Master Linux Boot: From GRUB to systemd on CentOS 5/6/7

grub and boot

CentOS 5/6 boot process

GRUB

CentOS 7 boot process

CentOS 5/6 Boot Process

initrd / initramfs

Usually stored in the /boot directory as files ending with .img, initrd is a small root filesystem image containing essential modules needed before the kernel can mount the real root filesystem.

rc

In the inittab file there is a line si::sysinit:/etc/rc.d/rc.sysinit which passes the run‑level value to the rc script.

Below is the core part of the rc script:

# Main two loops:
    for i in /etc/rc$runlevel.d/K* ; do
        subsys=${i#/etc/rc$runlevel.d/K??}
        [ -f /var/lock/subsys/$subsys -o -f /var/lock/subsys/$subsys.init ] || continue
        check_runlevel "$i" || continue
        [ -n "$UPSTART" ] && initctl emit --quiet stopping JOB=$subsys
        $i stop
        [ -n "$UPSTART" ] && initctl emit --quiet stopped JOB=$subsys
    done

    for i in /etc/rc$runlevel.d/S* ; do
        subsys=${i#/etc/rc$runlevel.d/S??}
        [ -f /var/lock/subsys/$subsys ] && continue
        [ -f /var/lock/subsys/$subsys.init ] && continue
        check_runlevel "$i" || continue
        if [ "$do_confirm" = "yes" ]; then
            confirm $subsys
            rc=$?
            if [ "$rc" = "1" ]; then
                continue
            elif [ "$rc" = "2" ]; then
                do_confirm="no"
            fi
        fi
        update_boot_stage "$subsys"
        [ -n "$UPSTART" ] && initctl emit --quiet starting JOB=$subsys
        if [ "$subsys" = "halt" -o "$subsys" = "reboot" ]; then
            export LC_ALL=C
            exec $i start
        fi
        $i start
        [ -n "$UPSTART" ] && initctl emit --quiet started JOB=$subsys
    done

The script selects the appropriate run‑level, then stops services prefixed with K and starts those prefixed with S.

chkconfig updates the services enabled for each run‑level.

chkconfig [--add | --del] name          # add or delete a service
chkconfig --list name                 # list service status per run‑level
chkconfig [--level levels] [--type type] [--no-redirect] name <on|off|reset|resetpriorities>  # set service state for a specific run‑level

GRUB

After BIOS reads the boot order, the MBR (first 446 bytes) contains the boot loader, which in CentOS is typically GRUB.

GRUB consists of three stages: stage1 (in the MBR), stage1.5 (usually in /boot), and stage2 (the full loader).

GRUB configuration file: /boot/grub/grub.conf (or /boot/grub/menu.lst as a symlink).

default=0          # default boot entry (0 = first)
timeout=5          # wait time in seconds
splashimage=(hd0,6)/boot/grub/splash.xpm.gz
hiddenmenu         # hide the menu

title Fedora Core (2.6.11-1.1369_FC4)
    root (hd0,6)
    kernel /boot/vmlinuz-2.6.11-1.1369_FC4 ro root=LABEL=/
    initrd /boot/initrd-2.6.11-1.1369_FC4.img

title WinXP
    rootnoverify (hd0,0)
    chainloader +1

Manual boot commands in GRUB:

grub> find /PATH               # locate a path
grub> root (hd0,1)            # set root device
grub> kernel /vmlinux... ro   # specify kernel
grub> initrd /initrd...img    # specify initramfs
grub> boot                    # start booting

GRUB installation and repair:

grub-install --root-directory=/boot /dev/sda   # write stage1 to MBR

In the GRUB console:

grub> chroot /mnt/sysimage   # mount real rootfs
grub> root (hd0,1)          # set boot partition
grub> setup (hd0)           # write stage1 to MBR

CentOS 7 Boot Process

CentOS 7 replaces the traditional init with systemd, which also manages daemons and provides many new features.

Parallel service startup with dependency ordering

On‑demand activation

Snapshot and state restoration

Cgroup resource limits

Parallel service startup and dependency

systemd groups services into units (e.g., .service, .mount, .socket, .device, .swap, .path, .target, .timer) and starts them according to declared dependencies.

On‑demand activation

Unlike init, which starts all enabled services at boot, systemd activates services only when they are needed, improving boot speed and resource usage.

Snapshot and state restoration

systemd can save the current system state as a snapshot and restore it later.

Targets

systemd replaces SysV run‑levels with targets, offering finer granularity.

SysV runlevel   systemd target          description
0               runlevel0.target, poweroff.target   halt
1, s, single    runlevel1.target, rescue.target   single‑user mode
2, 4            runlevel2.target, runlevel4.target, multi-user.target   multi‑user (no GUI)
3               runlevel3.target, multi-user.target   multi‑user (no GUI)
5               runlevel5.target, graphical.target   multi‑user with GUI
6               runlevel6.target, reboot.target   reboot
emergency       emergency.target   emergency shell

systemd startup diagram

systemctl commands

Common systemctl usage:

systemctl show [NAME.unittype]          # show status or list
systemctl status [NAME.unittype]        # detailed status
systemctl list-units [--type TYPE] [--all]
systemctl list-unit-files --type TYPE [--all]
systemctl start [NAME.unittype]
systemctl stop [NAME.unittype]
systemctl restart [NAME.unittype]
systemctl reload [NAME.unittype]
systemctl enable [NAME.unittype]
systemctl disable [NAME.unittype]
systemctl isolate [NAME.target]
systemctl get-default
systemctl set-default [NAME.target]
systemctl snapshot
systemctl hibernate
systemctl suspend
systemctl hybrid-sleep
systemctl reboot
systemctl halt

systemd unit files

/etc/systemd/system.conf                # systemd configuration file
/lib/systemd/system/NAME.UNIT           # unit files location
/etc/systemd/system/NAME.UNIT.wants     # services wanted by a target

# Example of /etc/systemd/system/ directory listing
[root@xiao ~]# ll /etc/systemd/system/
 total 32
 drwxr-xr-x. 2 root root 4096 Nov 21 2014 basic.target.wants
 lrwxrwxrwx. 1 root root 37 Nov 21 2014 default.target -> /lib/systemd/system/multi-user.target
 drwxr-xr-x. 2 root root 4096 Nov 21 2014 default.target.wants
 drwxr-xr-x. 2 root root 4096 Nov 21 2014 getty.target.wants
 drwxr-xr-x. 2 root root 4096 Apr 8 14:29 multi-user.target.wants
 drwxr-xr-x. 2 root root 4096 Nov 21 2014 printer.target.wants
 drwxr-xr-x. 2 root root 4096 Nov 21 2014 sockets.target.wants
 drwxr-xr-x. 2 root root 4096 Mar 22 17:48 sysinit.target.wants
 drwxr-xr-x. 2 root root 4096 Nov 21 2014 system-update.target.wants

# Switch default target manually
[root@xiao system]# rm -f default.target
[root@xiao system]# ln -s /lib/systemd/system/graphical.target default.target