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#Let us Learn Systemd
The systemd is the new system and service manager in Linux mainly from Fedora 18 and now adopted by many major Linux distro. The Systemd replaces the Upstart init daemon while providing backward compatibility for legacy System V ( Run Control scripts - RC ) service scripts.
The "systemd" offers the following benefits over init:
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Services are started in parallel wherever possible using socket-based activation and D-Bus.
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Daemons can be started on demand.
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Processes are tracked using control groups (cgroups).
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Snapshotting of the system state and restoration of the system state from a snapshot is supported.
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Mount points can be configured as systemd targets.
The "systemd" is the first process that starts after the system boots, and is the final process that is running when the system shuts down. systemd controls the final stages of booting and prepares the system for use. systemd also speeds up booting by loading services concurrently.
When a Linux system boots, it performs the following operations:
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The computer's BIOS performs a power-on self-test (POST), and then locates and initializes any peripheral devices including the hard disk.
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The BIOS reads the Master Boot Record (MBR) into memory from the boot device. The MBR stores information about the organization of partitions on that device. On a computer with x86 architecture, the MBR occupies the first 512 bytes of the boot device. The first 446 bytes contain boot code that points to the boot loader program, which can be on the same device or on another device. The next 64 bytes contain the partition table. The final two bytes are the boot signature, which is used for error detection (MBR = 512 Bytes. 446 (Boot code) + 64 (Partition Table) + 2 (Signature)= 512.
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The default boot loader program used on most of the Linux is GRUB 2, which stands for GRand Unified Bootloader version 2. The boot loader loads the vmlinuz kernel image file into memory and extracts the contents of the initramfs image file into a temporary, memory-based file system (tmpfs).
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The kernel loads the driver modules from the initramfs file system that are needed to access the root file system.
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The kernel starts the systemd process with a process ID of 1 (PID 1). systemd is the ancestor of all processes on a system. systemd reads its configuration from files in the /etc/systemd directory. The /etc/systemd/system.conf file controls how systemd handles system initialization.
Example:
[root@gcpadman-laptop ~]# ps 1
PID TTY STAT TIME COMMAND
1 ? Ss 0:02 /usr/lib/systemd/systemd --switched-root --system --deserialize 24
- systemd reads the file linked by /etc/systemd/system/default.target, for example /usr/lib/systemd/system/multi-user.target, to determine the default system target. The system target file defines the services that systemd starts. systemd brings the system to the state defined by the system target, performing system initialization tasks such as:
Example:
[root@gcpadman-laptop ~]# ls -l /etc/systemd/system/default.target
lrwxrwxrwx. 1 root root 36 Jul 25 12:24 /etc/systemd/system/default.target -> /lib/systemd/system/graphical.target
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Setting the host name.
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Initializing the network.
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Initializing SELinux based on its configuration.
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Printing a welcome banner.
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Initializing the system hardware based on kernel boot arguments.
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Mounting the file systems, including virtual file systems such as the /proc file system.
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Cleaning up directories in /var.
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Starting swapping.
###Note: Check this inittab file. This means no more inittab and only it's dead body present.
[root@gcpadman-laptop ~]# cat /etc/inittab
# inittab is no longer used.
#
# ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.
#
# Ctrl-Alt-Delete is handled by /usr/lib/systemd/system/ctrl-alt-del.target
#
# systemd uses 'targets' instead of runlevels. By default, there are two main targets:
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# multi-user.target: analogous to runlevel 3
# graphical.target: analogous to runlevel 5
#
# To view current default target, run:
# systemctl get-default
#
# To set a default target, run:
# systemctl set-default TARGET.target
Now let us try to understand this by doing some reverse engineering rather then just reading. Please take example of any disabled service ( I have taken cups ):
[root@gcpadman-laptop ~]# systemctl list-unit-files --type service | grep cups
cups-browsed.service disabled
cups.service disabled
Let me try to enable this service using systemctl and carefully check the output;
[root@gcpadman-laptop ~]# systemctl enable cups.service
Created symlink from /etc/systemd/system/printer.target.wants/cups.service to /usr/lib/systemd/system/cups.service.
Created symlink from /etc/systemd/system/sockets.target.wants/cups.socket to /usr/lib/systemd/system/cups.socket.
Created symlink from /etc/systemd/system/multi-user.target.wants/cups.path to /usr/lib/systemd/system/cups.path.
So systemctl created below 3 soft links for me..!!
[root@gcpadman-laptop sockets.target.wants]# ls -l /etc/systemd/system/*/cups*
lrwxrwxrwx 1 root root 33 Aug 4 21:25 /etc/systemd/system/multi-user.target.wants/cups.path -> /usr/lib/systemd/system/cups.path
lrwxrwxrwx 1 root root 36 Aug 4 21:25 /etc/systemd/system/printer.target.wants/cups.service -> /usr/lib/systemd/system/cups.service
lrwxrwxrwx 1 root root 35 Aug 4 21:25 /etc/systemd/system/sockets.target.wants/cups.socket -> /usr/lib/systemd/system/cups.socket
When /etc/inittab itself is not available then their must not be any run levels !!!! that is 100% right, looks like someone copied this concept from Solaris 10 where they use SMF concept to manage system runlevel and to manage services.
Systemd has replaced runlevels with target. Let us understand important targets;
[root@gcpadman-laptop ~]# systemctl list-units --type target --all
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
bluetooth.target loaded active active Bluetooth
cryptsetup.target loaded active active Encrypted Volumes
emergency.target loaded inactive dead Emergency Mode
getty.target loaded active active Login Prompts
graphical.target loaded active active Graphical Interface
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network-online.target loaded active active Network is Online
network-pre.target loaded inactive dead Network (Pre)
network.target loaded active active Network
nfs-client.target loaded active active NFS client services
nss-lookup.target loaded inactive dead Host and Network Name Lookups
nss-user-lookup.target loaded active active User and Group Name Lookups
paths.target loaded active active Paths
remote-fs-pre.target loaded active active Remote File Systems (Pre)
remote-fs.target loaded active active Remote File Systems
rescue.target loaded inactive dead Rescue Mode
shutdown.target loaded inactive dead Shutdown
slices.target loaded active active Slices
sockets.target loaded active active Sockets
sound.target loaded active active Sound Card
swap.target loaded active active Swap
sysinit.target loaded active active System Initialization
● syslog.target not-found inactive dead syslog.target
time-sync.target loaded inactive dead System Time Synchronized
timers.target loaded active active Timers
umount.target loaded inactive dead Unmount All Filesystems
After seeing the above example I feel for a sysadmin below are some of the important targets;
- basic.target
- emergency.target
- graphical.target
- multi-user.target
- rescue.target
- shutdown.target
- sysinit.target