Perl's "-a" command-line option allows you to pretend that you're using something that resembles "awk" (that's why it's "-a") -- differences between "perl -a" and "awk" are that perl uses @F where awk uses $1,$2,..., and the first token on the line is $F[0] instead of $1: 
 df -k | perl -lane 'print $F[3]'

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(the "-l" option helps make line-feed handling more like awk as well). I presume this still doesn't do exactly what the OP intended, since it prints a line of output for every line of input from "df -k"; but the OP's use of "grep '/^' " seems odd -- not sure what's intended by that.

To limit the output to a particular line, one can cite the target path as an arg to "df", and/or use a condition in the perl script: 

 df -k / | perl -lane 'print $F[3] if /\d/' # skips column headings

# or just:

 df -k | perl -lane 'print $F[3] if m{/$}'  # only prints value for "/
+"

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In reply to Re^2: disk space utilization by graff
in thread disk space utilization by ministry

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