How to split a file into 2 chunks in bash

I have a task to take a file with millions of lines, and to split it in exactly two files, with a controlled top `head` portion part A file, and the remainder of the lines in part B file.

It actually turns out to be somewhat non-trival to split a file in 2 chunks, with a small defined top portion and a large arbitrary bottom portion.

The split [OPTION]... [INPUT [PREFIX]] function is really designed to make N chunks out of your file, or M chunks each containing N lines; I debated using it and merging all but the first file back together, but decided to look up sed examples instead.

I ended up with the below function. There’s almost certainly a faster/better way to achieve it but this seems to work.

Sample usage

remainderof bigfile.txt 1000
When that runs you will have this result

1000_bigfile.txt // this is the top 1000 lines
1000R_bigfile.txt // this is the remainder of the file after splitting off the top 1000 lines
bigfile.txt // the original untouched file
Using optional 3rd argument

remainderof bigfile.txt 1000 true
When that runs you will have this result

1000_bigfile.txt // this is the top 1000 lines
bigfile.txt // this is the remainder of the file after splitting off the top 1000 lines
// the original file no longer exists
Here’s the function

alias ll='ls -larth'

function remainderof() {

    thefile=$1
    batchsize=$2
        if [ ! -z $3 ]
                then
        : # $1 was given
        replaceoriginal=$3
        else
        : # $1 was not given
        replaceoriginal=false
        fi

    if [ $# -lt 2 ]
    then
         echo "Usage: $0 filename_to_split rows_to_chop [opt_bool_process_in_place_destructively]"
         exit 1
    fi

    extension="${thefile##*.}"
    filename="${thefile%.*}"
    length=$( wc -l < $thefile )
    buffer=$(($length-$batchsize))
    echo Splitting $batchsize lines off the top of $filename $extension leaving $buffer from $length  ...
    #  The top chunk filename
    topfile=${filename}_${batchsize}.${extension}

    # The bottom chunk = remainder of file
    startofnext=$(($batchsize+1))
    remainder=${filename}_${startofnext}-${length}.${extension}
    echo "Writing $topfile and $remainder"

    # Split off the first N lines of the file
    head -n $batchsize $thefile > $topfile

    # split off the bottom LENGTH - N lines of the file
    sed "1,${batchsize}d" $thefile > $remainder

    # whether to leave a copy
    if [ "$replaceoriginal" = true ] ; then
        rm -rf $thefile
        mv $remainder $thefile
    fi

    echo `wc -l < $topfile` lines in $topfile
    echo `wc -l < $remainder` lines in $remainder


    echo Done
}

If you have improvements, leave a comment!