I'll describe here how to automatically capture images with Canon A75 using gphoto2 software.

After connecting camera first step is to enable capture mode:

gphoto2 --set-config /main/settings/capture=1

Next capture image and download it:

gphoto2 \
--set-config /main/capturesettings/shootingmode=Manual \
--set-config /main/capturesettings/shutterspeed=1/50 \
--set-config /main/capturesettings/aperture=4.5 \
--capture-image-and-download

To see list of available configuration settings execute:

gphoto2 --list-config

To see possible values of certain setting:

gphoto2 --get-config /main/capturesettings/shutterspeed

To use a GUI to set configuration:

gphoto2 --config