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README

daemon - turns other processes into daemons

DESCRIPTION

Daemon turns other processes into daemons. There are many tasks that need to be performed to correctly set up a daemon process. This can be tedious. Daemon performs these tasks for other processes. This is useful for writing daemons in languages other than C, C++ or Perl (e.g. /bin/sh, Java).

If you want to write daemons in languages that can link against C functions (e.g. C, C++), see libslack which contains the core functionality of daemon.

DETAILS

Daemon turns other processes into daemons. There are many tasks that need to be performed to correctly set up a daemon process. This can be tedious. Daemon performs these tasks for other processes.

The preparatory tasks that daemon performs for other processes are:

First revoke any setuid or setgid privileges that daemon may have been installed with (by system administrators who laugh in the face of danger).

Process command line options.

Change the root directory if the --chroot option was supplied.

Change the process uid and gid if the --user option was supplied. Only root can use this option. Note that the uid of daemon itself is changed, rather than just changing the uid of the client process.

Read the system configuration file(s) (/etc/daemon.conf and /etc/daemon.conf.d/* by default, or specified by the --config option) unless the --noconfig option was supplied. Then read the user's personal configuration file(s) (~/.daemonrc and ~/.daemonrc.d/*), if any. Note: The root directory and the user must be set before access to the configuration file(s) can be attempted so neither --chroot nor --user options may appear in the configuration file.

On BSD systems (except macOS), the system configuration file(s) are /usr/local/etc/daemon.conf and /usr/local/etc/daemon.conf.d/* by default.

On macOS, when installed via macports, the system configuration file(s) are /opt/local/etc/daemon.conf and /opt/local/etc/daemon.conf.d/*.

Disable core file generation to prevent leaking potentially sensitive information in daemons that are run by root (unless the --core option was supplied).

Become a daemon process:

  • If daemon was not invoked by init(8) (i.e. pid 1) or inetd(8):

    • Ignore SIGHUP signals in case the current process session leader terminates while attached to a controlling terminal, causing us to receive a SIGHUP signal before we start our own process session below. This can happen when daemon was invoked interactively via the shell builtin exec. When this initial process terminates below, the terminal emulator that invoked the shell also terminates, so daemon needs to protect itself from that.

    • Background the process to lose process group leadership.

    • Start a new process session.

    • Background the process again to lose process session leadership. Under SVR4 this prevents the process from ever gaining a controlling terminal. This is only necessary under SVR4, but is always done for simplicity. Note that ignoring SIGHUP signals earlier means that when the newly created process session leader terminates, then even if it has a controlling terminal open, the newly backgrounded process won't receive the corresponding SIGHUP signal that is sent to all processes in the process session's foreground process group, because it inherited signal dispositions from the initial process.

  • Change the current directory to the root directory so as not to hamper umounts.

  • Clear the umask to enable explicit file creation modes.

  • Close all open file descriptors. If daemon was invoked by inetd(8), stdin, stdout and stderr are left open, because they are open to a socket.

  • Open stdin, stdout and stderr to /dev/null, in case something requires them to be open. Of course, this is not done if daemon was invoked by inetd(8).

  • If the --name option was supplied, create and lock a file containing the process id of the daemon process. The presence of this locked file prevents two instances of a daemon with the same name from running at the same time. The default location of the pidfile is /var/run for root (/etc on Solaris, /opt/local/var/run on macOS when installed via macports), and /tmp for normal users. If the --pidfiles option was supplied, its argument specifies the directory in which the pidfile will be placed. If the --pidfile option was supplied, its argument specifies the name of the pidfile and the directory in which it will be placed.

If the --umask option was supplied, set the umask to its argument, which must be a valid three-digit octal mode. Otherwise, set the umask to 022, to prevent accidentally creating group- or world-writable files.

Set the current directory if the --chdir option was supplied.

Spawn the client command and wait for it to terminate. The client command can be specified as command line arguments, or as the argument of the --command option. If both the --command option and command line arguments are present, the client command is the result of appending the command line arguments to the argument of the --command option.

If the --output, --stdout and/or --stderr option were supplied, the client's standard output and/or standard error are captured by daemon, and sent to the respective syslog destinations.

When the client terminates, daemon respawns it if the --respawn option was supplied. If the client ran for less than 300 seconds (or the value of the --acceptable option), then daemon sees this as a failure. It will attempt to restart the client up to five times (or the value of the --attempts option), before waiting for 300 seconds (or the value of the --delay option). This gives the system administrator the chance to correct whatever is preventing the client from running successfully without overloading system resources. If the --limit option was supplied, daemon terminates after the specified number of respawn attempt bursts. The default is zero, which means never give up, never surrender.

When the client terminates, and the --respawn option wasn't supplied, daemon terminates as well.

If the --foreground option was supplied, the client process is run as a foreground process, and is not turned into a daemon at all. If daemon is connected to a terminal, then the client process will also be connected to it. If daemon is not connected to a terminal, but the client needs to be connected to a terminal, use the --pty option.

If the --bind option was supplied, on systems with systemd-logind or elogind, the client process will be terminated when the user logs out.

The --stop option sends a SIGTERM signal to a currently running named daemon, which causes it to terminate its client process (with a SIGTERM signal), and to then terminate itself.

The --restart option sends a SIGUSR1 signal to a currently running named daemon, which causes it to terminate its client process (with a SIGTERM signal). If the named daemon was started with the --respawn option, it will then restart the client. Otherwise, it will terminate itself.

The --signal option sends a user-specified signal directly to a currently running named daemon's client process.

The --running option reports whether or not a given named daemon process is currently running.

The --list option reports all the currently running named daemon processes.

URL: https://libslack.org/daemon
URL: https://raf.org/daemon
GIT: https://github.com/raforg/daemon
GIT: https://codeberg.org/raforg/daemon
Date: 20230824
Author: raf <raf@raf.org>