Version : 2021.01
Updated : 2021/11/01 15:52
The most commonly used boot loader is called GRUB (Grand Unified Boot Loader), however historically there have been others:
The Boot Loader is generally placed in the MBR (Master Boot Record) of the disk on which the system to-be-booted resides. The MBR format is as follows:
Important - Note that you can also install the boot loader in what is known as the PBR (Partition Boot Record).
Since 2011, the BIOS is being steadily replaced by UEFI (Unified Extensible Firmware Interface). Systems using a CPU other than the x86 or the x86-64 use non-BIOS software such as OpenFirmware or EFI .
EFI relies on boot loaders stored in a disk partition called the EFI System Partition or ESP. This partition is normally mounted by Linux at /boot/efi. The boot loaders reside in files having a .efi extension stored in subdirectories named after the OS to be booted.
The EFI firmware includes a boot manager that enables you to choose which OS to boot. In order for EFI to work each boot loader must be registered with the firmware.
GRUB 2 is a complete rewrite of GRUB LEGACY.
GRUB 2 is modular in design.
Grub2 reads its entries from the /boot/grub/grub.cfg file:
<code>
[root@centos8 ~]# cat /boot/grub2/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#
### BEGIN /etc/grub.d/00_header ###
set pager=1
if [ -f ${config_directory}/grubenv ]; then
load_env -f ${config_directory}/grubenv
elif [ -s $prefix/grubenv ]; then
load_env
fi
if [ "${next_entry}" ] ; then
set default="${next_entry}"
set next_entry=
save_env next_entry
set boot_once=true
else
set default="${saved_entry}"
fi
if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
else
menuentry_id_option=""
fi
export menuentry_id_option
if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi
function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}
function load_video {
if [ x$feature_all_video_module = xy ]; then
insmod all_video
else
insmod efi_gop
insmod efi_uga
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
fi
}
terminal_output console
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=5
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=5
fi
### END /etc/grub.d/00_header ###
### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
set tuned_initrd=""
### END /etc/grub.d/00_tuned ###
### BEGIN /etc/grub.d/01_menu_auto_hide ###
if [ "${boot_success}" = "1" -o "${boot_indeterminate}" = "1" ]; then
set last_boot_ok=1
else
set last_boot_ok=0
fi
# Reset boot_indeterminate after a successful boot
if [ "${boot_success}" = "1" ] ; then
set boot_indeterminate=0
# Avoid boot_indeterminate causing the menu to be hidden more then once
elif [ "${boot_indeterminate}" = "1" ]; then
set boot_indeterminate=2
fi
set boot_success=0
save_env boot_success boot_indeterminate
if [ x$feature_timeout_style = xy ] ; then
if [ "${menu_show_once}" ]; then
unset menu_show_once
save_env menu_show_once
set timeout_style=menu
set timeout=60
elif [ "${menu_auto_hide}" -a "${last_boot_ok}" = "1" ]; then
set orig_timeout_style=${timeout_style}
set orig_timeout=${timeout}
if [ "${fastboot}" = "1" ]; then
# timeout_style=menu + timeout=0 avoids the countdown code keypress check
set timeout_style=menu
set timeout=0
else
set timeout_style=hidden
set timeout=1
fi
fi
fi
### END /etc/grub.d/01_menu_auto_hide ###
### BEGIN /etc/grub.d/01_users ###
if [ -f ${prefix}/user.cfg ]; then
source ${prefix}/user.cfg
if [ -n "${GRUB2_PASSWORD}" ]; then
set superusers="root"
export superusers
password_pbkdf2 root ${GRUB2_PASSWORD}
fi
fi
### END /etc/grub.d/01_users ###
### BEGIN /etc/grub.d/10_linux ###
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1' 2ae4c035-9244-458c-82c5-a49ae169cdb6
else
search --no-floppy --fs-uuid --set=root 2ae4c035-9244-458c-82c5-a49ae169cdb6
fi
insmod part_msdos
insmod ext2
set boot='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=boot --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1' 2ae4c035-9244-458c-82c5-a49ae169cdb6
