CODEWITHSUNDEEP
awkgetlinegawk
ysap
ysap

Reputation: 8115

How to "roll back" a getline call?

If I use getline in my awk script, it reads the next line from the stream and updates the $0 and NR variables (as it should). Is there a way to un-getline?

For example, I want to use getline to determine the EOF and then take an action on that. But if the condition is false (i.e., not EOF), then the script should continue normally.

#!/bin/gawk -f

{
    print $0;

    if (getline == 0)
    {
        print "EOF";
    } else {
        ungetline;
    }
}

Without the ungetline, the above script will only print every other input line.

One can wrap the whole script body with a while() construct, but for this single use, it is just cleaner to undo the operation.

Upvotes: 4

Views: 1395

Answers (1)

karakfa
karakfa

Reputation: 67507

Not sure that's the problem you're solving but for example, to print a line before the last line of the input you can follow "delaying printing" method. Here it's just printing the line but any processing can be similarly added.

$ seq 5 | awk 'NR>1 {print line} 
                    {line=$0} 
               END  {print "and finally..."; print line}'

1
2
3
4
and finally...
5

based on your updated posting, perhaps your use case is just the simulate the END block.

$ seq 5 | awk '{print $1*$1} 
           END {print "EOF"}'

1
4
9
16
25
EOF

either you do something before the last line or after the last time; both cases are covered in these scripts. You can have a combination of these two as well. I'm still not sure about your use case...

Upvotes: 1

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