How to understand the command awk '{for(x=1;$x;++x)print $x}'?

Question

I know this command

awk '{for(x=1;$x;++x)print $x}' 

will print out all columns in a line.

wouldn't this ++x change x to 2, and thus print $2 first? As I understood based on this: https://stackoverflow.com/questions/1812990/incrementing-in-c-when-to-use-x-or-x

And what does the $x do in for(x=1;$x;++x) ?

Answer 1

No. The for(i=0;i<10;i++) is a classic programming construct (see Traditional for loops) that is present in many languages. It can be broken down to:

start-expression; end-condition; end-of-iteration-expression

In other words, what I wrote above means "initialize i to 0 and, while i is less than 10, do something and then increment i by 1. Yes the syntax is confusing but that's just the way it is. The end-of-iteration-expression (++x in this case) is executed once at the end of each loop. It is equivalent to writing:

while(i<10){print i; ++i}

As for the $x, I believe that just checks that a field of that number exists and that its contents do not evaluate to false (as explained in Mathias's answer below). $N will return true if the field number N exists and is not a type of false. For example:

$ echo "a b c d" | awk '($4){print "yes"}'
yes
$ echo "a b c d" | awk '($14){print "yes"}' ## prints nothing, no $14
$ echo "a b c 0" | awk '($4){print "yes"}' ## prints nothing, $4 is 0

As you can see above, the first command prints yes because there is a $4. Since there is no $14, the second prints nothing. So, to get back to your original example:

awk '{for(x=1;$x;x++)print $x}' 
          ___ __ ___
           |   |  |
           |   |  |-----> increment x by 1 at the end of each loop.
           |   |--------> run the loop as long as there is a field number x
           |------------> initialize x to 1

Answer 2

Since terdon provided a comprehensive answer I just want to add that if any column evaluates to false, the for statement ends the loop, as you can see in this example:

$ echo 1 2 3 4 5 0 6|awk '{for(x=1;$x;++x)print $x}' 
1
2
3
4
5

Answer 3

1. ++x and x++ are functionally equivalent when used stand-alone.  As discussed in the Stack Overflow question that you referenced,

   • There may be performance (i.e., timing) differences.
   • The results of something = ++x; and something = x++; are different – but your example isn’t doing that.

   So, as far as the incrementing of x is concerned, your example is equivalent to

   awk '{for(x=1;$x;x++) print $x}'
   

2. The standalone $x is equivalent to $x != "", so the loop will iterate until it encounters a blank field.  This is a lazy shortcut for x <= NF, where NF is the number of fields in the current record (line).  For the purposes of your example, this is harmless, AFAICT.  But, if you specify a non-default field separator,

   awk -F, '{for(x=1;$x;++x) print $x}'
   

   this will try to do the same as your example, but splitting lines at commas.  If you type a b, it will output a b.  If you type a,b, it will print a and b on separate lines.  But if you input a,,b, it will output a and then stop, because $2 is null.