Append text to file from command line without using io redirection

Question

How can we append text in a file via a one-line command without using io redirection?

Answer 1

You can use the --append feature of tee:

cat file01.txt | tee --append bothFiles.txt 
cat file02.txt | tee --append bothFiles.txt 

Or shorter,

cat file01.txt file02.txt | tee --append bothFiles.txt 

I assume the request for no redirection (>>) comes from the need to use this in xargs or similar. So if that doesn't count, you can mute the output with >/dev/null.