Example of split/join of fields with AWK

awk_split_join.txt

This example shows how $0 and the other $n fields change during the execution
of an AWK script. In a sense this is what using AWK is all about.

Consider the following awk script, with "     1  2  3   4     5" as input.

{
    print_fields();

    $1 = "";
    $3 = "";
    print_fields();

    $0 = $0;
    print_fields();

    $1 = $1;
    print_fields();
}

Suppose print_fields() prints the value of $0 and each of the fields as an
array. The output the script would be this:

$0:     1  2  3   4     5
[1,2,3,4,5]

$0: 2  4 5
[,2,,4,5]

$0: 2  4 5
[2,4,5]

$0:2 4 5
[2,4,5]

Understanding why this output looks like it does explains A LOT about how AWK
works. A couple things to note here:

 - $0 is the _original_ record, which is different to "fields joined by FS".
 - Assigning to a field different to $0 causes $0 to be recompiled. In some
   fake Python syntax this means something like

       $0 = "{FS}".join(fields)

 - Assigning to $0 causes field splitting to be executed. Field splitting in
   AWK repopulates field variables by separating $0 by the current value of FS,
   ignoring consecutive occurences of FS.

       [$1..$NF] = $0.split("{FS}")

This essentially explains why there is no split/join function for fields in
AWK. AWK is actually executing one of these behaviors implicitly, every time
we assign to field variables.

===========
FULL SOURCE

field_test.awk
---------------------------------------
function print_fields()
{
    res = $1;
    for (i=2; i<=NF; i++) {
        res = res ","$i;
    }
    printf("$0:%s\n[%s]\n", $0, res);
    print "";
}
{
    print_fields();

    $1 = "";
    $3 = "";
    print_fields();

    // Force field splitting
    $0 = $0;
    print_fields();

    // Force record compilation
    $1 = $1;
    print_fields();
}
---------------------------------------

Run with:
 $ echo "     1  2  3   4     5" | awk -f field_test.awk