else
search --no-floppy --fs-uuid --set=boot 2ae4c035-9244-458c-82c5-a49ae169cdb6
fi
# This section was generated by a script. Do not modify the generated file - all changes
# will be lost the next time file is regenerated. Instead edit the BootLoaderSpec files.
#
# The blscfg command parses the BootLoaderSpec files stored in /boot/loader/entries and
# populates the boot menu. Please refer to the Boot Loader Specification documentation
# for the files format: https://www.freedesktop.org/wiki/Specifications/BootLoaderSpec/.
set default_kernelopts="root=UUID=4c0cc28c-0d59-45be-bd73-d292b80be33c ro crashkernel=auto resume=UUID=c8bb3f47-d67f-4b21-b781-766899dc83d4 rhgb quiet "
insmod blscfg
blscfg
### END /etc/grub.d/10_linux ###
### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###
### BEGIN /etc/grub.d/20_ppc_terminfo ###
### END /etc/grub.d/20_ppc_terminfo ###
### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###
### BEGIN /etc/grub.d/30_uefi-firmware ###
### END /etc/grub.d/30_uefi-firmware ###
### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###
### BEGIN /etc/grub.d/41_custom ###
if [ -f ${config_directory}/custom.cfg ]; then
source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###
</file>
==The /etc/default/grub file==
This file contains the default global configuration for GRUB 2:
<code>
[root@centos8 ~]# cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto resume=UUID=c8bb3f47-d67f-4b21-b781-766899dc83d4 rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true
Important : Any change made to this file requires the execution of the grub2-mkconfig command in order for the changes to become effective.
The most important directives in the above file are as follows:
| Directive | Description |
|---|---|
| GRUB_TIMEOUT | Indicates the time to wait for user input prior to booting the default OS |
| GRUB_DISTRIBUTOR | Set by distributors of GRUB to their identifying name. This is used to generate more informative menu entry titles. |
| GRUB_DEFAULT | Indicates the default OS to boot. A value of 0 indicates the OS referenced by the first stanza commencing with the menuentry keyword |
| GRUB_CMDLINE_LINUX | Command-line arguments to add to menu entries for the Linux kernel. |
The files in this directory are executed in a numerical order and are used to build the stanzas in the /boot/grub/grub.cfg file:
[root@centos8 ~]# ls -l /etc/grub.d total 92 -rwxr-xr-x. 1 root root 8958 Mar 2 15:51 00_header -rwxr-xr-x. 1 root root 1043 Jun 15 2020 00_tuned -rwxr-xr-x. 1 root root 232 Mar 2 15:51 01_users -rwxr-xr-x. 1 root root 832 Mar 2 15:51 08_fallback_counting -rwxr-xr-x. 1 root root 14088 Mar 2 15:51 10_linux -rwxr-xr-x. 1 root root 830 Mar 2 15:51 10_reset_boot_success -rwxr-xr-x. 1 root root 889 Mar 2 15:51 12_menu_auto_hide -rwxr-xr-x. 1 root root 11696 Mar 2 15:51 20_linux_xen -rwxr-xr-x. 1 root root 2559 Mar 2 15:51 20_ppc_terminfo -rwxr-xr-x. 1 root root 10670 Mar 2 15:51 30_os-prober -rwxr-xr-x. 1 root root 1412 Mar 2 15:51 30_uefi-firmware -rwxr-xr-x. 1 root root 214 Mar 2 15:51 40_custom -rwxr-xr-x. 1 root root 216 Mar 2 15:51 41_custom -rw-r--r--. 1 root root 483 Mar 2 15:51 README
The Initramfs INITial Ram File System file is a minimal system image which is initialised upon system boot.
The file format is cramFS that is to say archived using cpio and compressed with gzip.
