How does "cat << EOF" work in bash?

linux bash scripting heredoc
hasen picture hasen · Mar 23, 2010 · Viewed 1M times · Source

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

kennytm picture kennytm · Mar 23, 2010

This is called heredoc format

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