Is it mandatory to have a variable in awk's first statement?

Question

I am having some hard time with awk today. If I try to do:

df|awk '{print $2; $some=$2; print $some}'

It works as expected and I get the size of the disks twice but if I do:

df|awk '{$some=$2; print $some}'

I just get blank lines. Why is this happening? Something is maybe wrong in my understanding but why the usage of a field mandatory for subsequent fields to work? I also tried doing:

df|awk '{print "hello"; $some=$2; print $some}'

and I got some "hello", each separated by a newline. Where is $some=$2 lost?

My df command outputs:

Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/sda5       38448788 32098732   4396932  88% /
udev             1914564        4   1914560   1% /dev
tmpfs             768744      984    767760   1% /run

Answer 1

You should not use the $-sign in front of some. What happens is that with your first command $2 gets printed and then $some=$2 translates to $0=$2 since some is unitialized (in some awk implementations some gets converted from "" to 0, not all (then you get an error message)), so you are replacing the record ($0) with $2. Then you print $some, which means $0 So it is identical to

df|awk '{print $2; $0=$2; print $0}'

and

df|awk '{$0=$2; print $0}'

What you probably meant to do was:

df|awk '{print $2; some=$2; print some}'

Answer 2

I get the expected result. The first command prints each item twice. The second command prints it once.

This is rather unconventional - "some" will have the value of zero by default since it is undefined. $some is therefore $0, which represents the entire line, so basically you are replacing the line with whatever you find in $2 when you use $some=$2

Compare it with this result:

johan@computer:~$ df|awk '{$some=$2; print}'
1K-blocks
30827524
4041028
1621692
5120
4054228
102400
4054228
60555340
209548784

Why would you not get any output or a different result: I expect the behaviour of using uninitiated variables to be potentially undefined, at least on some systems.

On Solaris I have access to the "standard" awk as well as the supposedly POSIX compliant version installed in /usr/xpg4/bin/awk. Both of these produce the same result for the following

They produce varying results depending on whether I add print "hello" in front or not, and depending on whether I replace the last command with "print" (in stead of print $some) or not.

All of them print SOMETHING, not blanks, though - I have not managed to find a version that reproduces that effect.

So I expect this to be some kind of undefined behaviour due to using an un-initialized variable - you are assigning a value to a column, which is potentially far past the end of the possible range of columns in the input stream. $some refers to a column identified by the value of "some"

You should just use "some" as variable, eg

df | awk '{print "hello";some=$2; print some}'

Answer 3

I think the reason this happened is because Ubuntu does not use the standard GNU implantation but something else known as mawk which breaks a lot of code which worked under standard unix.

Please refer to this link for a similar discussion.