ASCII vs Binary

Zen and the Art of the Internet

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 ASCII vs Binary
In the example above, the file newthisweek.Z was transferred, but
supposedly not correctly.  The reason is this: in a normal ASCII
transfer (the default), certain characters are translated between
systems, to help make text files more readable.  However, when binary
files (those containing non-ASCII characters) are transferred, this
translation should not take place.  One example is a binary
program---a few changed characters can render it completely useless.
To avoid this problem, it's possible to be in one of two modes---ASCII
or binary.  In binary mode, the file isn't translated in any way.
What's on the remote system is precisely what's received.  The
commands to go between the two modes are:
ftp> ascii
200 Type set to A.   (Note the A, which signifies ASCII mode.)
ftp> binary
200 Type set to I.   (Set to Image format, for pure binary transfers.)
Note that each command need only be done once to take effect; if the
user types binary, all transfers in that session are done in
binary mode (that is, unless ascii is typed later).
The transfer of newthisweek.Z will work if done as:
ftp> binary
200 Type set to I.
ftp> get newthisweek.Z
200 PORT command successful.
150 Opening BINARY mode data connection for newthisweek.Z (42390 bytes).
226 Transfer complete.
local: newthisweek.Z remote: newthisweek.Z
42390 bytes received in 7.2 seconds (5.8 Kbytes/s)
Note: The file size (42390) is different from that done
in ASCII mode (42553) bytes; and the number 42390 matches the one
in the listing of UUNET's top directory.  We can be relatively sure
that we've received the file without any problems.