How to exclude results from grep

  Sometimes you may wish to further filter grep output, such as in the following situation:  
# zfs get all | grep compressratio backup01         compressratio         1.23x                  – backup01         refcompressratio      1.00x                  – backup01/data    compressratio         1.50x                  – backup01/data    refcompressratio      1.50x                  – backup01/photos  compressratio         1.05x                  – backup01/photos  refcompressratio      1.05x                  –
Here we only really want to see the compressratio results, not the refcompressratio. We can pipe the grep output to another grep command, this time inverting the results with the -v switch.  
# zfs get all | grep compressratio | grep -v refcompressratio backup01         compressratio         1.21x                  – backup01/data    compressratio         1.50x                  – backup01/photos  compressratio         1.05x                  –
This excludes any line containing refcompressratio, making the results easier to read. This is particularly helpful when you have a large number of results from grep.