如何比较两个目录中的时间戳

Question

我有一个问题，该如何检查备份目录中是否已存在文件，比较两个文件的更新时间，然后根据需要进行更新。

谢谢！

#!/bin/bash

echo "Enter a directory name" 
read dirtydan

if [ -d "$dirtydan" ]; then
echo "directory exists."
else
echo "Error: directory does not exist."

echo -n "Do you want to make $dirtydan as new directory (y/n)? "
read answer

if [ "$answer" != "${answer#[Yy]}" ] ;then
echo yes
echo "Making $dirtydan as directory"
mkdir -p $dirtydan
else
echo No
echo "Goodbye"
fi

fi

find $pwd -name '*.txt' -exec cp -t $dirtydan "{}" +