[root@centos8 ~]# cp /boot/initramfs-4.18.0-240.22.1.el8_3.x86_64.img /tmp/custom [root@centos8 ~]# cd /tmp [root@centos8 tmp]# ls cpio.list custom dateref incremental.tar mbr.save systemd-private-9af7a2f7444849578f55b306bfd9f820-chronyd.service-iQiNzF tblpart.save test.cpio test.print test.tar tmp.iso vg0_backup [root@centos8 tmp]# mv custom custom.gz [root@centos8 tmp]# gunzip custom.gz [root@centos8 tmp]# mkdir initramfs [root@centos8 tmp]# cd initramfs [root@centos8 initramfs]# cpio -cid -I ../custom 216 blocks [root@centos8 initramfs]# ls bin dev etc init lib lib64 proc root run sbin shutdown sys sysroot tmp usr var
To examine the current image, you need to use the lsinitrd command:
[root@centos8 tmp]# lsinitrd custom | more Image: custom: 25M ======================================================================== Early CPIO image ======================================================================== drwxr-xr-x 3 root root 0 Feb 22 10:57 . -rw-r--r-- 1 root root 2 Feb 22 10:57 early_cpio drwxr-xr-x 3 root root 0 Feb 22 10:57 kernel drwxr-xr-x 3 root root 0 Feb 22 10:57 kernel/x86 drwxr-xr-x 2 root root 0 Feb 22 10:57 kernel/x86/microcode -rw-r--r-- 1 root root 109568 Feb 22 10:57 kernel/x86/microcode/GenuineIntel.bin ======================================================================== Version: dracut-049-95.git20200804.el8_3.4 Arguments: -f --kver '4.18.0-240.22.1.el8_3.x86_64' dracut modules: bash systemd systemd-initrd nss-softokn rngd i18n network-legacy network ifcfg drm plymouth prefixdevname kernel-modules kernel-modules-extra kernel-network-modules resume rootfs-block terminfo udev-rules biosdevname dracut-systemd usrmount base fs-lib memstrack microcode_ctl-fw_dir_override shutdown ======================================================================== drwxr-xr-x 12 root root 0 Feb 22 10:57 . crw-r--r-- 1 root root 5, 1 Feb 22 10:57 dev/console crw-r--r-- 1 root root 1, 11 Feb 22 10:57 dev/kmsg crw-r--r-- 1 root root 1, 3 Feb 22 10:57 dev/null crw-r--r-- 1 root root 1, 8 Feb 22 10:57 dev/random crw-r--r-- 1 root root 1, 9 Feb 22 10:57 dev/urandom lrwxrwxrwx 1 root root 7 Feb 22 10:57 bin -> usr/bin drwxr-xr-x 2 root root 0 Feb 22 10:57 dev drwxr-xr-x 11 root root 0 Feb 22 10:57 etc -rw-r--r-- 1 root root 30 Nov 10 2020 etc/centos-release drwxr-xr-x 2 root root 0 Feb 22 10:57 etc/cmdline.d drwxr-xr-x 2 root root 0 Feb 22 10:57 etc/conf.d -rw-r--r-- 1 root root 124 Feb 22 10:57 etc/conf.d/systemd.conf --More--
RHEL/CentOS 8 uses systemd. For this reason the init script is a soft link to /usr/lib/systemd/systemd :
[root@centos8 tmp]# lsinitrd custom | grep usr/lib/systemd/systemd | grep init lrwxrwxrwx 1 root root 23 Feb 22 10:57 init -> usr/lib/systemd/systemd
The Kernel Booting Process is divided into 6 stages:
| Stage | Description |
|---|---|
| Kernel loader loading, setup and configuration | In this step, the bootsect.s file is loaded into the memory by the BIOS. When the bootsect.s file sets up, it loads the rest of the kernel into the memory. |
| Parameter setup and switch to 32-bit mode | When the kernel has been loaded, the boot.s file sets up a temporary IDT and GDT and handles the switch to 32-bit mode. |
| Kernel decompression | The head.s file decompresses the kernel. |
| Kernel setup | After the kernel is decompressed, the real GDT and IDT are created by the head.s (second file). |
| Kernel and memory initialisation | In this step, the kernel sets up all memory constraints and virtual memory is completely set up. |
| Init process creation | In the final step of booting, the init process is created, which switches a Linux computer to different runlevels. |
The init_post() function then tries to execute one of the following in the order shown:
An error at this stage results in a Kernel Panic.
