how to get the available space left deterministically?

Question

When I use the cut command, everytime I get a different result which makes sense. So I wonder how to get the answer similar to this but consistent across different Ubuntu nodes?

ubuntu@ip:~/aws_script$ df -h / | grep '/dev'
/dev/xvda1       32G   21G  9.8G  68% /
ubuntu@ip:~/aws_script$ df -h / | grep '/dev' | cut -d' ' -f13
9.8G

I really don't care what command you would use if not cut as long as it gets the job done deterministically.

Answer 1

You want second line, fourth column. So use awk for that:

$ df -h / | awk 'NR==2{print $4}'                                                                                                     
41G

Answer 2

To get the available space on / with df, use:

df --output=avail -h / | grep -v ^Avail

How it works

--output=avail tells df to show just the available space.

grep -v ^Avail removes the header line.

Answer 3

awk is more powerfull than cut

df -h / |awk '/dev/{print $4}'

Modified as requested by Serg