Why is TAB-completion in shell not working for some commands?

[Marius Hofert]

Hi,

I have a strange problem with OS: Mac OS X 10.7.2 and Emacs: GNU Emacs 
24.0.91.1.

If I open a shell with M-x shell, I can TAB-complete several commands, such as 
ps2pdf and many others. However, I can't seem to TAB-complete all commands. One 
exception is a2ps. I installed it, it works perfectly fine, I can TAB-complete 
it from the Mac's terminal, however, I can't TAB-complete it from the shell 
opened with M-x shell. The simple question is: Why?

Note that if I type "echo $PATH", I get:
a) from the Mac's terminal: 
/Applications/Emacs.app/Contents/MacOS/:/Applications/Emacs.app/Contents/MacOS/bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/usr/texbin:/opt/local/bin:/opt/local/sbin
b) from the emacs shell: 
/Applications/Emacs.app/Contents/MacOS/:/Applications/Emacs.app/Contents/MacOS/bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/usr/texbin:/opt/local/bin:/opt/local/sbin
=> exactly the same. 

If I type "which a2ps" from the emacs shell, I get: /opt/local/bin/a2ps so the 
correct place is found, but why is there no TAB-completion?

Cheers,

Marius