Bash script that paginates a pdf file

paginate.sh

#! /bin/bash

# If you use MacOS, download the latest version of pdftk from here :
#        https://www.pdflabs.com/tools/pdftk-the-pdf-toolkit/pdftk_server-2.02-mac_osx-10.11-setup.pkg

function exists(command){
    if ! [ -x "$(command -v $command)" ]; then
    echo "Error: $command is not installed." >&2
    exit 1
    fi
}
# Check if required tools exist
exists(pdflatex)
exists(pdftk)

input=$1 #Input PDF filename
output_name=$2 # Output PDF filename

# Retrieve the number of pages from the original
n_page=$(pdftk $input dump_data|grep NumberOfPages| awk '{print $2}')

# Build a stamp
echo " \documentclass[12pt,a4paper]{article}
 \usepackage{multido}
 \usepackage[hmargin=.8cm,vmargin=1.5cm,nohead,nofoot]{geometry}
 \begin{document}
 \multido{}{$n_page}{\vphantom{x}\newpage}
 \end{document}" > number.tex
pdflatex number.tex

# Add the "numbered" stamp to the pdf
pdftk $input multistamp numbers.pdf output $output_name