What does <<< mean>output-filename)

Question

I am trying to understand a codebase where I see a line like below: socat /tmp/haproxy - <<< "show servers state" > /var/state/haproxy/global

What is socat doing here? What does <<< mean?

Answer 1

The socat command creates a bidirectional pipe between the file /tmp/haproxy and stdin which is expressed by passing - to socat.

In fact it appends stdin to /tmp/haproxy and writes the resulting output to /var/state/haproxy/global

<<< is a bash feature, a so called here string. It passes the string "show server state" as stdin to socat.

Answer 2

A posix shell version would be:

echo "show servers state" | socat /tmp/haproxy - > /var/state/haproxy/global

<<< is a bashism to put a string on stdin

Answer 3

The man pages of the socat command and bash may help (Note that <<< is a bash feature)

Try:

man socat
man bash
# Type the following when the bash man page is open
# it will point you right to the explanation of <<<
/<<<