I am trying to use su to execute a command as another user during
boot-up (it doesn't matter, but in this case I want to start the
postgreSQL postmaster as user postgres).

The best solution is to put a command into /etc/rc.local like the
following:

    su -l postgres -c 'run_postmaster_command'

This uniformly yields a segmentation fault.

I have traced the problem to the su command in the sense that it
doesn't matter what login I su to or what command is executed by su
(e.g., running pwd does the same thing), there is always a
segmentation fault.  This still occurs under a wide variety of
conditions: 1) if the command is run from /etc/rc.local, or 2) if the
su command is within a script run via at.  However, running the same
su command (or a script containing the same one) from a terminal after
logging in works fine.

For example, the following script works fine if run from a terminal:

     #!/bin/sh

     echo "testing su"
     pwd
     su -l postgres -c 'pwd'
     su -l brook -c 'pwd'
     pwd

But it yields the following if run via at:

     testing su
     /usr/local/pgsql/bin
     Segmentation fault
     Segmentation fault
     /usr/local/pgsql/bin

What is going on here?  How can I fix it to allow su from within
scripts run, say, from /etc/rc.local?

Cheers,
Brook