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linuxbashtelnet

Linux shell: why is "open" required in this telnet command

I tried to process telnet output in bash and i stumbled upon this syntax to send telnet commands to a server

( echo open $host $port
sleep 1
echo $cmd1
sleep 1
) | telnet

What i would like to know is why the "open" command is required and why

( echo $host $port
...
) | telnet

results in a "?Invalid command" error.

Solution

  • ...because a hostname is not a valid command name? There is a big difference between

    $ telnet host port

    and

    $ telnet
telnet> host port

    Where the latter is what your echo command is effectively doing.

    The one-liner automatically runs an open command, so it is basically equivalent to this:

    $ telnet
telnet> open host port

    But I'm not at all sure why you wouldn't just run telnet host port in the first place.