Use awk for values stored in a variable?

I want to do an if statement that compares the values of a command that reads header information of two image files. First I pass output of the command to the variables

~$ hd1=$(<command> <file1> | grep dim3) ~$ hd2=$(<command> <file2> | grep dim3) ~$ if [ $hd1 = $hd2 ]; ~$ then etc 

The output of my command has alot of fields

~$ <command> <file> ~$ sizeof_hdr 348 data_type INT16 dim0 3 dim1 256 dim2 256 dim3 70 dim4 1 dim5 1 dim6 1 dim7 1 vox_units mm time_units s datatype 4 nbyper 2 bitpix 16 pixdim0 0.000000 pixdim1 0.828125 pixdim2 0.828125 pixdim3 2.199998 pixdim4 4.177372 pixdim5 0.000000 pixdim6 0.000000 pixdim7 0.000000 vox_offset 352 file_type NIFTI-1+ 

So I use grep to get the one that I care about here. The problem is the output of my command includes a "dim3" field and a "pixdim3" field, and using grep prints both, like this:

~$ dim3 70 pixdim3 2.19 

I really just need compare the second column for both files. I tried using awk, but it wouldn't work because they are variables and not files. Is there anyway to print just the second column, or better yet only grep the dim3 field?