A script to incrementally poll a log file of timestamp-prefixed text records

retail.sh

#!/bin/bash

#
# A script to incrementally poll a log file.
# Subsequent invocations print lines added since the previous invocation.
# An assumption is that log lines begin with a timestamp 23 characters long (see/tweak the TS_LEN variable below).
# 
# The state is stored in the file <original_filename>.seen as a last_line_no + last_timestamp pair last seen.
# If the state file is not there, or the line_no in the target file has the last_timestamp
# different from the one in the state file, then the file is printed from the line 1.
#

set -eu

trap 'echo "bye"' EXIT 
echo "hi"

declare -i TS_LEN=23

declare -r LOG_FILE=$1
declare -r POS_FILE="${LOG_FILE}.seen"

if [[ -r ${LOG_FILE} ]]; then
    read -r -n${TS_LEN} log_stamp < "${LOG_FILE}"

    declare -r last_line=$(echo -n $(wc -l < "${LOG_FILE}"))
    first_line=1

    if [[ -s ${POS_FILE} ]]; then
        read seen_line seen_stamp < "${POS_FILE}"
        if [[ $log_stamp == $seen_stamp ]]; then
            first_line=$(( seen_line + 1 )) 
        fi
    fi

    echo "${last_line} ${log_stamp}" > "${POS_FILE}"
    sed -n "${first_line},${last_line}p" "${LOG_FILE}"
fi