how to pass variable with list of fields to be printed in awk?

Question

I would like to have list of fields, for example '$1, $2', in variable "dc". The variable is passed to awk, but doesn't work as I expected.

$ dc='$1, $2'          # variable with list of fields
$ fff="%-3s %-3s\n"    # format


$ echo "1 2 3 4 5"|awk -v fff="$fff" -v dc="$dc" '{printf fff, $1, $2}'     ### OK
1   2                      

$ echo "1 2 3 4 5"|awk -v fff="$fff" -v dc="$dc" '{printf fff, dc}'         ### NOT OK
  awk: (FILENAME=- FNR=1) fatal: not enough arguments to satisfy format string
          `%-3s %-3s
  '
            ^ ran out for this one

Answer 1

The only way to do what you're asking for without adding a lot of complexity is to jump back into shell to expand dc:

$ echo "1 2 3 4 5"|awk -v fff="$fff" '{printf fff, '"$dc"'}'
1   2

but in 30+ years of UNIX programming and 20+ years of using awk I've never come across a situation where this would be the best approach so if you tell us more about what you're doing we can probably suggest a much better alternative.

Think about this - in a well designed program, you should be able to replace your whole awk script with a C program or something else in future with no change to the interface or calling code. That would clearly be impossible with your current approach so your software is tightly coupled which is highly undesirable.

Here's an example of an alternative:

$ dc='1 2'
$ fff="%-3s"
$ echo "1 2 3 4 5"|awk -v fff="$fff" -vdc="$dc" 'BEGIN{n=split(dc,dcA)} {for (i=1; i<=n; i++) printf fff, $dcA[i] (i<n?FS:RS) }'
1   2

It's probably still not the best approach but maybe it'll give you something to think about.

Answer 2

In awk you cannot store $1, $2, $3 etc in strings variable as you're trying to do. $ here is an operator, which based on the integer value n next to it represents the n-th column from the input.

You can do something like this if you want:

echo "1 2 3 4 5"|awk -v fff="$fff" -v p="1" -v q="2" '{printf fff, $p, $q}'
1   2  

That is rather than pass $1, $2 etc from shell just pass field/column numbers.