If you use the shell you surely know about redirection:

  # echo 'hello world' >output
  # cat <output

The first line writes "hello world" to the file "output", the second reads it back and writes it to standard output (normally the terminal).

Then there are "here" documents:

  # cat <<EOF
  > hello
  > world
  > EOF

A "here" document is essentially a temporary, nameless file that is used as input to a command, here the "cat" command.

A less commonly seen form of here document is the "here" string:

  # cat <<<'hello world'

In this form the string following the "<<<" becomes the content of the "here" document.

Another less commonly seen form of redirection is redirecting to a specific file descriptor:

  # echo 'Error: oops' >&2

This redirects the output of the "echo" command to file descriptor 2, aka standard error. This is useful if you want to keep the error output of your scripts from contaminating the normal output when the output of your script is redirected.

These features work in bash and may not be available in other shells.