cat to combine heredoc and file-CodePudding

I'm trying to and failing to understand why this doesn't work. I'm trying to use cat to combine an existing file and a heredoc into one file:

# test.txt
Hi there

$ cat test.txt <<END > /tmp/combined.txt
This is heredoc
END

I would have expected this to function in essence like:

cat file.txt file1.txt > /tmp/combined.txt

But what I actually see is just the contents of test.txt:

#/tmp/combined.txt
Hi there

If I do just the heredoc it works. If i do just the file it works obviously. What is it about combining them that makes cat ignore the heredoc?

(I know I can use tee and a variety of other approaches to do this but I want to fill in my understanding of cat, redirection etc)

CodePudding user response：

cat echos the contents of its input filename arguments or standard input if none are given, but not both. However, if one of the arguments is -, that one is treated as reading from standard input:

$ cat test.txt - <<EOF
This is heredoc
EOF
Hi there
This is heredoc