cat magic stdin/out redirection and mixing

cat.md

How to abuse cat stdin to redirect file content

cat with stdin EOF tag can be used to mix stdin input and file content

cat - << 'EOF' "file.txt"
text from stdin to prepend to file.txt content
EOF

would print to the stdout the line text from stdin to prepend to file.txt content and after the content of file.txt

optionnaly, the output of the cat command can be redirected to a file with the line cat - << 'EOF' "file.txt" > out.txt or event piped to another command with a similar line: cat - << 'EOF' "file.txt" | sed ...