ps -eo ppid,pid,sid,stat,tty,comm | awk '{ if ($2 == $3 && $5 == "?") {print $0}; }'
首先，要注意，守护进程（daemon）和后台进程（background process）有区别。
守护进程是一种后台进程，但是，同时，它必须具备以下特性：
1. 没有控制它的tty
2. 必须是一个session leader
3. 必须是一个进程组的leader
4. 执行后台任务
5. root目录即为工作目录
6. umask设置为0
7. 文件描述符（file descriptor）都必须先关闭，然后再关联相应文件（或者设备），如果有需要的话。
Wiki原文摘录如下：
On a Unix-like system, the common method for a process to become a daemon, when the process is started from the command line or from a startup script such as an init script or a SystemStarter script, involves:
-
Dissociating from the controlling tty
-
Becoming a session leader
-
Becoming a process group leader
-
Executing as a background task by forking and exiting (once or twice). This is required sometimes for the process to become a session leader. It also allows the parent process to continue its normal execution.
-
Setting the root directory (/) as the current working directory so that the process does not keep any directory in use that may be on a mounted file system (allowing it to be unmounted).
-
Changing the umask to 0 to allow open(), creat(), et al. operating system calls to provide their own permission masks and not to depend on the umask of the caller
-
Closing all inherited files at the time of execution that are left open by the parent process, including file descriptors 0, 1 and 2 for the standard streams (stdin, stdout and stderr). Required files will be opened later.
-
Using a logfile, the console, or /dev/null as stdin, stdout, and stderr
实际上，当我们查找守护进程的时候，没有必要用到以上那么多的标准，我们只需要用两条：
1. 没有控制终端
2. session leader
所以我们可以用文中最开始提到的命令来查找：
ps -eo ppid,pid,sid,stat,tty,comm | awk '{ if ($2 == $3 && $5 == "?") {print $0}; }'
E.g
chenqi@pek-qchen1-d1:~/test [1] $ ps -eo ppid,pid,sid,stat,tty,comm | awk '{ if ($2 == $3 && $5 == "?") {print $0}; }'
0 1 1 Ss ? init
1 401 401 Ss ? systemd-udevd
1 435 435 Ssl ? rsyslogd
1 446 446 Ss ? dbus-daemon
1 520 520 Ss ? rpc.idmapd
1 530 530 Ss ? systemd-logind
1 575 575 Ss ? bluetoothd
1 750 750 Ss ? smbd
1 762 762 Ss ? rpcbind
1 779 779 Ss ? rpc.statd
1 829 829 Ssl ? ModemManager
1 950 950 Ssl ? NetworkManager
1 993 993 Ss ? cups-browsed
1 1424 1424 Ss ? nmbd
1 1550 1550 Ss ? acpid
1 1559 1559 Ss ? cron
1 1560 1560 Ss ? atd
1 1565 1565 Ss ? sshd
1 1612 1612 Ssl ? automount
1 1622 1622 Ssl ? named
1 1627 1627 Ssl ? whoopsie
1 1664 1664 Ss ? gdomap
1 1670 1670 Ss ? kerneloops
1 1721 1721 Ss ? rpc.mountd
1 1829 1829 Ss ? master
1 1998 1998 Ss ? apache2
1 2016 2016 SLsl ? lightdm
2561 2588 2588 Ss ? init
2588 2653 2653 Ss ? dbus-daemon
2588 2656 2656 Ss ? ssh-agent
2588 2663 2663 Ss ? upstart-event-b
2588 2667 2667 Ss ? window-stack-br
2588 2668 2668 Ssl ? ibus-daemon
2588 2681 2681 Ssl ? unity-settings-
2588 2686 2686 Ssl ? hud-service
2588 2687 2687 Ssl ? at-spi-bus-laun
2588 2688 2688 Ssl ? gnome-session
2588 2693 2693 Ssl ? unity-panel-ser
2588 2786 2786 Ssl ? indicator-messa
2588 2789 2789 Ssl ? indicator-bluet
2588 2792 2792 Ssl ? indicator-power
2588 2799 2799 Ssl ? indicator-datet
2588 2800 2800 Ssl ? indicator-sound
2588 2803 2803 Ssl ? indicator-print
2588 2866 2866 Ssl ? indicator-sessi
2588 2879 2879 Ssl ? indicator-appli
1 24409 24409 Ss ? cupsd