Tcp socket read linux

How to open a TCP/UDP socket in a bash shell

Suppose you want to open a TCP/UDP socket on a Linux server for various reasons. For example, you want to check if a specific address/port is reachable. Or you want to fetch a remote web page or invoke a restful API for testing. Or you want to connect to a remote IRC server, etc. However, what if the Linux server you are on is very restrictive? On that server, none of standard tools such as netcat , curl or wget may be available, and you are pretty much left with the bash shell only.

In fact, one of built-in features of bash shell is to open TCP/UDP sockets via /dev/tcp (and /dev/udp ) device file. In the rest of this tutorial, let’s find out how to open a TCP/UDP socket, and read to and write from the socket in bash shell.

Open or Close a TCP/UDP Socket in Bash Shell

In a nutshell, you can open a TCP/UDP socket using the following syntax in bash shell.

The file descriptor is a unique non-negative integer associated with each socket. File descriptors 0, 1 and 2 are reserved for stdin , stdout and stderr , respectively. Thus you must specify 3 or higher (whichever is unused) as a file descriptor.

The protocol field can be either tcp or udp. The host and port fields are self-explanatory.

For example, to open a bi-directional TCP socket for xmodulo.com with HTTP port and file descriptor 3 :

Once opened, a read/write socket can be closed using the following syntax. The first command close an input connection, while the latter closes an output connection.

Read from or Write to a TCP/UDP Socket in Bash Shell

Once a socket is opened, you can write a message to or read a message from the socket.

To write a message stored in $MESSSAGE to a socket:

$ echo -ne $MESSAGE >&3 $ printf $MESSAGE >&3

To read a message from a socket and store it in $MESSAGE :

$ read -r -u -n $MESSAGE /dev/null)

TCP/UDP Socket Examples in Bash Shell

Here I present several shell script examples that open and use a TCP socket.

1. Fetch a remote web page and print its content.

#!/bin/bash exec 3<>/dev/tcp/xmodulo.com/80 echo -e "GET / HTTP/1.1rnhost: xmodulo.comrnConnection: closernrn" >&3 cat 
2. Display a remote SSH server version.
In fact, the above script can be shortened to the following one-liner:
3. Display the current time from nist.gov.
4. Check the Internet connectivity.
#!/bin/bash HOST=www.mit.edu PORT=80 (echo >/dev/tcp/$/$) &>/dev/null if [ $? -eq 0 ]; then echo "Connection successful" else echo "Connection unsuccessful" fi
 
5. Perform TCP port scanning against a remote host.
 
#!/bin/bash host=$1 port_first=1 port_last=65535 for ((port=$port_first; port/dev/tcp/$host/$port) >/dev/null 2>&1 && echo "$port open" done
 
Final Notes
 
Opening a socket in bash requires that the bash shell have net-redirections enabled (i.e., compiled with --enable-net-redirections ). Old distributions may have this feature disabled for bash , in which case you will encounter the following error.
 
/dev/tcp/xmodulo.com/80: No such file or directory
 
Besides bash , socket support is known to be available in other shells such as ksh or zsh .
 
If you find this tutorial helpful, I recommend you check out the series of bash shell scripting tutorials provided by Xmodulo.
 
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