in fact a program named
/clear/ that can be run from a shell command line.  Then figure
out how to set an appropriate PATH variable for your Python
program.

Note that if /clear/ does work, but you want this script to use
/cls/ so that it is portable to some silly OS where a /cls/
exists...  You can define a shell function (which will be
inherited by sub-shells) to look like this,

  function cls () {
     clear;
  }

And place it where ever is appropriate (/etc/profile is one place).

>i'm using os x.

I don't know anything about it... :-)

-- 
Floyd L. Davidson            <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)                         floyd at apaflo.com