Shell Programming and Scripting

Hi,

In a previous, now closed thread, I found the following awk script:

Code:

awk '{t[$1" "$2" "$3" "$4]=$5" "$6" "$7}END{for (i in t){print i,t[i]}}'

This code does a great job of removing duplicates by the the first four fields from a 7-field set of columns. I would very very much like to understand how this code works, but can't find anything in the awk documentation. Could someone explain it please? Is the t[ ]= some special function?

Thanks,
Pawel

The big { } code block without the END gets run for every line. The values of $1, $2, $3, etc are set to the values for the columns read. Putting variables and strings in a row sticks them together into a longer string.

So every line, it sets a value like this in the array:

Code:

T["a b c"]="d e f"

If there's a duplicate line, setting the same value in the array twice doesn't put two elements in the array. The previous contents of T["a b c"], for instance, would just get overwritten given another line starting with a b c

Once all the lines have been read, only then will awk run the END {} block, which goes through each thing in the array and prints them (in no particular order).
The syntax for(X in ARRAY) loops through every element in an array, with X being the array index, and ARRAY[X] being the contents of that index.

t[] is an associative array. In plainer terms, the index ( t[ index goes in here ] ) can be any characters or groups of fields. A lot of other languages use an integer to reference array elements. awk can use numbers but most times it is character strings

Thanks. That cleared it up!