awk:How return number of columns by awk?

Question

Data source:

a b c d

Expect:

$1 $2 $3 $4
a  b  c  d

I try to echo 'abc d'| awk 'END {for(i=1;i<=NF;i++) printf("$%d\\t",i)}' echo 'abc d'| awk 'END {for(i=1;i<=NF;i++) printf("$%d\\t",i)}' (but fail)

Answer 1

$ echo 'a b c d' |
    awk 'FNR==1{for (i=1;i<=NF;i++) printf "$%d%s", i, (i<NF ? OFS : ORS)} 1'
$1 $2 $3 $4
a b c d

$ echo 'a b c d' |
    awk 'FNR==1{for (i=1;i<=NF;i++) printf "$%d%s", i, (i<NF ? OFS : ORS)} 1' |
    column -t
$1  $2  $3  $4
a   b   c   d

Read Effective Awk Programming, 4th Edition, by Arnold Robbins - trying to do this in the END section and other things in your script indicate you are missing the fundamentals of awk.