How can I close a netcat connection after a certain character is returned in the response?

Question

We have a very simple tcp messaging script that cats some text to a server port which returns and displays a response.

The part of the script we care about looks something like this:

cat someFile | netcat somehost 1234 

The response the server returns is 'complete' once we get a certain character code (specifically &001C) returned.

How can I close the connection when I receive this special character?

(Note: The server won't close the connection for me. While I currently just CTRL+C the script when I can tell it's done, I wish to be able to send many of these messages, one after the other.)

(Note: netcat -w x isn't good enough because I wish to push these messages through as fast as possible)

Answer 1

Create a bash script called client.sh:

#!/bin/bash  cat someFile  while read FOO; do         echo $FOO >&3         if [[ $FOO =~ `printf ".*\x00\x1c.*"` ]]; then                 break         fi done 

Then invoke netcat from your main script like so:

3>&1 nc -c ./client.sh somehost 1234 

(You'll need bash version 3 for the regexp matching).

This assumes that the server is sending data in lines - if not you'll have to tweak client.sh so that it reads and echoes a character at a time.

Answer 2

How about this?

Client side:

awk -v RS=$'\x1c' 'NR==1;{exit 0;}'  < /dev/tcp/host-ip/port 

Testing:

# server side test script while true; do ascii -hd; done | { netcat -l 12345; echo closed...;} # Generate 'some' data for testing & pipe to netcat. # After netcat connection closes, echo will print 'closed...'  # Client side: awk -v RS=J 'NR==1; {exit;}' < /dev/tcp/localhost/12345 # Changed end character to 'J' for testing. # Didn't wish to write a server side script to generate 0x1C. 

Client side produces:

    0 NUL    16 DLE    32      48 0    64 @    80 P    96 `   112 p     1 SOH    17 DC1    33 !    49 1    65 A    81 Q    97 a   113 q     2 STX    18 DC2    34 "    50 2    66 B    82 R    98 b   114 r     3 ETX    19 DC3    35 #    51 3    67 C    83 S    99 c   115 s     4 EOT    20 DC4    36 $    52 4    68 D    84 T   100 d   116 t     5 ENQ    21 NAK    37 %    53 5    69 E    85 U   101 e   117 u     6 ACK    22 SYN    38 &    54 6    70 F    86 V   102 f   118 v     7 BEL    23 ETB    39 '    55 7    71 G    87 W   103 g   119 w     8 BS     24 CAN    40 (    56 8    72 H    88 X   104 h   120 x     9 HT     25 EM     41 )    57 9    73 I    89 Y   105 i   121 y    10 LF     26 SUB    42 *    58 :    74 

After 'J' appears, server side closes & prints 'closed...', ensuring that the connection has indeed closed.