A number of environment variables available in a tool session.  A few are discussed here.  A full list can be viewed by running the "env" command from a terminal in the workspace tool.

*Note: tools are invoked by the user's account and permissions set accordingly.  A tool can save files to a user's home directory because the tool runs as that user.

This variable stores the session ID or session number that is currently active.  It's the ID of the session you are currently using.

This variable stores the current username of the user running the tool.

This variable stores the current session directory of the open tool.  This session directory is a separate directory create for each new tool session that the tool can write to.  This is a good place for temporary files generated by your tool.

*This is the default path for a tool on invoke.

This variable stores the results directory located in the user's home directory. This is a good place to place simulation results for the user to access later. 

*Be mindful of the user's quota limits.

This variable stores the present working directory.

This variable stores the path of the user's home directory.  This is useful if a tool provided an option to save the user's current work. Please create a directory for the tool to save files here, to prevent cluttering the user's home directory too much.   A best practice would be to create a new directory for a tool in the user's data directory.  For example, "$HOME/data/toolname".