@c -*-texinfo-*-
@c This file is part of Guile-SSH Reference Manual.
@c Copyright (C) 2015 Artyom V. Poptsov
@c See the file guile-ssh.texi for copying conditions.

@node Remote Pipes
@section Remote Pipes
@cindex remote pipes

@code{(ssh popen)} provides API for working with remote pipes, akin to
@code{(ice-9 popen)} procedures (@pxref{Pipes,,, guile, The GNU Guile
Reference Manual})

@var{mode} argument allows to specify what kind of pipe should be created.
Allowed values are: @code{OPEN_READ}, @code{OPEN_WRITE}, @code{OPEN_BOTH}.

There is an additional value, @code{OPEN_PTY}, that allows to request a pseudo
terminal.  The terminal is needed to run such commands as @code{top}.  Thus,
to run @code{top} on the remote side you need to open a remote pipe with "t"
flag set.

@strong{Note} that when a PTY is used, a server merges stderr stream with
stdout.

Values of the aforementioned constants:

@table @samp
@item OPEN_READ
      ``r''
@item OPEN_WRITE
      ``w''
@item OPEN_BOTH
      ``r+''
@item OPEN_PTY
      ``t''
@end table

@deffn {Scheme Procedure} open-remote-pipe session command mode
Execute a @var{command} on the remote host using a @var{session} with a pipe
to it.  Returns newly created channel port with the specified @var{mode}.
@end deffn

@deffn {Scheme Procedure} open-remote-pipe* session mode prog [args...]
Execute @var{prog} on the remote host with the given @var{args} using a
@var{session} with a pipe to it.  Returns newly created channel port with the
specified @var{mode}.
@end deffn

@deffn {Scheme Procedure} open-remote-input-pipe session command
@deffnx {Scheme Procedure} open-remote-input-pipe* session prog [args...]
Equvalent to @code{open-remote-pipe} and @code{open-remote-pipe*} respectively
with mode @code{OPEN_READ}.
@end deffn

@deffn {Scheme Procedure} open-remote-output-pipe session command
@deffnx {Scheme Procedure} open-remote-output-pipe* session prog [args...]
Equvalent to @code{open-remote-pipe} and @code{open-remote-pipe*} respectively
with mode @code{OPEN_WRITE}.
@end deffn

@subsection Examples

@subsubsection Simple cases

Here's a self-explanatory little script that executes @code{uname -o} command
on the local host and prints the result:

@lisp
#!/usr/bin/guile \
-e main -s
!#

(use-modules (ice-9 rdelim)             ; @{read,write@}-line
             ;; Guile-SSH
             (ssh session)
             (ssh auth)
             (ssh popen))               ; remote pipes

(define (main args)
  ;; Make a session with local machine and the current user.
  (let ((session (make-session #:host "localhost")))

    ;; Connect the session and perform the authentication.
    (connect! session)
    (authenticate-server session)
    (userauth-agent! session)

    ;; Execute the command on the remote side and get the input pipe
    ;; to it.
    (let ((channel (open-remote-input-pipe session "uname -o")))
      ;; Read and display the result.
      (write-line (read-line channel)))))
@end lisp

@subsubsection Executing a command with a pseudo terminal

Surely we aren't limited to one-line outputs; for example, we can watch
@code{top} command executing on a remote side locally, by reading data from
the channel in a loop:

@lisp
(define OPEN_PTY_READ (string-append OPEN_PTY OPEN_READ))

(let ((channel (open-remote-pipe* session OPEN_PTY_READ
                                  "top" "-u avp")))
  (let r ((line (read-line channel)))
    (unless (eof-object? line)
      (write-line line)
      (r (read-line channel)))))
@end lisp

Or we can do the same, but this time with streams:

@lisp
(use-modules (srfi srfi-41)             ; streams
             (ssh session)
             (ssh auth)
             (ssh popen))

(define (pipe->stream p)
  (stream-let loop ((c (read-char p)))
    (if (eof-object? c)
        (begin
          (close-input-port p)
          stream-null)
        (stream-cons c (loop (read-char p))))))

(define OPEN_PTY_READ (string-append OPEN_PTY OPEN_READ))

(define (main args)
  (let ((s (make-session #:host "example.org")))
    (connect! s)
    (userauth-agent! s)
    (let ((rs (pipe->stream (open-remote-pipe* s OPEN_PTY_READ
                                               "top" "-u avp"))))
      (stream-for-each display rs))))
@end lisp

@subsubsection Controlling the pseudo terminal size

To set the size of a pseudo terminal, one may use @code{channel-set-pty-size!}
from @code{(ssh channel)}.  For example:

@lisp
(use-modules (ssh popen)
             (ssh auth)
             (ssh channel))

(define OPEN_PTY_READ (string-append OPEN_PTY OPEN_READ))

;; Opening of a Guile-SSH session goes here ...

(let ((p (open-remote-pipe* session OPEN_PTY_READ "top" "-u avp")))
  (channel-set-pty-size! p 80 50)
  ;; Reading output from a port ...
  )
@end lisp

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