Read from two files line by line and process them simultaneously

Question

Hello everyone I'm very new to the game so my question is probably rather simple but I'm stuck at this for a long time. I want to process two files from two list of files simultaneously line by line.

I'm trying currently:

 read file1 && read file2; 
    do 
    echo "$file1 and $file2"
    done

The echo is of course just a spaceholder for the rest of the script but i didn't manage to get any variables out of the read operation.

http://stackoverflow.com/a/15461247/1030675 – choroba Mar 09 '17 at 11:46 — choroba, Mar 09 '17 at 11:46
You are missing the `while` keyword. – chepner Mar 09 '17 at 12:55 — chepner, Mar 09 '17 at 12:55

chepner · Answer 1 · 2017-03-09T13:29:58.250

11

You need two separate file descriptors to read from two files at once. One of them can be standard input.

while IFS= read -r line1 && IFS= read -r line2 <&3; do
  echo "File 1: $line1"
  echo "File 2: $line2"
done < file1 3< file2

edited Mar 09 '17 at 13:29

answered Mar 09 '17 at 12:58

chepner

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score -1 · Answer 2 · answered Mar 09 '17 at 12:17

LINECOUNTER=1
while true; do
    FILE1INPUT="$(sed -n "${LINECOUNTER}p" file1.txt)"
    FILE2INPUT="$(sed -n "${LINECOUNTER}p" file2.txt)"
    echo "$FILE1INPUT and $FILE1INPUT"
    let LINECOUNTER=LINECOUNTER+1
done

The LINECOUNTER variable is just to memorize which line to output next.
Then you assign the output of the sed command $(sed ...) to the variable FILE1INPUT. This sed command just reads one line, specified by LINECOUNTER from file1.txt. Same with file2.txt
Then LINECOUNTER is incremented by one, so that the next time sed is executed the next line is returned.

Of course one needs an appropriate condition for the while loop to end.

There is no reason to run `sed` for every line of each file. — chepner, Mar 09 '17 at 12:56

Read from two files line by line and process them simultaneously

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