TIL: You can make HTTP requests without curl using Bash /dev/TCP
I needed to check that one container could reach another over an internal Docker network: a plain GET /health against a service on a shared network. The obvious move is curl http://service:8642/health . But this app image was stripped right down, with no curl or wget and nothing else around that I could use to open a socket.
As it turns out, bash can speak HTTP by itself bash can open a TCP socket, and you can write a small HTTP request to it by hand. Opening a connection to a host and port and writing the request needs nothing beyond the shell that’s already there:
service here is just the hostname of whatever you’re talking to. It has to resolve and be reachable from wherever you run this, so it needs to be set up first: a container or service name on a Docker network you’ve configured, or any DNS name that resolves. Swap in your own host and port.
That prints the whole response: the status line, the headers, the blank line, and the body. To add a header, such as an Authorization: Bearer token, put another \r\n -terminated line before the blank line that ends the request:
What caught me out the first time is that /dev/tcp isn’t a real device file. There’s no such path on disk; ls /dev/tcp finds nothing, and cat /dev/tcp/... from another shell just errors. It’s a redirection that bash handles internally. From the Bash manual :
/dev/tcp/host/port – If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corresponding TCP socket.