Ok, you shouldn't put the tag in to the script but you should call the script with it.

{cell-signal-strength?{script: sh signalbar.sh}}

This ties the execution of the script to changes in the property.

The $percent variable in your script has the exact same value as the {cell-signal-strentgh} property at that time, so use that.

P.S.

The image produced by my previous image tag is just for signal strength in bars (0 bars for no signal and 5 bars for max). Just like on the normal status bar.