Shell Programming and Scripting

Hi Franklin52 & Alister,

We would sort the file by key field first.

Alister,

Could you please explain your code a little bit?

Thanks

It's Franklin52's code, with a minor tweak, but I'll explain it

Quick primer, in case it's needed. AWK scripts consist of a series of pattern-action pairs. For each input line, awk will evaluate the pattern-action pairs in the order they occur in the script. The action (the code within curly braces) will only execute for a given input line if its pattern expression evalutes to a boolean true value. Either the pattern or the action may be absent, but not both. An absent pattern defaults to true and therefore an action without a pattern will execute for every line read. An absent action defaults to printing the current line, so a pattern without an action will print out the current line, $0, if the pattern evaluates to true.

Now, on to this particular script.

The first field of each input line is treated as a key value. The active key, the key for which values are being collected at any given time is stored in k.

i is a counter that keeps track of how many values we've collected for the active key.

s is a string containing the current key followed by its value (or two values delimited with a comma).

Given the way this code works, it makes sense to start with the last pattern-action pair before the END pattern-action pair.

Code:

{s=$0;k=$1;i=1}

This pattern-action pair is missing it's pattern, so it will match every line by default. This is the first action to execute. When the script first starts, the previous pattern-action pairs do not match because the values of k and s are still unset. This line initializes (and later, whenever there is a key change, reinitializes) s to the current line; the active key, k, to the first field of the line, $1; and the counter, i, that tracks how many values have been collected in s.

Code:

$1==k {if (++i<3) s=s "," $2; next}

If the current line's key ($1) is equal to the active key, k, and if less than three values (including the current line's value) have been seen for this key, then we want to append its value ($2) to the string s. Regardless of how many values have been seen for this key, skip the rest of the pattern-action pairs (the 'next' statement), read in the next line from the input, and continue repeating this first step as long as the new input line's key is equal to the active key.

Code:

s{print s}

If we reached this point in the awk script, it's because the current line's key is not the same as the active key. Time to print the old key and its values. The next step will be to execute the pattern-action pair with which I began this explanation (to reinitialize all of the script's variables).

Code:

END{print s}

All the input has been read, so print the key which was active when we reached the end of the file.

I hope that helped,
Alister

Hi alister,

Your right, I misread the question. Thanks for the fix and explanation.

Regards

Hi Alister & Franklin52,

Thanks so much to you both for the code and detailed explanation. Appreciate your help!