CODEWITHSUNDEEP
bashawk
robcore
robcore

Reputation: 3

Explanation of awk function

I am converting some bash-style (actually using busybox) scripts to c for usage in a custom kernel driver. Everything is going fine but I'm dreadfully unfamiliar with awk, and would really appreciate an explanation of what this one liner is doing. The function is here:

checksum=`echo $sum | busybox awk '{$NF *= -1; print}'`

checksum and sum are standard integers that have been accounted for, and can be either positive or negative. I just have no clue what happens when sum is piped into the awk function.

Upvotes: 0

Views: 103

Answers (1)

George Vasiliou
George Vasiliou

Reputation: 6345

This piece of code awk '{$NF *= -1; print}' multiplies the value of the last field $NF by -1 in all the lines and then it prints the whole line with the new value assigned to last field $NF.

This syntax is often called a shorthand assignment and is equivalent to $NF=$NF*-1. Similarilly we have more shorthand operations like addition and subtraction:

$ echo "1 2 3" |awk '{$NF *=10;print}' #Equivalent to $NF=$NF*10
1 2 30
$ echo "1 2 3" |awk '{$NF +=10;print}' #Equivalent to $NF=$NF+10
1 2 13
$ echo "1 2 3" |awk '{$NF -=10;print}' #Equivalent to $NF=$NF-10
1 2 -7
$ echo "1 2 3" |awk '{$NF /=10;print}' #Equivalent to $NF=$NF/10
1 2 0.3

In your case:

$ echo "1 2 3" |awk '{$NF *=-1;print}'
1 2 -3

Mind that in awk, each input line - each record, is by default separated by one or more spaces.
Then each line is split into fields starting from $1 (first field) up to the last field $NF.

$ echo "1 2 3" |awk '{print $1}'
1
$ echo "1 2 3" |awk '{print $2}'
2
$ echo "1 2 3" |awk '{print $3}'
3
$ echo "1 2 3" |awk '{print $NF}'
3

The whole record in awk is called $0:

$ echo "1 2 3" |awk '{print $0}'
1 2 3

A single print, by default prints the whole line $0:

$ echo "1 2 3" |awk '{print}'
1 2 3

Upvotes: 3

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