How to know if a system command doesn't exist?

[Nicolas Fleury]

Nicolas Fleury wrote:

> Hi,
>     How can I know if a system command doesn't exist?  All the ways I 
> have tried behave like if the system command was existing but returning 
> one.  I don't want to sound provocative, but open in Perl returns an 
> error in a command doesn't exist, so I'm sure we could have an exception 
> in the same case for popen in Python?

Small correction, as far as I can see, Perl's open returns the same 
error if the command returns 1 or doesn't exist.  Same problem there...