Print columns using string variable in AWK

Question

I am trying to use one variable in my AWK (or GAWK) program to print multiple columns.

I am taking the columns to print from the command line:

gawk -v cols=1,2,3 -f sample.awk -F,

I want to be able to set this variable in my BEGIN{} block, and use it in the main part of my program.

BEGIN{
  split(cols, col_arr, FS)

  i=1;
  col_str = "$"col_arr[1];
  for(col in col_arr){
    if (i > 1){ 
      col_str = col_str",$"col;
    }
    i++;
  } 
}

{
  print col_str
}

However, this will just print "$1,$2,$3". How can I change this to print columns 1, 2, and 3?

Answer 1

A BEGIN rule is executed once only, before the first input record is read.

Try something like this

awk '{cols = $1 OFS $2 OFS $5; print cols}' file

Update

Either you have to generate script like how Jonathan Leffler showed since unlike the shell (and PERL) AWK does not evaluate variables within strings, or something like this

BEGIN{
       sub(/,$/,"",cols)
       n=split(cols,C,/,/)
}
function _get_cols(i,s){
       for(i=1;i<=n;i++) s = length(s) ? s OFS $(C[i]) : $(C[i])
       return s  
}
{
     print _get_cols()
}

Execute

awk -v cols=2,3, -f test.awk infile

OR Else something like this you have to try

#!/bin/bash

# Usage : _parse <FS> <OFS> 1 2 3 ... n < file
_parse()
{
    local fs="$1"
    local ofs="$2"
    shift 2
    local _s=
    local f

    for f; do
        _s="${_s}\$${f},"
    done
    awk -F"$fs" -v OFS="$ofs" "{ print ${_s%,} }"
}

# Call function
_parse ' ' '\t' 1 3 < infile