~/src]$ cat get_file
#!/bin/bash
# By Ainsley Pereira

# This script is designed to be a primitive web/ftp client, to pull a file
# from somewhere on the Internet when you have nothing but the base
# LFS. Generally, it's recommended that you use it one time -- to pull
# a Web or FTP client to get the rest of the stuff you need.

# If somebody doesn't know how to call the script, tell them.
if [ x$1 = "x" ]
        then echo "USAGE: $0 <URL>"
fi

# Pick apart the URL, and break it into the pieces we need.
URL=${1##http://} &&
SERVER=${URL%%/*} &&
FULLFILENAME=${URL#$SERVER} &&
FILENAME=${FULLFILENAME##*/}

echo URL=$URL
echo SERVER=$SERVER
echo FULLNAME=$FULLFILENAME
echo FILENAME=$FILENAME

# There are two important (and somewhat unusual pieces to this
# next command (or series of commands -- it gets tricky trying to
# say which it is.)

# The first is the "3<>" part. This says to open file descriptor
# 3 for BOTH input and output.
# The second important piece is "/dev/tcp". There is no actual entry in /dev
# for this -- it's a construct of bash (actually, more properly
# it's a construct of readline). It allows you to do TCP interactions
# with any machine. The format is "/dev/tcp/<address>/<port>". <address> can
# be either an IP address or a hostname. A hostname gets resolved just like
# it does with any other TCP/IP application. Since we are using HTTP
# in this case, the port will be 80.

# First, we send the HTTP command to file descriptor 3. Note the "&" on
# the end. Without that, we'll never get out of the command so that we
# can execute the next one.
(echo -e "GET $FULLFILENAME HTTP/0.9\r\n\r\n" 1>&3 &

# Just like we rerouted stdout (file descriptor 1) to FD3 in the last
# command, we reroute stdin (FD0) from FD3 in this one.
# We also tell bash that we want to open FD3 for both input and output
# at the same time.
# Since our earlier command was written to it, what happens once FD3
# is built and associated with the remote server, is that that server
# gets sent "GET <path/filname> HTTP/0.9". The server very obediently
# sends back that file.
cat 0<&3) 3<> /dev/tcp/$SERVER/80 |\
(
# The "(" and ")" around all of these statements form a subshell.
# One reason it is done is that it only requires one output
# statement to handle all the commands in the subshell. In this
# case, it has a more important purpose: it allows us to feed info
# to all the commands with one INPUT. (The pipe, which we can only
# have one of.
# Without the subshell, the pipe would read into the first "read i",
# and that's all that would ever happen.

# Prime the variable "i" so the while will work properly.
read i
# As long as there's HTTP text coming back from the GET request, keep
# reading.
while [ x"$(echo $i | tr -d '\r')" != "x" ]
do
        read i
done
# Copy whatever's coming in (from fd3) to stdout (which is about to
# be redirected to $FILENAME).
cat
) >$FILENAME