snap

#!/bin/bash

# $ wmctrl -d
# 0  * DG: 1920x1200  VP: 0,0  WA: 0,31 1920x1169  Workspace 1
# 1  - DG: 1920x1200  VP: N/A  WA: 0,31 1920x1169  Workspace 2

xorigin=0
yorigin=0
width=640
height=480
while read desktop current dg resolution vp vporigin wa waorigin waresolution name; do
  if test "$current" = "*"; then
    IFS='x' read width height <<<"$waresolution"
    IFS=',' read xorigin yorigin <<<"$waorigin"
    break
  fi
done <<<"$(wmctrl -d)"
echo "workarea is @${width}x${height}+$xorigin+$yorigin"
half=$((width/2))
margin=8    # allow for window borders

case $1 in
left)
    coords=0,$xorigin,$yorigin,$((half-margin)),-1
    ;;
right)
    coords=0,$((xorigin+half)),$yorigin,$((half-margin)),-1
    ;;
*)
    echo "Usage: snap {left|right}" 1>&2
    exit 1
    ;;
esac
wmctrl -r :ACTIVE: -b remove,maximized_horz
wmctrl -r :ACTIVE: -b add,maximized_vert
wmctrl -r :ACTIVE: -e $coords