how to download a file using just bash and nothing else (no curl, wget, perl, etc.)

If you have bash 2.04 or above with the /dev/tcp pseudo-device enabled, you can download a file from bash itself.

Paste the following code directly into a bash shell (you don't need to save the code into a file for executing):

function __wget() {
    : ${DEBUG:=0}
    local URL=$1
    local tag="Connection: close"
    local mark=0

    if [ -z "${URL}" ]; then
        printf "Usage: %s \"URL\" [e.g.: %s http://www.google.com/]" \
               "${FUNCNAME[0]}" "${FUNCNAME[0]}"
        return 1;
    fi
    read proto server path <<<$(echo ${URL//// })
    DOC=/${path// //}
    HOST=${server//:*}
    PORT=${server//*:}
    [[ x"${HOST}" == x"${PORT}" ]] && PORT=80
    [[ $DEBUG -eq 1 ]] && echo "HOST=$HOST"
    [[ $DEBUG -eq 1 ]] && echo "PORT=$PORT"
    [[ $DEBUG -eq 1 ]] && echo "DOC =$DOC"

    exec 3<>/dev/tcp/${HOST}/$PORT
    echo -en "GET ${DOC} HTTP/1.1\r\nHost: ${HOST}\r\n${tag}\r\n\r\n" >&3
    while read line; do
        [[ $mark -eq 1 ]] && echo $line
        if [[ "${line}" =~ "${tag}" ]]; then
            mark=1
        fi
    done <&3
    exec 3>&-
}

Then you can execute it as from the shell as follows:

__wget http://example.iana.org/

Source: Moreaki's answer upgrading and installing packages through the cygwin command line?

Update: as mentioned in the comment, the approach outlined above is simplistic:

• the read will trashes backslashes and leading whitespace.
• Bash can't deal with NUL bytes very nicely so binary files are out.
• unquoted $line will glob.

Use lynx.

It is pretty common for most of Unix/Linux.

lynx -dump http://www.google.com

-dump: dump the first file to stdout and exit

man lynx

Or netcat:

/usr/bin/printf 'GET / \n' | nc www.google.com 80

Or telnet:

(echo 'GET /'; echo ""; sleep 1; ) | telnet www.google.com 80

Adapted from Chris Snow answer This can also handle binary transfer files

function __curl() {
  read proto server path <<<$(echo ${1//// })
  DOC=/${path// //}
  HOST=${server//:*}
  PORT=${server//*:}
  [[ x"${HOST}" == x"${PORT}" ]] && PORT=80

  exec 3<>/dev/tcp/${HOST}/$PORT
  echo -en "GET ${DOC} HTTP/1.0\r\nHost: ${HOST}\r\n\r\n" >&3
  (while read line; do
   [[ "$line" == $'\r' ]] && break
  done && cat) <&3
  exec 3>&-
}

• i break && cat to get out of read
• i use http 1.0 so there's no need to wait for/send a connection:close

You can test binary files like this

ivs@acsfrlt-j8shv32:/mnt/r $ __curl http://www.google.com/favicon.ico > mine.ico
ivs@acsfrlt-j8shv32:/mnt/r $ curl http://www.google.com/favicon.ico > theirs.ico
ivs@acsfrlt-j8shv32:/mnt/r $ md5sum mine.ico theirs.ico
f3418a443e7d841097c714d69ec4bcb8  mine.ico
f3418a443e7d841097c714d69ec4bcb8  theirs.ico