How to pass a string variable in the addRings command?

Hi,

I am trying to pass my metal layer information for the addRing command using some string variables. However, the console prints an error saying variable not found.

I am using the following command:

addRing -nets {VDD VSS} -type core_rings -follow core -layer {top $horzPowerLayer bottom $horzPowerLayer left $vertPowerLayer right $vertPowerLayer} -width {top 4 bottom 4 left 4 right 4 } -spacing {top 1.8 bottom 1.8 left 1.8 right 1.8} -offset {top 1.8 bottom 1.8 left 1.8 right 1.8} -center 1 -threshold 0 -jog_distance 0 -snap_wire_center_to_grid None

Following is the log that I see on the terminal:
#% Begin addRing (date=02/04 02:06:12, mem=820.9M)
#% End addRing (date=02/04 02:06:12, total cpu=0:00:00.0, real=0:00:00.0, peak res=820.9M, current mem=820.9M)
can't read "horzPowerLayer": no such variable

The variables $horzPowerLayer and $vertPowerLayer are M7 and M6 in values respectively which are read through a file. The values hold well before the addRing command is executed but they are not recognized in the command itself. Can someone please let me know the right way to pass this information? I plan to do the same for the dimensions as well so it would be helpful if you could include that in your discussion as well.

Thank You
Vinay