Shell Programming and Scripting

Originally Posted by Scrutinizer

A[$1] = $1 is used to set up the array of patterns. It is set when NR is equal to FNR, in other words when the first file is read i.e. the pattern file. It means fill the associative array "$1" to the value of "$1", $1 being the first field of your pattern file So with the OP's provided inputs it gets filled like so:

Code:

A[cd003]=cd003
A[cd005]=cd005
A[cd007]=cd007
A[cd008]=cd008

So once the pattern file is done it starts reading the input file and it will print each line if field 3 exist as a key in the array.

The script is only testing the existence of the array elements, not using its contents. So IMO the use of $3 is a tiny bit superfluous. I think we could also just set it to 1 instead of $1:

Code:

awk 'NR == FNR { A[$1]=1; next } A[$3]' pattern_file input_file

or, since pattern file does not have a third column.

Code:

awk 'NR == FNR { A[$1]=1 } A[$3]' pattern_file input_file

Originally Posted by jp2542a

As Scrutinizer says, what is assigned to A[$1] is unimportant. It just has to be a value. Why are you changing the A[$1] assignment? what are you trying to do ?

BTW, then 'next' statement has a good side effect... it prevents the A[$3] clause from being executed. If a novice decides to modify the script, it will prevent some undesired behavior. And awk doesn't have to do useless work processing a clause that isn't useful for the pattern file....

Originally Posted by jp2542a

As Scrutinizer says, what is assigned to A[$1] is unimportant. It just has to be a value. [...]

Originally Posted by radoulov

$1 in A

Originally Posted by Scrutinizer

Hi Radoulov, you mean $3 in A, no?

Originally Posted by jp2542a

BTW, then 'next' statement has a good side effect... it prevents the A[$3] clause from being executed. If a novice decides to modify the script, it will prevent some undesired behavior. And awk doesn't have to do useless work processing a clause that isn't useful for the pattern file....