Why isn't "ssh localhost nohup echo hello" working?

Question

I think this sums it up:

# /usr/bin/nohup echo hello
/usr/bin/nohup: ignoring input and appending output to `nohup.out'

# ssh localhost /usr/bin/nohup echo hello
hello

What was happened?

Answer 1

Apparently,

recent versions of ssh started using pipes for non-TTY STDIO

so

there is no way for a bash script to tell whether a non-tty ssh command is being piped or not.

Since nohup only springs into action when it detects that stdout/stderr are connected to a terminal (and does nothing when the output goes to a pipe), and ssh's way of launching it looks like it's being piped, you get the behaviour you're seeing.

---

As muru points out, you can force ssh to allocate a tty by using the -t parameter:

$ ssh -t localhost "/usr/bin/nohup /bin/echo foo"
/usr/bin/nohup: ignoring input and appending output to ‘nohup.out’

Answer 2

From man nohup (POSIX):

   If  the  standard output is a terminal, all output written by the named
   utility to its standard output shall be appended to the end of the file
   nohup.out  in  the current directory. ...

   If the standard error is a terminal, all output written  by  the  named
   utility  to  its  standard  error  shall be redirected to the same file
   descriptor as the standard output.

To stress: If the standard output is a terminal.

With a simple ssh nohup echo foo, standard output isn't a TTY unless you tell SSH to create one:

ssh -t localhost nohup echo foo

From man ssh:

-T   Disable pseudo-tty allocation.
-t   Force pseudo-tty allocation.  This can be used to execute
     arbitrary screen-based programs on a remote machine, which can be
     very useful, e.g. when implementing menu services.  Multiple -t
     options force tty allocation, even if ssh has no local tty.

Also see:

• What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?