Shell Programming and Scripting

Hi,

I have a simple awk script:

Code:

BEGIN{}
{
$a=$2-$1;
print $a
}
END{if(NR==0){
print "0"
}
}

to which I provide the following input

Code:

2.9     14
22.2    27

When I invoke awk with

Code:

awk -f script input.txt

it returns me the following output

Code:

0
0

but if I change the awk script to

Code:

BEGIN{}
{
print $1
print $2
$a=$2-$1;
print $a
}
END{if(NR==0){
print "0"
}
}

I get the expected output, which is

Code:

14
2.9
11.1
27
22.2
4.8

I am not sure what the problem with the initial script is and why the two print statements in the beginning make any difference?

Is it something obvious that I am missing here?
Thanks.

The problem is the use of the variable name $a . Change this to a and all should be fine..

Thanks! Confusion between bash variable and awk..the surprising thing is that the same script works okay in cygwin..

You're welcome. It is not entirely surprising.. If the variable a is not initialized then it is the empty string ("").. So $a becomes $"".

Some awks may interpret it in a numerical context and change it to 0 and so it turns into $0 (which is the record), which would give the proper answer, but it would work differently from what you might expect...

Other awks may not accept $"" and raise an error message

Yet other awks may assign it to $0 (because it is a numerical context), but reference $"" which may return 0, etc...

It is an ambiguous situation and it really depends on the specific awk implementation how this improper use of the variable gets handled..

Hello jaime_123,

You can remove the use of variable here.

Code:

awk 'BEGIN{}
{
print $2 - $1;
}
END{if(NR==0){
print "0"
}
}'  Input_file

Thanks,
R. Singh