spawn* or exec* and fork, what should I use and how ?

[Lingyun Yang]

Hi,

   I want to use python as a "shell like" program,
and execute an external program in it( such as mv, cp, tar, gnuplot)
I tried:

os.execv("/bin/bash",("/usr/bin/gnuplot",'-c "gnuplot < plot.tmp"'))

since it's in a for-loop, it should be executed many times, but
It exits after the first time running.

so I have to use spawn* like this:

os.spawnlp(os.P_WAIT, 'gnuplot', 'gnuplot', 'plot.tmp')

It works very well.

My question is,

1. why my exec(..) command doesn't work?
2. exec* must be with fork ?
3. In what situation, we choose one over another ?

Thank you!

regards,
Lingyun