Blog Rss FeedWhen executing a command on a /dev/tcp/$host/$port pseudo-device, Bash opens a TCP connection to the associated socket.
Exemple using TCP:
cat </dev/tcp/nist1.symmetricom.com/13 54496 08-01-31 02:30:53 00 0 3 345.7 UTC(NIST) *
Exemple using UDP:
$ exec 4<>/dev/udp/172.16.2.200/7 $ echo -e "Networking with bash" >&4 && head -1 <&4 Networking with bash $ exec 4>&- $ exec 4<&-
In this example I open a datagram (UDP) connection to my home server (172.16.2.200) on the echo service (port 7). The file descriptor is open in read/write mode.
An almost useful program
This is an example of a small stripped down version of wget (yes very stripped down) written in bash.
The line `exec 6<>/dev/tcp/$1/80` opens a socket in read write mode. Since we are using HTTP protocol 1.1 we need to pass the host name in the header with the line `Host: $1`. Also by default HTTP version 1.1 keeps all the connections alive unless specified otherwise. The line `Connection: close` tells the http server to close the connection once the response has been sent. I we forget this line the script will stay connected until the server times out.
#!/bin/bash # PATH=${2-/} exec 6<>/dev/tcp/$1/80 echo -e "GET ${PATH} HTTP/1.1" >&6 echo -e "Host: $1\nConnection: close\n" >&6 echo "Reading: http://${1}${PATH}" >&2 #read and discard the header while read <&6 do LINE=${REPLY//$'\r'/} if [ -z "$LINE" ]; then break fi done #read the page while read <&6 do echo -n $REPLY >&1 done # close the file descriptor exec 6<&- exec 6>&-
This program takes two arguments, the server name and the path to the file to download.
$ ./wget.sh www.google.com '/search?q=bsd' > ~/result.html Reading: http://www.google.com/search?q=bsd $
There is one caveat though. The version of bash you are using needs to be compiled with the flag --enable-net-redirections. This code should work on Mac, *BSD and most of the linux distributions (some Debian by default use a version of bash without net redirection). I don't know about the bash version running on Windows. The last version of windows I ever used was Windows 95.