Sending printable characters
If you send electronic mail to someone, the message you send
must consist only of printable characters. If you wish to send
other data you must encode it into a form containing only ordinary
text. The reason for this is that some networks interpret some
non-printing characters as instructions, which could cause messages
to go astray or their contents to be changed. The command
uuencode takes a file and writes to standard output a
representation of that file containing only ASCII characters; the
command uudecode takes a file and performs the reverse
operation. Either one or two arguments are needed by
uuencode - the second one is the name of the file as
it will be known when decoded (which is not necessarily the same as
the name of the file you are encoding). The first argument, if
there are two, is the file to be encoded (standard input is encoded
if there is only one argument). The format of the file after
encoding is a sequence of lines commencing with a header
line and terminating with end on a line of its
own. The header line consists of three fields - the word
begin, the access permissions the file should have
after it is decoded, and the name of the file after decoding. For
example, suppose we have a file A containing an
'alert' character (ctrl-G), and we wish to mail it to
sam, and to be received with name
chris_file. We can check what the file contains using
od, which will confirm that \a (i.e.
ctrl-G) is indeed included in chris_file
$ od -t c A
0000000 h e l l o \a \n
0000007
This file can now be coded using uuencode. Note
that the output is sent to standard output:
$ uuencode A chris_file
begin u=rw,go= chris_file
':&5L;&\'"@CP
(line containing a single blank space)
end
So, to send the encoded file to sam, we merely pipe
the output to mailx:
$ uuencode A chris_file | mailx -s "Binary
file" sam
The resulting file can then be recreated by sam
storing the mail message in (say) mailfile and
typing:
$ uudecode mailfile
Any lines before begin and after end
are ignored by uudecode, so you don't need to worry
about any extra header lines the mailer inserts into your message.
Try this yourself - choose a file, uuencode it, mail
it to a friend, and get them to uudecode it. Have a
look at the encoded version, and the final decoded file, and
convince yourself that it does in fact work.
Try now encoding a large file, say
/usr/dict/words:
$ uuencode /usr/dict/words
tmpfile
Look at the output - it consists of lines of fixed width (61
characters) commencing with the letter M:
...
M=&4*87)B;E86P*87)B;W)E='5M"F%R8G5T=7,*87)C"F%R8V%D90I!<F-A
M9&EA"F%R8N80IA<F-A;F4*87)C8V]S"F%R8V-O<VEN90IA<F-H"F%R8VAA
M90IA<F-H8C"F%R8VAA:7-M"F%R8VAA;F=E; IA<F-H8FES:&]P"F%R8VAD
...
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