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I have lines of data that contain single column and two columns. What I want to do is to extract lines that contain only 2 columns.

0333 foo
23243 qux

yielding only:

0333 foo
23243 qux

Note that they are tab separated, even for lines with only one column you have tab at the beginning.

What's the way to do it?

I tried this but fail:

awk '$1!="";{print $1 "\t" $2}' myfile.txt

enter code here
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3 Answers

up vote 5 down vote accepted

You need to use the NF (number of fields) variable to control the actions, such as in the following transcript:

pax> echo '0333 foo
>  bar
> 23243 qux' | awk 'NF==2{print}{}'
0333 foo
23243 qux

This will print the line if the number of fields is two, otherwise it will do nothing. The reason I have the (seemingly) strange construct NF==2{print}{} is because some implementations of awk will print by default if no rules are matched for a line. The empty command {} guarantees that this will not happen.

If you're lucky enough to have one of those that doesn't do this, you can get away with:

awk 'NF==2'

but the first solution above will work in both cases.

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Why not awk 'NF==2{print}{}{}{}{}{}{}' ?? NF==2 suffices. –  Mark Edgar Aug 3 '10 at 8:54
@Mark, some implementations of awk will print by default if you don't specify a default action. My code works on those ones as well. You bods are spoilt with your GNU awk, some of us have to write portable code :-) I'll clarify. –  paxdiablo Aug 3 '10 at 9:13
@ pax, An awk implementation which behaves the way you describe would be pretty broken. As far as I know, awk dates to v7, and the manual there pretty clearly describes its behavior: "for example, the program length > 72 prints all input lines whose length exceeds 72 characters." –  Mark Edgar Apr 26 '12 at 16:24
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awk 'NF==2' file
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awk '(NF==2){print}' test.txt
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