Systemd uses Units and Targets. A unit can be:
Examples of Targets are:
Systemd uses Targets in a similar way to the use of run-levels by SysVinit. To facilitate the transition to Systemd, certain Targgets are soft links:
[root@centos8 ~]# ls -l /usr/lib/systemd/system/runlevel* lrwxrwxrwx. 1 root root 15 Apr 7 16:55 /usr/lib/systemd/system/runlevel0.target -> poweroff.target lrwxrwxrwx. 1 root root 13 Apr 7 16:55 /usr/lib/systemd/system/runlevel1.target -> rescue.target lrwxrwxrwx. 1 root root 17 Apr 7 16:55 /usr/lib/systemd/system/runlevel2.target -> multi-user.target lrwxrwxrwx. 1 root root 17 Apr 7 16:55 /usr/lib/systemd/system/runlevel3.target -> multi-user.target lrwxrwxrwx. 1 root root 17 Apr 7 16:55 /usr/lib/systemd/system/runlevel4.target -> multi-user.target lrwxrwxrwx. 1 root root 16 Apr 7 16:55 /usr/lib/systemd/system/runlevel5.target -> graphical.target lrwxrwxrwx. 1 root root 13 Apr 7 16:55 /usr/lib/systemd/system/runlevel6.target -> reboot.target /usr/lib/systemd/system/runlevel1.target.wants: total 0 /usr/lib/systemd/system/runlevel2.target.wants: total 0 /usr/lib/systemd/system/runlevel3.target.wants: total 0 /usr/lib/systemd/system/runlevel4.target.wants: total 0 /usr/lib/systemd/system/runlevel5.target.wants: total 0
To list unit files, use the systemctl command with the list-units sub-command:
[root@centos8 ~]# systemctl list-units UNIT LOAD ACTIVE SUB DESCRIPTION proc-sys-fs-binfmt_misc.automount loaded active waiting Arbitrary Executable File Formats File System Automount Point sys-devices-pci0000:00-0000:00:01.1-ata2-host1-target1:0:0-1:0:0:0-block-sr0.device loaded active plugged CD-ROM sys-devices-pci0000:00-0000:00:03.0-net-enp0s3.device loaded active plugged 82540EM Gigabit Ethernet Controller (PRO/1000 MT Desktop Adapter) sys-devices-pci0000:00-0000:00:05.0-sound-card0.device loaded active plugged 82801AA AC'97 Audio Controller sys-devices-pci0000:00-0000:00:0d.0-ata3-host2-target2:0:0-2:0:0:0-block-sda-sda1.device loaded active plugged VBOX_HARDDISK 1 sys-devices-pci0000:00-0000:00:0d.0-ata3-host2-target2:0:0-2:0:0:0-block-sda-sda2.device loaded active plugged VBOX_HARDDISK 2 sys-devices-pci0000:00-0000:00:0d.0-ata3-host2-target2:0:0-2:0:0:0-block-sda-sda3.device loaded active plugged VBOX_HARDDISK 3 sys-devices-pci0000:00-0000:00:0d.0-ata3-host2-target2:0:0-2:0:0:0-block-sda.device loaded active plugged VBOX_HARDDISK sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb1.device loaded active plugged VBOX_HARDDISK 1 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb10.device loaded active plugged VBOX_HARDDISK 10 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb11.device loaded active plugged VBOX_HARDDISK my_ext4 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb12.device loaded active plugged VBOX_HARDDISK 12 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb2.device loaded active plugged VBOX_HARDDISK 2 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb3.device loaded active plugged VBOX_HARDDISK 3 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb4.device loaded active plugged VBOX_HARDDISK 4 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb5.device loaded active plugged VBOX_HARDDISK 5 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb6.device loaded active plugged VBOX_HARDDISK 6 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb7.device loaded active plugged VBOX_HARDDISK 7 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb8.device loaded active plugged VBOX_HARDDISK 8 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb-sdb9.device loaded active plugged VBOX_HARDDISK 9 sys-devices-pci0000:00-0000:00:0d.0-ata4-host3-target3:0:0-3:0:0:0-block-sdb.device loaded active plugged VBOX_HARDDISK sys-devices-platform-serial8250-tty-ttyS0.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS0 sys-devices-platform-serial8250-tty-ttyS1.