Custom TCP Monitor

The echo command should be working okay. You won't see a null character echoed to standard out, as it's not a printable character. But xxd shows the null character as 00 in hex. od shows it as the escape sequence \0. You'll also probably want to suppress the trailing newline character echo adds by default by adding \c to the end of the echo string. See below for some examples.

 

 

If you view the monitor script with a hex editor like xxd or od, are the ^@ chars actually 00 (null in hex)? Or are they literally ^ and @? Can you upload the file?

 

 

You might try converting the entire send string into either octal \000 or hex \x00 to avoid any encoding issues.

 

 

Example using hex format of \x00

 

echo -e "0123\x00456"| xxd

 

0000000: 3031 3233 0034 3536 0a 0123.456.

 

 

Example using octal format of \0000

 

$ echo -e "0123\0000456"| xxd

 

0000000: 3031 3233 0034 3536 0a 0123.456.

 

 

od (utility to dump files in octal and other formats) shows the escape sequences for the unprintable character(s):

 

 

Example using hex format of \x00

 

$ echo -e "0123\x00456"| od -t c

 

0000000 0 1 2 3 \0 4 5 6 \n

 

0000011

 

 

Example using octal format of \0000

 

$ echo -e "0123\0000456"| od -t c

 

0000000 0 1 2 3 \0 4 5 6 \n

 

0000011

 

 

You can prevent echo -e from appending a newline character to the end of the string using \c:

 

 

$ echo -e "0123\x00456\c"| od -t c

 

0000000 0 1 2 3 \0 4 5 6

 

0000010

 

 

Aaron