When would sage -sh not work?

edit

The script at $SAGE_ROOT/local/bin/sage-env sets the shell environment up for building spks's. Here's what it reads at the top:

###########################################################################
#
#  Set environment variables for building and/or running Sage.
#  You must *source* this instead of executing; see below!
#
#  AUTHORS:   William Stein                            2005-12
#             David Kirkby <david.kirkby@onetel.net>   2005-12-17
#
###########################################################################

##########################################################################
#
#  If you want to set all environment variables for your shell like
#  they are during the build of Sage packages, type
#
#             source local/bin/sage-env
#
#  from the SAGE_ROOT directory.   To do the same from a /bin/bash
#  script use ". local/bin/sage-env".
#
##########################################################################

Note that you should run source local/bin/sage-env from $SAGE_ROOT for it to work.

Essentially, sage -sh calls your shell (say /bin/bash) with an option to prevent loading of the user's .profile or rc file (for /bin/bash it runs the shell without sourcing .profile, .bash_profile, .bashrc) to ensure that the paths are correct. If you are using Bash, then running ./sage -sh from $SAGE_ROOT should be equivalent to doing:

> /bin/bash --norc
> source local/bin/sage-env