What does "./sage -cleaner" do?

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From the file SAGE_ROOT/local/bin/sage-cleaner (this is the script that gets run when you execute sage -cleaner):

# This is the sage monitor *daemon*, which cleans up after SAGE. 
# Some things that it cleans up:
#   * $HOME/.sage/temp/pid directories
#   * Processes that SAGE spawns.  If a copy of SAGE isn't
#     running, then any process it spawned should have its 
#     process group killed