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I'm using Arch Linux. I have read about systemd, and as I understand it, systemd is the first process, and it starts the rest of the processes.
But when I use:
ps -aux
The result shows that /sbin/init has PID 1. And when I use:
pstree -Apn
The result shows that systemd has PID 1. Which is correct? Is /sbin/init starting systemd?
867
Init and Systemd are both init daemons but it is better to use the latter since it is commonly used in recent Linux Distros. Init uses service whereas Systemd uses systemctl to manage Linux services.
The init daemon is going to be replaced with daemon systemd on some of the Linux Distributions, while a lot of them have already implemented it. This is/will be creating a huge gap between traditional Unix/Linux Guard and New Linux Guard – programmers and System Admins.
Before systemd , the mainstream default for the init process was a reworking of the Unix System V init. There were other choices available, but System V init was the standard option in most non-Berkeley Software Distribution (BSD) derived distributions.
The /sbin/init program (also called init ) coordinates the rest of the boot process and configures the environment for the user. When the init command starts, it becomes the parent or grandparent of all of the processes that start up automatically on the system.
They're probably both right.
$ sudo ls -ltrh /proc/1/exe
[sudo] password for user:
lrwxrwxrwx 1 root root 0 May 30 21:22 /proc/1/exe -> /lib/systemd/systemd
$ echo $(tr '\0' ' ' < /proc/1/cmdline )
/sbin/init splash
$ stat /sbin/init
File: '/sbin/init' -> '/lib/systemd/systemd'
Size: 20 Blocks: 0 IO Block: 4096 symbolic link
Device: 801h/2049d Inode: 527481 Links: 1
Access: (0777/lrwxrwxrwx) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2017-05-30 21:27:12.058023583 -0500
Modify: 2016-10-26 08:04:58.000000000 -0500
Change: 2016-11-19 11:38:45.749226284 -0600
Birth: -
The commands above show us:
exec()) when pid 1 was started?On my system, /sbin/init is a symlink to "/lib/systemd/systemd". This is likely similar to your system. We can see what information ps -aux is using by straceing it.
$ strace ps -aux
...
open("/proc/1/cmdline", O_RDONLY) = 6
read(6, "/sbin/init\0splash\0", 131072) = 18
read(6, "", 131054) = 0
close(6) = 0
...
and likewise for pstree:
$ strace pstree -Apn
...
getdents(3, /* 332 entries */, 32768) = 8464
open("/proc/1/stat", O_RDONLY) = 4
stat("/proc/1", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
fstat(4, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
read(4, "1 (systemd) S 0 1 1 0 -1 4194560"..., 8192) = 192
read(4, "", 7168) = 0
open("/proc/1/task", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 5
...
So the difference in output is because they use different sources of information. /proc/1/cmdline tells us how the process was invoked. Whereas /proc/1/stat shows that the process' name is systemd.
$ cat /proc/1/stat
1 (systemd) S 0 1 1 0 -1 4194560 34371 596544 1358 3416 231 144 298 1758 20 0 1 0 4 190287872 772 18446744073709551615 1 1 0 0 0 0 671173123 4096 1260 0 0 0 17 2 0 0 12188 0 0 0 0 0 0 0 0 0 0
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