In /usr/bin/slideshow line 5:
bindir="/usr/bin"
^----^ SC2034: bindir appears unused. Verify use (or export if used externally).


In /usr/bin/slideshow line 76:
   if [ "${1+n}" != 'n' ] ; then echo $0: missing argument for standard X flag $oneargflag ; exit 1 ; fi
                                      ^-- SC2086: Double quote to prevent globbing and word splitting.
                                                                               ^---------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
   if [ "${1+n}" != 'n' ] ; then echo "$0": missing argument for standard X flag "$oneargflag" ; exit 1 ; fi

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- bindir appears unused. Verify use...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...