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Why does this awk statement work?

 
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# 1  
Old 02-28-2011
Why does this awk statement work?
So, I have an awk statement that does a little filtering and formats the output conveniently. Here's what I had originally:

Code:
<input> | awk -F "\t" 'BEGIN{OFS=","} {sub(" ","_",$2)} (NR == 1) || (substr($2,9,2) >= 19 && substr($2,1,7) == "2011-02") {print}'

That did what I wanted, except that the header line was not properly formatted; specifically, it was still tab-delimited, rather than comma-delimited. I did some hunting about and found a solution I don't understand:

Code:
<input> | awk -F "\t" 'BEGIN{OFS=","} {sub(" ","_",$2)} (NR == 1) || (substr($2,9,2) >= 19 && substr($2,1,7) == "2011-02") {$1 = $1 ; print}'

Why does that $1 = $1 fix the first line? Thanks in advance.
# 2  
Old 02-28-2011
A modification of a field causes awk to recompile the current record and all occurrences of FS get changed to OFS.
Assigning any field to itself will do that without changing the actual data.
# 3  
Old 03-01-2011
Ah, good. Thanks for the reply. That makes sense.
 
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