daemonize sh function

daemonize_function.sh

_daemonize()
{   #daemonize an external command
    #http://blog.n01se.net/blog-n01se-net-p-145.html
    [ -z "${1}" ] && return 1
    (   # 1. fork, to guarantee the child is not a process
        # group leader, necessary for setsid) and have the
        # parent exit (to allow control to return to the shell)

        # 2. redirect stdin/stdout/stderr before running child
        [ -t 0 ] && exec  </dev/null
        [ -t 1 ] && exec  >/dev/null
        [ -t 2 ] && exec 2>/dev/null
        if ! command -v "setsid" >/dev/null 2>&1; then
            # 2.1 guard against HUP and INT (in child)
            trap '' 1 2
        fi

        # 3. ensure cwd isn't a mounted fs so it does't block
        # umount invocations
        cd /

        # 4. umask (leave this to caller)
        # umask 0

        # 5. close unneeded fds
        #XCU 2.7 Redirection says: open files are represented by
        #decimal numbers starting with zero. The largest possible
        #value is implementation-defined; however, all
        #implementations shall support at least 0 to 9, inclusive,
        #for use by #the application.
        i=3; while [ "${i}" -le "9" ]; do
            eval "exec ${i}>&-"
            i="$(($i + 1))"
        done

        # 6. create new session, so the child has no
        # controlling terminal, this prevents the child from
        # accesing a terminal (using /dev/tty) and getting
        # signals from the controlling terminal (e.g. HUP, INT)
        if command -v "setsid" >/dev/null 2>&1; then
            exec setsid "$@"
        else
            if [ ! -f "${1}" ]; then
                "$@"
            else
                exec "$@"
            fi
        fi
    ) &
    # 2.2 guard against HUP (in parent)
    if ! command -v "setsid" >/dev/null 2>&1; then
        disown -h "${!}"
    fi
}