6.13 uuencode and uudecode
6.13.1 Description
There is still a need for UNIX administrators to understand the
conversion process of uuencoding binary files into
ASCII text and uudecoding them back to binary.
Example One: Encoding a Binary File
Flavors: AT&T, BSD
Syntax:
uuencode [file] name
When encoding a binary file into an ASCII file, three filenames
get passed to the uuencode command. The first is
the name of the actual file being uuencoded. The second is the name that the
file will become after the uuencoded file is decoded. This can be the same as
the first name. The third is the filename the uuencoded file is stored as. The
last filename usually has the .uu extension tacked to the end of the file. It
might all sound confusing, but it'll all make sense in a minute.
For example, suppose there's a new compiled version of a
program called top in binary format. You would have
to uuencode the file before you could email it to a friend. It works like this:
rocket 82% uuencode top top.new > top.uu
rocket 83% more top.uu
begin 755 top.new
M?T5,1@$"`0`````````````"``@````!$``9=````#0``'C0(```)``T`"``
M!P`H`!@`%P````8````T$```-!```#0```#@````X`````0````$`````P``
M`100``$4$``!%````!$````1````!`````%P```"```!*!```2@0``$H````
.
.
.
M`*$````(`````Q`!:"```&@@```%Y```````````````$`````````%M<```
M'@@```````````!P````!S````````````````$````````!E`````,`````
<````````=S````&>``````````````````````$`
`
end
You can see that the uuencoded file top.uu can be viewed because
it is now an ASCII file. The file is also marked by begin
and end markers. Following the begin
marker, on the first line of the file is the permission that the file top.new
has when it is decoded. This filename (top.new) is the same as the second name
that was used at the time uuencode was run on the
binary file. The file is now ready to be emailed to a friend, posted to a Usenet
newsgroup, or about anything else you want to do with it.
rocket 84% mail friend@foo.com < top.uu
It is possible to uuencode a file and email it all in one command.
As you might remember from section 4.5, "Mail a Process," you can
pipe the output of a command or process to an email address.
rocket 85% uuencode top top.new | mail friend@foo.com
When the file arrives at the destination email address it is ready
to be uudecoded, or the email client interprets it for the recipient.
It
is possible to uuencode a series of files within a particular directory by creating
a quick script that processes each file and uuencodes it.
vi uuall
#! /bin/sh
for uufile in `ls -1`
do
uuencode $uufile $uufile > $uufile.uu
done
Line 1: Define the shell to be used.
Line 2: Process each file in the current directory.
Line 4: While processing through each file, uuencode it with its
current name and place a .uu extension on the uuencoded file.
When
the uuall script is executed, it progresses through
a directory and all the files are uuencoded.
rocket 64% ls -al
total 458
drwxr-xr-x 2 ugu user 512 Jan 2 16:46 ./
drwxr-xr-x 31 ugu user 2048 Jan 2 16:45 ../
-rw-r--r-- 1 ugu user 45898 Jan 2 16:46 ugu.gif
-rw-r--r-- 1 ugu user 81920 Jan 2 16:46 uguboard.tar
-rwxr-xr-x 1 ugu user 3854 Jan 2 16:46 ugugrab*
rocket 65% uuall
rocket 66% ls -la
total 1084
drwxr-xr-x 2 ugu user 512 Jan 2 16:48 ./
drwxr-xr-x 31 ugu user 2048 Jan 2 16:45 ../
-rw-r--r-- 1 ugu user 45898 Jan 2 16:46 ugu.gif
-rw-r--r-- 1 ugu user 71042 Jan 2 16:48 ugu.gif.uu
-rw-r--r-- 1 ugu user 81920 Jan 2 16:45 uguboard.tar
-rw-r--r-- 1 ugu user 112899 Jan 2 16:48 uguboard.tar.uu
-rwxr-xr-x 1 ugu user 3854 Jan 2 16:46 ugugrab*
-rw-r--r-- 1 ugu user 5336 Jan 2 16:48 ugugrab.uu
Example Two: Decoding a Binary File
Flavors: AT&T, BSD
Syntax:
uudecode [file]
Most of the email clients today can interpret an encoded message
that was made with the uuencode command and translate
the ASCII text back into a binary format. For those readers that can't,
you have to do it manually.
The manual process is a simple task of writing the email message
out to a file, stripping the header information and running uudecode.
