How does “cat << EOF” work in bash?

Question

I needed to write a script to enter multi-line input to a program (psql).

After a bit of googling, I found the following syntax works:

cat << EOF | psql ---params
BEGIN;

`pg_dump ----something`

update table .... statement ...;

END;
EOF

This correctly constructs the multi-line string (from BEGIN; to END;, inclusive) and pipes it as an input to psql.

But I have no idea how/why it works, can some one please explain?

I'm referring mainly to cat << EOF, I know > outputs to a file, >> appends to a file, < reads input from file.

What does << exactly do?

And is there a man page for it?

Answer 1

In your case, "EOF" is known as a "Here Tag". Basically <<Here tells the shell that you are going to enter a multiline string until the "tag" Here. You can name this tag as you want, it's often EOF or STOP.

Some rules about the Here tags:

1. The tag can be any string, uppercase or lowercase, though most people use uppercase by convention.
2. The tag will not be considered as a Here tag if there are other words in that line. In this case, it will merely be considered part of the string. The tag should be by itself on a separate line, to be considered a tag.
3. The tag should have no leading or trailing spaces in that line to be considered a tag. Otherwise it will be considered as part of the string.

example:

$ cat >> test <<HERE
> Hello world HERE <-- Not by itself on a separate line -> not considered end of string
> This is a test
>  HERE <-- Leading space, so not considered end of string
> and a new line
> HERE <-- Now we have the end of the string