The purpose of this script is to analyze plain text files (with or without the ".txt" extension) looking for broken paragraphs, i.e., paragraphs splited in more than one line, and join them in a single very long line. It's markdown friendly.

paragrapher.sh

#!/usr/bin/env zsh
################################################################################
# PARAGRAPHER revision 19
#     By Daniel "Nerun" Rodrigues
#     June 19, 2024
#     https://gist.github.com/nerun/8318924aa35f3f27231f86468804cc8c
#
# The purpose of this script is to analyze plain text files (with or without the
# ".txt" extension) looking for broken paragraphs, i.e., paragraphs splited in
# more than one line, and join them in a single very long line.
#
# This program is free software; you can redistribute it and/or modify it under
# the terms of the Creative Commons Zero 1.0 Universal (CC0 1.0) Public Domain
# Dedication (https://creativecommons.org/publicdomain/zero/1.0/).
################################################################################
if [ -z "$*" ] || [[ "$1" = "-h" ]] || [[ "$1" = "--help" ]]; then
    echo """The purpose of PARAGRAPHER is to analyze plain text files (with or without the
\".txt\" extension) looking for broken paragraphs, i.e., paragraphs splited in
more than one line, and join them in a single very long line (a.k.a. paragraph).

Usage: paragrapher [OPTION] <FILE>

  -c, --columns    Sets the width of a paragraph. The program will look for all
                   lines with a width greater than this number and consider
                   them to be a paragraph. The default is 72 columns: ideal for
                   paragraphs with more than 72 columns, such as 80 or 100. If
                   your text has smaller paragraphs, try reducing this value to
                   50 or 60. Ex.: \"paragrapher -c 60 filename\".

  -h, --help       Show this help.

  -v, --version    Show version, author, URL and license.

The output file will always be 'filename_paragraphed'.

Paragrapher is markdown friendly, and can recognize headings and unordered
lists. You can use it directly over a markdown file: \"paragrapher filename.md\".
"""
    exit 0
elif [[ "$1" = "-v" || "$1" = "--version" ]]; then
    echo """PARAGRAPHER revision 17 (August 1st, 2023)
By Daniel \"Nerun\" Rodrigues
https://gist.github.com/nerun/8318924aa35f3f27231f86468804cc8c

This program is free software; you can redistribute it and/or modify it under
the terms of the Creative Commons Zero 1.0 Universal (CC0 1.0) Public Domain
Dedication (https://creativecommons.org/publicdomain/zero/1.0/).
"""
    exit 0
elif [[ "$1" = "-c" || "$1" = "--columns" ]]; then
    columns=$2
    filename="$3"
else
    columns=72
    filename="$*"
fi

# Check if file exists
if [[ ! -f "$filename" ]]; then
    echo "Error: there's no file \"$filename\"."
    exit 1
else
    # Check if file is a plain text file
    file_test=$(file $filename)
    echo $file_test

    if [[ "$file_test" != *"text"* ]]; then
        echo "Error: \"$filename\" is not a plain text file."
        exit 1
    else
        # Check if it is UTF-8
        if [[ "$file_test" != *"UTF-8"* || "$file_test" == *"with CRLF line terminators"* ]]; then
            is_dos2unix=$(which dos2unix; echo $?)

            if [ ${is_dos2unix: -1} -eq 0 ]; then
                dos2unix $filename
            else
                echo "Consider installing \"dos2unix\"."
            fi
        fi
    fi
fi

# Verify filename extension, if any
if [[ "$filename" == *"."* ]]; then
    ext=$(echo ${filename##*.} | sed 's/.*/\L&/')
    output="${filename%.*}_paragraphed.$ext"
else # filename has no extension
    ext="txt"
    output="${filename}_paragraphed.$ext"
fi

# Add an empty line to the end of the file to avoid paragrapher not reading the
# last line
last_line=$(tail -1 $filename)

if [[ $(expr length "$last_line") != 0 ]]; then
    echo "" >> $filename
fi

# Create or clean the output file
:> $output

paragraph=''

# get string length
_strSize(){
    if [ -n "$1" ]; then
        echo $(expr length "$1")
    else
        echo 0
    fi
}

_dump_paragraph(){
    echo "$paragraph" >> $output
    paragraph=''
    echo "" >> $output
}

# https://mywiki.wooledge.org/BashFAQ/001#Trimming
# while IFS= read -r line; do
while read -r line; do
    size=$(_strSize "$line")

    if [ -z "$paragraph" ]; then
        if [ $size -ge $columns ]; then
            paragraph="$line"
        else # 0 <= size <= 71
           echo "$line" >> $output
        fi
    else # [ -n "$paragraph" ]
        if [[ ! "${paragraph: -1}" =~ [\.:] ]]; then
            if [[ "$ext" = "md" ]]; then
                if [[ ! "${line: 0:2}" =~ (#[# ]|•[ \t]|-[ \t]) ]]; then
                    paragraph+=" $line"
                else # starts with # (it's a heading) or •/- (unordered list)
                    _dump_paragraph
                    
                    if [[ "${line: 0:1}" = "#" ]]; then
                        echo "$line" >> $output
                        echo "" >> $output
                    else # unordered list (• or -)
                        paragraph=''
                        paragraph+="$line"
                    fi
                fi
            else
                paragraph+=" $line"
            fi
        else
            _dump_paragraph

            if [ $size -ge $columns ]; then
                paragraph+="$line"
            else # 0 <= size <= 71
                if [ $size -ne 0 ]; then
                   echo "$line" >> $output
                fi
            fi
        fi
    fi
done < $filename

# Clean output file
sed -ri "s/’|‘/'/g" $output
sed -ri "s/“|”/\"/g" $output
sed -i s/$'\u000c'//g $output # Form feed (U+000C)
sed -i "s/ -- / – /g" $output # En Dash (U+2013)
sed -i '/ --[[:digit:]]/ s/--/–/g' $output # En Dash (U+2013) representing a minus sign
if [[ "$ext" = "md" ]]; then
    sed -i 's/^[•-] [ \t]*/  - /g' $output # Bullet (U+2022) or list
fi
squeeze_empty=$(/usr/bin/cat -s $output)
echo "$squeeze_empty" > $output
echo "" >> $output # i like to end files with empty line

It is fully compatible with BASH 5+. Just change the shebang to #!/usr/bin/env bash