Append an 'offset' command to the script. The set value of this object should be the offset from the beginning of the data area of the TCP/UDP packet to start matching the string configured in the last bexpect command. The 'offset ' characters will be automatically appended. Also a 'null terminator'(\0) will be automatically appended. The null terminator and the 'offset ' characters will be counted as characters in the script. When read, the return value has no significant meaning, but a 1 is always returned.