After you write out the email message it should have all the header information
attached to the uuencoded file.
planet 20% vi /usr/tmp/top.tmp
>From xinu@ugu.com Sat Nov 28 23:34:44 1998
Received: from mail.ugu.com (0@mail.ugu.com [204.128.192.15]) by foo.com (8.8.8/8.7.6)
with ESMTP id XAA07347 for <friend@foo.com>; Sat, 28 Nov 1998 23:34:42 -0500 (EST)
Received: from ugu.com (ugu.com [206.18.19.10])
by mail.ugu.com (8.9.1/8.9.1) with ESMTP id UAA12667
for <friend@foo.com>; Sat, 28 Nov 1998 20:34:41 -0800 (PST)
Received: (from xinu@localhost)
by ugu.com (8.9.1/8.9.0) id UAA110274
for friend@foo.com; Sat, 28 Nov 1998 20:35:05 -0800 (PST)
Date: Sat, 28 Nov 1998 20:35:05 -0800 (PST)
From: UGU <xinu@ugu.com>
Message-Id: <199811290435.UAA110274@ugu.com>
Status: O
begin 755 top.new
M?T5,1@$"`0`````````````"``@````!$``9=````#0``'C0(```)``T`"``
M!P`H`!@`%P````8````T$```-!```#0```#@````X`````0````$`````P``
M`100``$4$``!%````!$````1````!`````%P```"```!*!```2@0``$H````
.
.
.
M`*$````(`````Q`!:"```&@@```%Y```````````````$`````````%M<```
M'@@```````````!P````!S````````````````$````````!E`````,`````
<````````=S````&>``````````````````````$`
`
end
Bring the email message file into an editor, and delete all email
header information that exists before the line that starts with the word begin.
begin 755 top.new
M?T5,1@$"`0`````````````"``@````!$``9=````#0``'C0(```)``T`"``
M!P`H`!@`%P````8````T$```-!```#0```#@````X`````0````$`````P``
M`100``$4$``!%````!$````1````!`````%P```"```!*!```2@0``$H````
.
.
.
M`*$````(`````Q`!:"```&@@```%Y```````````````$`````````%M<```
M'@@```````````!P````!S````````````````$````````!E`````,`````
<````````=S````&>``````````````````````$`
`
end
Now that the email message looks like a uuencoded file, write
the file out with the .uu extension appended to the end.
:w! /usr/tmp/top.uu
From the vi editor the w! forces the
file to be written out as /usr/tmp/top.uu, as long as you have the permission
in the directory to do so.
planet 21% ls -l top*
-rw------- 1 xinu 44694 Nov 28 23:59 top.tmp
-rw-r--r-- 1 xinu 43963 Nov 29 00:00 top.uu
Run the uudecode command on the file
/usr/tmp/top.uu to decrypt the file into its original binary state.
planet 22% uudecode top.uu
planet 23% ls -al top*
-rwxr-xr-x 1 xinu 31888 Nov 29 00:00 top.new*
-rw------- 1 xinu 44694 Nov 28 23:59 top.tmp
-rw-r--r-- 1 xinu 43963 Nov 29 00:00 top.uu
When the file is uudecoded the file binary file takes on the name
and the permissions passed on to it from the uuencode
command.
|
Note - In some flavors of UNIX,
the uudecode command strips out the header
information for you. Try it on your flavor first.
|
There is a nice tool available on the Internet called uudeview
that not only strips out any header information and performs a uudecode,
but also handles various decompression formats.
Reason
Many administrators forget about the use and benefit of uuencoding
files. Because newer mail clients handle the translation automatically, the
newer administrators get lazy and forget about the uuencode
and uudecode commands.
Real World Experience
There are no real tools in DOS to extract a uuencoded
file. When binary files are emailed across the network they are uuencoded.
Decoding the file back to its original state is all handled by the mail client.
Most administrators working in the PC world are unaware that these files are
actually encrypted by uuencoding them. They understand
only that it is a binary program that has been encrypted into an ASCII format.
If a malformed email ends up hanging your SMTP mail gateway, a UNIX system administrator
would have to figure out that a 50MB movie file of a pornographic nature is
holding up the entire company from getting its email.
The uuencode command can work well
by adding another level of security. You can take a compressed binary file and
uuencode it, split the file every 1000 lines (see section 6.11 "Splitting
Files"), and email all the split pieces to a destination address. The file
will have to be appended back together in the correct order, uudecoded, and
uncompressed back to its original state.
Other Resources
Man pages:
uuencode, uudecode,
mail
World Wide Web:
uudeview--http://www.informatik.uni-frankfurt.de/~fp/uudeview/
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