Multiple fifos over SSH

Question

Shell script allow multiple file descriptors (up to 9 for POSIX compliance, but even more for bash):

$ install -m 755 /dev/stdin /tmp/test <<\EOF
#!/bin/sh

echo hi1 >&1
echo hi2 >&2
echo hi3 >&3
echo hi4 >&4
EOF
$ /tmp/test 1> /tmp/1 2> /tmp/2 3> /tmp/3 4> /tmp/4
$ cat /tmp/1
hi1
$ cat /tmp/2
hi2
$ cat /tmp/3
hi3
$ cat /tmp/4
hi4

I want to achieve the same behaviour over SSH, but what happens is:

$ ssh user@remote_machine /tmp/test 1> /tmp/1 2> /tmp/2 3> /tmp/3 4> /tmp/4
$ cat /tmp/1
hi1
$ cat /tmp/2
hi2
/tmp/test: 5: /tmp/test: 3: Bad file descriptor
/tmp/test: 6: /tmp/test: 4: Bad file descriptor
$ cat /tmp/3
$ cat /tmp/4

Is it possible to have multiples output pipes over SSH other than stdout and stderr?

It is important to note I am not looking for port forwarding, since I will have network issues.

Kusalananda · Accepted Answer · 2019-05-04 15:43:19Z

1

No, that's not doable. The ssh client explicitly closes every open file descriptor larger than 2:

/*
 * Discard other fds that are hanging around. These can cause problem
 * with backgrounded ssh processes started by ControlPersist.
 */
closefrom(STDERR_FILENO + 1);

This is almost the first thing that happens when the client starts (see main() in the source code for OpenSSH 8.0 available here).

answered May 4, 2019 at 15:43

Kusalananda♦

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