Kindle accepts .mobi files directly, just copy them to the root documents folder. To convert from the more common .epub, we can use the calibre CLI utility ebook-convert.
To run multiple instances in parallel, converting all .epub files in current dir, and log each process into a .log with the same name, do:
foreach x (*.epub) {
echo -e "\n$x";
ebook-convert $x .mobi > "$x".log 2>&1 &
}
In the 3rd line, > redirects stdin to the file "$x".log, and 2>&1 redirects stderr to stdin to log them together. The final & runs ebook-convert as a background process, that can be monitored in htop with the printed PID, or using the jobs utility.