0
\$\begingroup\$

This script is running indefinitely as the Linux background process.

I have put an enormous effort to make this POSIX shell script containing an infinite loop shut down tidily along with the operating system (TERM signal) and by me sending it a HUP signal.

Ok, we don't call it exception-handling, but I didn't find any other suitable tag for it. Also did not find an appropriate tag for the script's termination.

We have reviewed two big pieces of the puzzle already, so I cut it out.


readonly script_one_instance_lockfile="${HOME}/.PROGRAM/$(basename "${0}").lock"

# this is long and not relevant to this question, it does as its name says
# you can find a review on it here: https://codereview.stackexchange.com/q/204828/104270
print_error_and_exit() { ... }

is_number()
{
    [ "${1}" -eq "${1}" ] 2> /dev/null
}

# purpose is clear, i.e. to clean up some temp, lock files
# which were created during script execution and should not be left in place
cleanup_on_exit()
{
    [ -f "${script_one_instance_lockfile}" ] && rm "${script_one_instance_lockfile}"
}

# this function is merely for future expansions of the script
# it might very well be different from cleanup_on_exit
cleanup_on_signal()
{
    cleanup_on_exit
}

# here we define a generic function to handle signals
# treat them as errors with appropriate messages
# example calls:
#    kill -15   this_script_name    # POSIX; all shells compatible
#    kill -TERM this_script_name    # Bash and alike; newer shells
signal_handler_generic()
# expected arguments:
# $1 = signal code
{
    # check if exactly one argument has been passed
    [ "${#}" -eq 1 ] || print_error_and_exit "signal_handler_generic()" "Exactly one argument has not been passed!\\n\\tPassed: ${*}"

    # check if the argument is a number
    is_number "${1}" || print_error_and_exit "signal_handler_generic()" "The argument is not a number!\\n\\Signal code expected.\\n\\tPassed: ${1}"

    number_argument=${1}
    signal_code=${number_argument}
    case "${number_argument}" in
        1 ) signal_human_friendly='HUP'  ;;
        2 ) signal_human_friendly='INT'  ;;
        3 ) signal_human_friendly='QUIT' ;;
        6 ) signal_human_friendly='ABRT' ;;
        15) signal_human_friendly='TERM' ;;
        * ) signal_human_friendly=''     ;;
    esac

    if [ "${signal_human_friendly}" = "" ]
    then
        print_error_and_exit "signal_handler_generic()" "Given number code (${signal_code}) does not correspond to supported signal codes."
    else
        # tidy up any temp or lock files created along the way
        cleanup_on_signal
        # print human friendly and number signal code that has been caught
        print_error_and_exit "\\ntrap()" "Caught ${signal_human_friendly} termination signal ${signal_code}.\\n\\tClean-up finished. Exiting. Bye!"
    fi
}

# use the above function for signal handling;
# note that the SIG* constants are undefined in POSIX,
# and numbers are to be used for the signals instead
trap 'signal_handler_generic 1' 1
trap 'signal_handler_generic 2' 2
trap 'signal_handler_generic 3' 3
trap 'signal_handler_generic 6' 6
trap 'signal_handler_generic 15' 15

# this is long and not relevant to this question, it does as its name says
# you can find a review on it here: https://codereview.stackexchange.com/q/213156/104270
is_java_program_running() { ... }

####################
### MAIN PROGRAM ###
####################

if [ -f "${script_one_instance_lockfile}" ]
then
    # if one instance of this script is already running, quit to shell
    print_error_and_exit "\\nmain()" "One instance of this script should already be running.\\n\\tLock file: ${script_one_instance_lockfile}\\n\\tMore than one instance is not allowed. Exiting."
else
    # create a .lock file for one instance handling
    touch "${script_one_instance_lockfile}"
fi

# keep the PROGRAM alive forever, check in 5 seconds interval
while true
do
    sleep 5s

    if ! is_java_program_running "${PROGRAM_java_process_identifier}"
    then
        my_date_time=$(date +%Y-%m-%d_%H:%M:%S)
        printf '%s  %s\n' "${my_date_time}" "(re-)starting PROGRAM"
        ( /full/path/to/program > /dev/null 2>&1 & )
    fi
done

# ordinary scripts don't have infinite loops
# this code shall be unreachable, but it is good to have it here since I will be
# copying / reusing the script and I would definitely forget on this
cleanup_on_exit
\$\endgroup\$
  • \$\begingroup\$ Have you actually found a Unix or kill that doesn't deal with signal names? man 7 signal says that POSIX does define those names. \$\endgroup\$ – Oh My Goodness Feb 11 at 11:19
  • \$\begingroup\$ @OhMyGoodness Renamed it shortly after posting the question. \$\endgroup\$ – Vlastimil Feb 11 at 11:28

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