Read n lines at a time using Bash

Question

I read the help read page, but still don\'t quite make sense. Don\'t know which option to use.

How can I read N lines at a time using Bash?

Answer 1

to read n+2 Lines from a file:-

2

4

6

8

.

.

so on

you can try this way:-

cat fileName | awk '!((NR - 0) % 2)'

Answer 2

I don't think there is a way to do it natively in bash, but one can create a convenient function for doing so:

#
# Reads N lines from input, keeping further lines in the input.
#
# Arguments:
#   $1: number N of lines to read.
#
# Return code:
#   0 if at least one line was read.
#   1 if input is empty.
#
function readlines () {
    local N="$1"
    local line
    local rc="1"

    # Read at most N lines
    for i in $(seq 1 $N)
    do
        # Try reading a single line
        read line
        if [ $? -eq 0 ]
        then
            # Output line
            echo $line
            rc="0"
        else
            break
        fi
    done

    # Return 1 if no lines where read
    return $rc
}

With this one can easily loop over N-line chunks of the data by doing something like

while chunk=$(readlines 10)
do
    echo "$chunk" | ... # Whatever processing
done

In this loop $chunk will contain 10 input lines at each iteration, except for the last one, which will contain the last lines of input, which might be less than 10 but always more than 0.

Answer 3

The echo simulates a file with two lines input, use head -2 before paste if needed:

IFS=\; read A B < <(echo -en "X1 X2\nY1 Y2\n" | paste -s -d\;)

If you want to read lines in a loop and create pairs and lines have only single word in them use:

while read NAME VALUE; do 
    echo "$NAME=$VALUE"; 
done < <(echo -en "M\n1\nN\n2\nO\n3\n" | xargs -L2 echo)