Your bash history file is in your home directory:
Zeitgeist user history is stored in
To reset the Zeitgeist history, open a terminal and copy/paste the following commands:
Or navigate to that directory in Nautilus or your file manager and manually delete the sqlite database "activity.sqlite". It will be created again by Zeitgeist minus your previous user history.
I do not feel comfortable telling users to use some commands unless they understand the command completely and are careful to double check that the command is written correctly, especially any command beginning with rm since a user could easily do irreparable damage to their system if the command was incorrectly formatted, but to better answer the OP's question as suggested in comments, the system logs are stored in
But I have heard of people causing problems by deleting subdirectories inside the log directory
as is mentioned here Can I delete /var/log files due to low root space?
But keep in mind that if you need to troubleshoot some system problem the logs will be very useful.
You can delete the contents of the journal folder by opening a terminal and copy and paste the following command:
sudo rm /var/log/journal/*
To see how much space it is using on your computer you can open a terminal and issue the query:
My system is currently is using 3.9GB.