How to change the working directory for a shell script (newbie here)

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http://unix.stackexchange.com – I have a python script that looks files up in a relative directory. For example: the python script is in /home/username/projectname/. I have a file that is being called within the python script that is in /home/username/projectname/subfolder. If I run the script from the shell as python scriptname.py it runs perfectly fine. However, i'm trying to run the script as a startup service. I'm setting it up in webmin, and I believe its creating a shell script to call it. Through the startup script, I'm doing something like this to call the script: execute python home/username/projectname/scrip (HowTos)