Question

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
 bar
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

Answer 1

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

$ 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.