shell-process

#!/bin/sh -efu
### This file is covered by the GNU General Public License,
### which should be included with libshell as the file LICENSE.
### All copyright information are listed in the COPYING.

if [ -z "${__included_shell_process-}" ]; then
__included_shell_process=1

. shell-error

### This function checks the PID from a pidfile and writes a new one.
### Usage: daemon_write_pid /path/to/pidfile [force]
daemon_write_pid()
{
	local pid='' pidfile
	pidfile="$1"

	if [ -z "${2-}" ]; then
		read -r pid < "$pidfile" ||:

		if [ -n "$pid" ] && [ -z "${pid##[0-9]*}" ]; then
			if kill -0 "$pid"; then
				verbose "Process with pid '$pid' still exists."
				return 1
			fi
		fi
	fi 2>/dev/null

	printf '%s\n' "$$" >"$pidfile"
}

### Closes descriptors
### Usage: daemon_close_fd [fd0 fd1 ...]
daemon_close_fd()
{
	while [ $# -gt 0 ]; do
		eval "exec $1>&-"
		shift
	done
}

daemon_noclose=
daemon_nokill=
daemon_nolock=
daemon_err_file=
daemon_log_file=
daemon_pid_file=

### The function is for programs wishing to detach themselves
### from the controlling terminal and run in the background as
### system daemons.
###
### This function forks and if the fork(2) succeeds, the parent
### calls "exit 0", so that further errors are seen by the child only.
###
### On success daemon() returns 0 or 1 if an error occurred.
###
### Variables:
### daemon_noclose   - do not close first 3-9 descriptors;
### daemon_nokill    - do not check previous pid;
### daemon_nolock    - do not lock the pid file;
### daemon_err_file  - specifies error log file (default /dev/null);
### daemon_log_file  - specifies log file (default /dev/null);
### daemon_pid_file  - specifies pid file.
###
### Usage: daemon [program args]
daemon()
{
	local i fd

	if [ -z "${__shell_process_daemon-}" ]; then
		local sid=
		! type setsid >/dev/null 2>&1 ||
			sid=setsid

		### Call fork and setsid.
		__shell_process_daemon=1 $sid "$0" ${1:+"$@"} &

		exit 0
	fi

	### Close unneeded file descriptors.
	if [ -z "${daemon_noclose-}" ]; then
		if [ -d /proc/self/fd ]; then
			# shellcheck disable=SC2035
			# Disable `Use ./*glob* or -- *glob* so names with dashes won't become options`
			# because printf does not regard the arguments as options.
			fd="$(set +f; cd /proc/self/fd; printf '%s ' *;)"
			fd="${fd#\* }"
			for i in $fd; do
				[ $i -lt 3 ] ||
					daemon_close_fd $i
			done
		else
			### All shell implementations shall support at least 0 to 9,
			### inclusive, for use by the application.
			daemon_close_fd 3 4 5 6 7 8 9
		fi
	fi

	if [ -n "${daemon_pid_file-}" ]; then
		if [ -z "${daemon_nolock-}" ]; then
			fd="${daemon_lock_fd:-3}"

			eval "exec $fd<>$daemon_pid_file"

			type flock >/dev/null 2>&1 ||
				fatal "Unable to lock pid file without 'flock' utility."

			if ! flock -n $fd; then
				verbose "Another process has locked pidfile."
				return 1
			fi
		fi

		### Write our pid.
		if ! daemon_write_pid "$daemon_pid_file" "${daemon_nokill-}"; then
			verbose "Unable to write pidfile."
			return 1
		fi
	fi

	### Redirect std{out,err} to files or /dev/null.
	exec 0</dev/null 1>>${daemon_log_file:-/dev/null} 2>>${daemon_err_file:-/dev/null}

	### Guard against terminal signals.
	trap : HUP INT QUIT

	### Change working directory to rootfs.
	cd /

	### Set umask to 0 to clear file mode creation mask.
	umask 0
}

fi #__included_shell_process