Based on Bash Scripting Tutorial for Beginners by Nana https://www.youtube.com/watch?v=PNhq_4d-5ek

nanalyse_logs.sh

#!/usr/bin/env bash

nanalyse_logs() {
# always good to declare the variable with declare here, or local inside a function
local LOG_DIR
local -a ERROR_PATTERNS #set it as an array since the start
local REPORT_FILE
local -a LOG_FILES # set it as an array since the start
local PATTERN_LIST
local WHEN

LOG_DIR="${HOME}/Downloads/Nana-youtube"
ERROR_PATTERNS=(
"ERROR"
"FATAL"
"CRITICAL"
)
# as te reports would be daily using just Y-m-d format
WHEN="$(date '+%F')"
# not recommended to be in the same directory, but for the example is good
REPORT_FILE="${LOG_DIR}/log_analysis_report-${WHEN}.txt"
# mapfile or readarray to store the found files in side the array
mapfile -t LOG_FILES < <(find "${LOG_DIR}" -name "*.log" -mtime -1)

#
printf "%s\n%s\n" "Analysing log files" "==============" | tee --append "${REPORT_FILE}"
printf "%s\n" "List of log files updated in last 24 hours" | tee --append "${REPORT_FILE}"
for LF in "${!LOG_FILES[@]}"; do
    printf "\t%s - %s\n" "$((LF+1))" "${LOG_FILES[LF]}" | tee --append "${REPORT_FILE}"
done

for LOG_FILE in "${LOG_FILES[@]}"; do

    # printf "\n%s\n"

    for PATTERN in "${ERROR_PATTERNS[@]}"; do
        PATTERN_LIST="$(grep --fixed-strings "${PATTERN}" "${LOG_FILE}")"
        printf "\nSearching %s logs in %s file...\n" "${PATTERN}" "${LOG_FILE}" | tee --append "${REPORT_FILE}"
        printf "%s\n" "${PATTERN_LIST}" | tee --append "${REPORT_FILE}"
        printf "Number of %s logs found in %s file is: %s\n" "${PATTERN}" "${LOG_FILE}" "${#PATTERN_LIST}" | tee --append "${REPORT_FILE}"
        
        if [[ "${#PATTERN_LIST}" -gt 10 ]];then
            printf "\n ⚠ ACTION REQUIRED: too many %s issues (%s) in log file %s.\n" "${PATTERN}" "${#PATTERN_LIST}" "${LOG_FILE}"
        fi
        unset PATTERN_LIST        
    done
 
done
}
nanalyse_logs || return 1