How can I concatenate two files in Unix?

How can I create a new file "new.txt" that is a concatenation of "file1.txt" and "file2.txt" in Unix?

cat file1.txt file2.txt > new.txt

if the file new.txt is an empty file, you can simply use the cat command :

cat file1.txt file2.txt > new.txt

if new.txt is not empty, and you want to keep its content as it is, and just want to append the concatenated output of two files into it then use this:

cat file1.txt file2.txt >> new.txt

If you want to append two or more files to an existing file without overwriting the file's (file4.txt) content, then below is an example:

cat file1.txt file2.txt file3.txt >> file4.txt

Even if the file file4.txt is not present, it would get created. If it is present, the other files' contents would get appended to it.