One way is to use xcape.

xcape [-d] [-t timeout] [-e map-expression]

This should do the trick:

xcape -e 'Shift_L=parenleft;Shift_R=parenright'

xcape is included as a package for some distributions.

###Notes:

It has a default delay of 500ms after which the mapped key will not be emitted. It can be set by the -t option.

Each map-expression run as a daemon and has it's own PID.

One way is to use xcape.

xcape [-d] [-t timeout] [-e map-expression]

This should do the trick:

xcape -e 'Shift_L=parenleft;Shift_R=parenright'

xcape is included as a package for some distributions.

Notes:

It has a default delay of 500ms after which the mapped key will not be emitted. It can be set by the -t option.

Each map-expression run as a daemon and has it's own PID.