More on Using Bash's Built-in /dev/tcp File (TCP/IP)

If you saw yesterday's Tech Tip and were looking for more on using TCP/IP with bash's built-in /dev/tcp device file then read on. Here, we'll both read from, and write to a socket.

Before I go any further, let me state that this is based on something I discovered here on Dave Smith's Blog. All I've done here is added a few improvements based on the comments to the original post. I've also added a bit of additional explanation.

The following script fetches the front page from Google:

exec 3<>/dev/tcp/www.google.com/80
echo -e "GET / HTTP/1.1\r\nhost: http://www.google.com\r\nConnection: close\r\n\r\n" >&3
cat <&3

Pretty simple, just 3 lines. The first line may be a bit confusing if you haven't seen this type of thing before. This line causes file descriptor 3 to be opened for reading and writing on the specified TCP/IP socket. This is a special form of the exec statement. From the bash man page:

exec [-cl] [-a name] [command [arguments]]

... If command is not specified, any redirections take effect in the current shell, and the return status is 0.

So using exec without a command is a way to open files in the current shell.

After the socket is open we send our HTTP request out the socket with the echo ... >&3 command. The request consists of:

GET / HTTP/1.1
host: http://www.google.com
Connection: close

Each line is followed by a carriage-return and newline, and all the headers are followed by a blank line to signal the end of the request (this is all standard HTTP stuff).

Next we read the response out of the socket using cat <&3, which reads the response and prints it out. The response being the main HTML page from Google:

$ bash tcp.sh
HTTP/1.1 200 OK
Date: Wed, 30 Sep 2009 17:28:36 GMT
Expires: -1
Cache-Control: private, max-age=0
Content-Type: text/html; charset=ISO-8859-1
Set-Cookie: PREF=ID=...
Set-Cookie: NID=27=...
Server: gws
X-XSS-Protection: 0
Transfer-Encoding: chunked
Connection: close

fef
<!doctype html><html><head><meta ...

And that's it, with just a few more lines of code you could have your own bash based browser... well maybe not.

author

Mitch Frazier is an embedded systems programmer at Emerson Electric Co. Mitch has been a contributor to and a friend of Linux Journal since the early 2000s.

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