In /usr/bin/xvfb-run line 74:
        i=$(($i + 1))
             ^-- SC2004: $/${} is unnecessary on arithmetic variables.


In /usr/bin/xvfb-run line 158:
    tries=$(( $tries - 1 ))
              ^----^ SC2004: $/${} is unnecessary on arithmetic variables.


In /usr/bin/xvfb-run line 165:
    (trap '' USR1; exec Xvfb ":$SERVERNUM" $XVFBARGS $LISTENTCP -auth $AUTHFILE >>"$ERRORFILE" 2>&1) &
                                           ^-------^ SC2086: Double quote to prevent globbing and word splitting.
                                                     ^--------^ SC2086: Double quote to prevent globbing and word splitting.
                                                                      ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
    (trap '' USR1; exec Xvfb ":$SERVERNUM" "$XVFBARGS" "$LISTENTCP" -auth "$AUTHFILE" >>"$ERRORFILE" 2>&1) &

For more information:
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2004 -- $/${} is unnecessary on arithmeti...