Объяснение кода UNIX при создании файла управления

Question

Я новичок в разработке UNIX, и у меня есть следующий код, я считаю, что он связан с файлом управления, но я его вообще не понимаю, если кто-нибудь может мне объяснить, i «буду очень благодарен

Это код

ARCH_CTRL_LOCK="$RUTA_CFG/servicioPortabilidad.ctl" 
if [ ! -f $ARCH_CTRL_LOCK ]; then 
    (echo "$$" > $ARCH_CTRL_LOCK) 2> /dev/null 
else 
    PID_GUARDADO=`cat $ARCH_CTRL_LOCK` 
    if [ -d /proc/${PID_GUARDADO} ]; then 
    echo "Proceso no iniciado, el archivo $ARCH_CTRL_LOCK existe y su PID esta activo (${PID_GUARDADO}), puede estar ejecutandose otra instancia de este proceso." 
    exit 2 
    fi 
    kill -0 ${PID_GUARDADO} 2> /dev/null 
    echo "" 
    if [ "$?" = "0" ]; then 
    echo "Proceso no iniciado, el archivo $ARCH_CTRL_LOCK existe y su PID responde a senhales (${PID_GUARDADO}), puede estar ejecutandose otra instancia de este proceso." 
    exit 2 
    fi 
    (echo "$$" > $ARCH_CTRL_LOCK) 2> /dev/null 
fi 

спасибо

Answer 1

# Store the name of the lock file in a variable, so we can use 
# it repeatedly. 
ARCH_CTRL_LOCK="$RUTA_CFG/servicioPortabilidad.ctl" 

# See if the lock file exists. 
if [ ! -f $ARCH_CTRL_LOCK ]; then 
    # The lock file doesn't exist. 

    # Write my own process ID into the file, creating it 
    (echo "$$" > $ARCH_CTRL_LOCK) 2> /dev/null 

else 
    # The lock file DOES exist. 

    # Get the process ID from the existing lock file. This should be 
    # the ID of the process that created the lock file. 
    PID_GUARDADO=`cat $ARCH_CTRL_LOCK` 

    # Looking for a directory named /proc/PID is a way of checking 
    # whether that process exists. This only works on operating systems 
    # that have a /proc filesystem. Linux always has /proc, but many 
    # other systems (like Mac OS X for example) don't. 

    if [ -d /proc/${PID_GUARDADO} ]; then 
    # The process still exists. 

    # Print an error message. 
    echo "Proceso no iniciado, el archivo $ARCH_CTRL_LOCK existe y su PID esta activo (${PID_GUARDADO}), puede estar ejecutandose otra instancia de este proceso." 

    # Exit with status 2. 
    # All statuses except 0 mean there was an error. 
    exit 2 
    fi 

    # The process doesn't still exist. But the kill command that 
    # follows checks for the process in a different way, that works 
    # on any POSIX system. The kill command exits with status 0 
    # (meaning success) if the process exists, and exists with some 
    # other status (meaning error) if the process doesn't exist. 
    kill -0 ${PID_GUARDADO} 2> /dev/null 

    # The following echo command prints a blank line. It also makes 
    # the shell forget about the exit status of the kill command. 
    echo "" 

    # Now see if the exit status from the last command is zero. I 
    # guess this was intended to check the exit status of kill, but 
    # someone inserted that echo command, which should always exit 
    # with status 0, so the test will always pass! 
    if [ "$?" = "0" ]; then 
    # The exit status of the last command was 0. This should always 
    # be the case. The echo command above should always succeed. 

    # Print an error message. 
    echo "Proceso no iniciado, el archivo $ARCH_CTRL_LOCK existe y su PID responde a senhales (${PID_GUARDADO}), puede estar ejecutandose otra instancia de este proceso." 

    # Exit with status 2. 
    exit 2 
    fi 

    # We can never reach this point, because the echo above should 
    # always succeed, so we always print the error message and exit 
    # in the previous if statement. 

    # Overwrite contents of the lock file with my own process ID. 
    (echo "$$" > $ARCH_CTRL_LOCK) 2> /dev/null 
fi 

Technically, команда echo "" может потерпеть неудачу, но только если стандартный вывод put был закрыт или перенаправлен в файл на полную файловую систему или каким-либо другим способом настроен таким образом, что он не подлежит написанию. Это очень маловероятно.