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

What you're asking about is called a "here document" ...

it's a very useful feature of all UNIX/Linux shells. It's considered a type of IO redirection that allows the developer to create a code/script block that gets passed into an interactive program: such as

1. ftp / telnet
2. sqlplus
3. mail (or any command-line email client)
4. any script interpreter (like python)
5. cat ( cat? why cat??)

So the motivation to use cat in this context is not so obvious?

What does "cat" do? Fundamentally, it echos out what ever it reads from stdin or files.

Executing "cat" with no parameters, and proceeding to type "hello" and then "there" at it will yield the following experience:

$ cat

hello

hello

there

there

So we can use cat within a here document to generate some specified multi-lined text content on demand ...

$ cat <<EOF

> Close your eyes and I'll kiss you

> Tomorrow I'll miss you

> Remember I'll always be true

> And then while I'm away

> I'll write home every day

> And I'll send All My Loving to you

> EOF

Close your eyes and I'll kiss you

Tomorrow I'll miss you

Remember I'll always be true

And then while I'm away

I'll write home every day

And I'll send All My Loving to you

We can even intermix shell substitute-able values - like this:

$ cat <<EOF

> My HOME directory on $(hostname)

> is $HOME

> EOF

My HOME directory on lji-laptop

is /home/lji

The basic structure of a here document is as follows:

interactive-command <<START_AND_TOKEN

line 1 to be processed by interactive-command

line 2 to be processed by interactive-command

...

line N to be processed by interactive-command

START_AND_TOKEN

Here Documents are quite often placed within shell functions - deferring their execution, and making them ultimately re-usable ... for example:

function sendAlert () {

mailx -s"Alert" $DISTRO_LIST <<-MAILER

Hello, all - the following file systems are above

their utilization high-water-mark of ${TOP_LIMIT}:

$FILE_SYSTEMS_IN_TROUBLE

MAILER

}