The difference between awk $1 and $NF? - linux

Question

case 1:
echo 'ABC-dev-test.zip' | awk -F'^ABC' {print $1}
output : null
case 2:
echo 'ABC-dev-test.zip' | awk -F'^ABC' {print $NF}
output : -dev-test.zip
I wonder. Why it comes out like this. $1 is the first record, $NF is the number of records. In the end they both point to the first, but I think running it should give you the same value.
Why is it different?

Answer 1

You are getting 1st results empty because you don't have anything before ABC in your input(and its going to be always empty since we are making specifically starting of the line ABC as field separator NOT any ABC in line), when you are making it field separator then it means things coming before it will be considered as 1st field($1) which is NOT there, hence your first command is not printing anything.
Let's run following command to see how many fields we have with shown samples and what are their respective values:
echo 'ABC-dev-test.zip' | awk -F'^ABC' '{for(i=1;i<=NF;i++){print "Field Number:"i " Field value is:" $i}}'
Field Number:1 Field value is:
Field Number:2 Field value is:-dev-test.zip
You could clearly see that 1st field is empty after making ABC as field separator with your shown samples, while $NF(which means last field of current line) works because we have -dev-test.zip after ABC in your shown samples.
Additional note: Looks like you are making ABC which is starting from line, in case you want to make ABC as field separator then if you have like: XYZ-ABC-dev-test.zipABC you will get XYZ- as 1st field value here.
Let's test this for string ABC-dev-test.zipABC-resvalues where we have 2 ABC values in it.
When we run it with making ^ABC as field separator see this: First field is empty and moreover 2nd ABC is not getting caught as a field separator here.
echo 'ABC-dev-test.zipABC-resvalues' | awk -F'^ABC' '{for(i=1;i<=NF;i++){print "Field Number:"i " Field value is:" $i}}'
Field Number:1 Field value is:
Field Number:2 Field value is:-dev-test.zipABC-resvalues
When we change field separator to ABC then see this: Its catching all ABC occurrences in whole value and treating them as a field separator.
echo 'ABC-dev-test.zipABC-resvalues' | awk -F'ABC' '{for(i=1;i<=NF;i++){print "Field Number:"i " Field value is:" $i}}'
Field Number:1 Field value is:
Field Number:2 Field value is:-dev-test.zip
Field Number:3 Field value is:-resvalues