Shell Programming and Scripting

I have a file sample.txt with the following contents:


the following gives output as

Code:

awk 'NF{s=$0; print s}' sample.txt

but,

Code:

awk 'NF{s=$0}{print s}' sample.txt

gives output as
why this difference, can someone explain me?

To write your first line of code shorter, you can do:

Code:

awk 'NF' infile

There is no need to store anything in a variable; print is not needed in this case since the default action of awk is to print.


For your question what the difference is:

Code:

awk 'NF{s=$0}{print s}' sample.txt

In difference to your 1st line of code, this 2nd line does check for 2 conditions. The 1st is, that if there is a number of fields (NF) greater than zero, it will do the action in the curled brackets. It does this for every line of your input file. It also does this with all other actions and your next action for every line is to print the value of your variable s.
If s is not filled since NF was not true, it still has the former value stored and prints it. So it will give more output than the 1st command line where everything is inside one curled brackets.
With 1st command it will just pass the lines with NF being not true ie. pass the empty lines. In the 2nd command it will print out current value of s even if it hits a new line, since the second curled bracket has no condition in front of it and will always be executed.

I hope it was understandable

But zaxxon There is a need to store the $0 in a variable if the $0 contain spaces in the beginning ...the restoration process will eliminate the heading spaces if any.

Hi Ahmad, I notice no difference if $0 is stored in a variable.

Code:

$ test="      First line\n\nThe  third     line \n"
$ printf "$test"
        First line

The  third     line

Code:

$ printf "$test" | awk NF
        First line
The  third     line

Code:

$ printf "$test" | awk 'NF{s=$0; print s}'
        First line
The  third     line

There is a difference if we do this to remove all whitespace:

Code:

$ printf "$test" | awk '$1=$1'
First line
The third line

The difference:

Code:

awk 'NF{s=$0; print s}' sample.txt

This command prints the variable s if the current line contains one or more fields, the print command is within the same brace (block).

Code:

awk 'NF{s=$0}{print s}' sample.txt

This command prints the variable s even if the current line is blanc (print command in a separate block).
If the current line is blanc it prints the previous line s.

Hope this helps

I'm assuming either IFS has been modified from its usual value or double-quotes around $test were lost while pasting, as that result is not what one would expect from a typical shell environment.

Regards,
alister

Correct! The "" somehow did not make it. I'll correct the post.