While loop - reading two lines at a time

Question

I am trying to parse data from a log file.

Here is the datafile in:

Data1
Data2
Data3
Data4

My code is as follow:

data=$(cat datafile)
echo "$data" | while read line; read line1;
do
    echo "$line and $line1"
done

My desired output is:

Data1 and Data2
Data2 and Data3
Data3 and Data4

But instead I get:

Data1 and Data2
Data3 and Data4

I do understand why I get this behavior, however I am short on ideas about how to do as I want. Does anybody happen to know a trick?

Thanks

That code sample will fail, and can't possibly produce the output you say it does. Please double-check what you've given us. — Chris Davies, Jan 28 '20 at 14:56
You could store the datafile into an array and do a loop to output "line[i] and line[i+1]". — dr_, Jan 28 '20 at 15:00
I made the final edit on the post but it does work though.The idea of an array could be useful, i'll try that! — elfamosomojito, Jan 28 '20 at 15:02
@dr01 there's a dup of this - I saw it only a couple of days ago - just trying to find it — Chris Davies, Jan 28 '20 at 15:03

score 4 · Answer 1 · answered Jan 28 '20 at 15:25

4

awk 'NR > 1 {print prev, "and", $0}; {prev = $0}' < datafile

Or with sed:

sed 'x; 1d; G; s/\n/ and /' < datafile

answered Jan 28 '20 at 15:25

Thanks for your answer. I'm currently reading all of your good practices answers in others posts. You could write a book named "you are using bash wrong" :) – elfamosomojito Jan 30 '20 at 08:07

score 2 · Accepted Answer · edited May 10 '23 at 14:15

2

How far would

{ read line
  while read line1
  do
     echo $line and $line1
     line=$line1
  done
 } < datafile
Data1 and Data2
Data2 and Data3
Data3 and Data4

get you?

edited May 10 '23 at 14:15

cantik memekmu

answered Jan 28 '20 at 15:09

RudiC

Bingo. Are the brackets the key to this? By the way it works for me now, but I'm considering investigating Stephane Chazelas answer furthermore, it seems to be the best practice one. – elfamosomojito Jan 30 '20 at 08:10
The braces enclose a "compund command" which in turn makes sure both leading read and while loop use the same , redirected input. Without them, the while loop would reopen it at line 1. I didn't use awk as I thought you need to perform other shell tasks on each data pair. – RudiC Jan 30 '20 at 08:59

Jetchisel · Answer 3 · 2020-01-29T00:02:45.577

1

One more while read loop...

while read -r line; do 
  printf '%s and %s\n' "$line" "$line"
done < data

Or

while read -r line <&3; do 
  read -r line1
  printf '%s and %s\n' "$line" "$line1"
done 3< data < data

or bash4+ mapfile

mapfile -t array < data

for i in "${!array[@]}"; do 
  printf '%s and %s\n' "${array[$i]}" "${array[$i]}"
done

edited Jan 29 '20 at 00:02

answered Jan 28 '20 at 23:32

Jetchisel

user9101329 · Answer 4 · 2023-05-11T12:43:39.953

1

I would simply paste the file to its clone with the first line removed:

$ tail -n+2 datafile | paste datafile -

edited May 11 '23 at 12:43

answered May 10 '23 at 19:25

user9101329

4 Answers4