Substitute ratio when using butter instead of shortening
1/2 cup of shortening is 110 g, while 1/2 cup of butter is 114 g. Also, butter is only 82% fat. So if you want to be precise, use 134 g of butter: (100/82)*110 = 134. You may want to reduce the liquid by 24 g in this case. 

If you don't care for precision, you can also substitute 1:1 and use 1/2 cup of butter. Most recipes for quickbreads are flexible enough for that, and you are changing the texture anyway by substituting a fat with a different melting profile.