2014-05-12

Want to use SFTP?

I've added SFTP library (rfc sftp) recently so let me introduce it.

If you are a lazy programmer, you would have thought, at least once, that avoiding GUI operations to upload files to SFTP server. (Yes, that was my motivation!) I've been thinking that for already couple of months (or even close to a year). SFTP is a protocol that depending on SSH so first I needed to write the SSH library. From that, for some reason, I didn't have much need to do. (Basically SFTP operation was reduced at my work.) Then a week ago or so, I needed to collect log files from multiple servers and I simply disliked that task. So I wrote it.

Following is the basic usage.
(import (rfc sftp))

(call-with-sftp-connection "hostname" "23"
  (lambda (conn)
    ;; downloading is very straight forward
    (sftp-read conn "a/file" (sftp-file-receiver "a/local/file"))

    ;; uploading is a bit complicated
    ;; firstly, you need to open the file handle
    (let ((handle (sftp-open conn "a/file2" 
                    (bitwise-ior +ssh-fxf-creat+ +ssh-fxf-write+))))
      ;; then you need to use the handle
      ;; uploading uses binary input port as it's input.
      (sftp-write! conn handle 
        (open-bytevector-input-port (string->utf8 "Hello SFTP from Sagittarius")))
      ;; finally close the handle
      (sftp-close conn handle))

    ;; want the contents of a directory?
    ;; (sftp-readdir conn ".") ;; >- this returns a list of sftp-name object
    ;; this returns a list of string representing directory contents (file names)
    (sftp-readdir-as-filenames conn "."))
  :username "user" :password "pass")
sftp-close may not be needed as long as the server can handle it correctly but it's better to do it. You can also write a very thin wrapper for this. (I just didn't have any good API for this so maybe for future.) If your SFTP server doesn't need any authentication then you don't have to specify the keyword arguments (never seen such a server though). For now, the library doesn't support any other authentication method such as login with certificate.

One important note; the underlying SSH library doesn't support key re-exchange properly so if the uploading/downloading file is more than 1 GB in total then it would most likely fail. However I don't need this yet so not sure when this will be fixed. (patches are always welcome!)

This library will be available from 0.5.4 (will be release very soon).

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