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Exchanging Files via Mail

  Miguel Marquina IT/User Support


Nowadays, access to remote computers is, in general, no longer a problem. The classical and recommended file exchange is made using net protocols like ftp, or using modern interfaces through WWW (see the article "Anonymous ftp access using WWW" in CNL 223).

However there may be occasions when you wish to send or receive files at your ordinary electronic mailbox. This article is intended to give you some hints on how to do that.

The first thing to remember is that e-mail only enables you to transmit properly ASCII text (the one you can produce from an International keyboard, composed by so-called "quoted-printable" characters), and this is guaranteed to work and be readable across computers. If you want to transmit binary files (images, drawings, etc.) you need to convert them to ASCII first. There are several methods to achieve this and we will present you the two most common ones:

Exchanging MIME Attached Files

Mail Agents may use different standards for encoding and decoding attachments, but today the world is converging on the Internet standard called MIME (Multipurpose Internet Mail Extensions) which is used, for example, by pine, Netscape, Quickmail and the CERN MSmail/Internet gateway. As for natural languages, the mail agents at both sides must be using the same standard to understand each other. It is not enough to know that "you got an attachment". Useful information that you or your correspondant should include when transmitting a file is:

If your mail client is properly configured and it uses the same standard as the one of your correspondent, it will automatically identify the type of the received file and launch the appropriate file browser for it (if available at your system).

Details of usage vary from client to client, but for example, using pine, an incoming mail with an attachment looks like this:

	   1 Shown    2 lines  Text                                                   
	   2         17 KB     Application                                            

	Hi John, please find here the paper I am preparing for the next
	conference, as MIME attachment.

	  [Part 2, Application/MSWORD  23KB]
  	[Cannot display this part. Press "V" then "S" to save in a file]

	MS-Word document received as an attachment
To send a file as an attachment, position the cursor in the header of the mail and use the Attach (ctrl-J) command.

Exchanging UUencoded Files

For systems that don't yet have MIME support, the most common format used in file exchange via e-mail is "uuencoding". You recognize it immediately because the incoming e-mail looks like this:

	Hi John, please find here the paper I am preparing for the next
	conference, in "uuencode" format...

	begin 644 myfile

	File "myfile" encoded with the "uuencode" program

Simply extract the text of your mail as a file (using the corresponding "Export" feature of your Mail Agent) and then execute the "uudecode" program on it (if available on your system); e.g. on UNIX:

        uudecode mailtext
It is very easy to send uuencoded files. On UNIX, one would encode "myfile" with the command:
        uuencode myfile myfile >mailtext
and then incorporate the generated file "mailtext" into your outgoing mail.
N.B. Please notice that "incorporate" really means embedding the text. If you happen to "attach" it using your mail agent, you would end up encoding the information twice, forcing your correspondent to go through two decoding methods to obtain at last the original file you sent. In brief, if you "uuencode" it, do not "attach" it.
next up previous
Next: Unsolicitated Electronic Mail Up: cnl227.html Previous: Expansion of the Central Mail Server