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS1 sys-devices-platform-serial8250-tty-ttyS2.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS2 sys-devices-platform-serial8250-tty-ttyS3.device loaded active plugged /sys/devices/platform/serial8250/tty/ttyS3 sys-devices-virtual-block-dm\x2d0.device loaded active plugged /sys/devices/virtual/block/dm-0 sys-devices-virtual-block-dm\x2d1.device loaded active plugged /sys/devices/virtual/block/dm-1 sys-devices-virtual-net-virbr0.device loaded active plugged /sys/devices/virtual/net/virbr0 sys-devices-virtual-net-virbr0\x2dnic.device loaded active plugged /sys/devices/virtual/net/virbr0-nic sys-module-configfs.device loaded active plugged /sys/module/configfs sys-subsystem-net-devices-enp0s3.device loaded active plugged 82540EM Gigabit Ethernet Controller (PRO/1000 MT Desktop Adapter) sys-subsystem-net-devices-virbr0.device loaded active plugged /sys/subsystem/net/devices/virbr0 sys-subsystem-net-devices-virbr0\x2dnic.device loaded active plugged /sys/subsystem/net/devices/virbr0-nic -.mount loaded active mounted Root Mount boot.mount loaded active mounted /boot dev-hugepages.mount loaded active mounted Huge Pages File System dev-mqueue.mount loaded active mounted POSIX Message Queue File System run-user-1000.mount loaded active mounted /run/user/1000 sys-kernel-config.mount loaded active mounted Kernel Configuration File System sys-kernel-debug.mount loaded active mounted Kernel Debug File System sys-kernel-tracing.mount loaded active mounted /sys/kernel/tracing var-lib-nfs-rpc_pipefs.mount loaded active mounted RPC Pipe File System cups.path loaded active running CUPS Scheduler systemd-ask-password-plymouth.path loaded active waiting Forward Password Requests to Plymouth Directory Watch systemd-ask-password-wall.path loaded active waiting Forward Password Requests to Wall Directory Watch init.scope loaded active running System and Service Manager session-96.scope loaded active running Session 96 of user trainee atd.service loaded active running Job spooling tools auditd.service loaded active running Security Auditing Service avahi-daemon.service loaded active running Avahi mDNS/DNS-SD Stack chronyd.service loaded active running NTP client/server crond.service loaded active running Command Scheduler cups.service loaded active running CUPS Scheduler dbus.service loaded active running D-Bus System Message Bus dracut-shutdown.service loaded active exited Restore /run/initramfs on shutdown firewalld.service loaded active running firewalld - dynamic firewall daemon lines 1-57
To see inactive units, use the following command:
[root@centos8 ~]# systemctl list-units --all | grep inactive | more ● boot.automount not-found inactive dead boot.automount proc-fs-nfsd.mount loaded inactive dead NFSD configuration filesystem proc-sys-fs-binfmt_misc.mount loaded inactive dead Arbitrary Executable File Formats File System sys-fs-fuse-connections.mount loaded inactive dead FUSE Control File System ● sysroot.mount not-found inactive dead sysroot.mount tmp.mount loaded inactive dead Temporary Directory (/tmp) var-lib-machines.mount loaded inactive dead Virtual Machine and Container Storage systemd-ask-password-console.path loaded inactive dead Dispatch Password Requests to Console Directory Watch ● apparmor.service not-found inactive dead apparmor.service auth-rpcgss-module.service loaded inactive dead Kernel Module supporting RPCSEC_GSS cpupower.service loaded inactive dead Configure CPU power related settings ● display-manager.service not-found inactive dead display-manager.service dm-event.service loaded inactive dead Device-mapper event daemon dnf-makecache.service loaded inactive dead dnf makecache dracut-cmdline.service loaded inactive dead dracut cmdline hook dracut-initqueue.service loaded inactive dead dracut initqueue hook dracut-mount.service loaded inactive dead dracut mount hook dracut-pre-mount.service loaded inactive dead dracut pre-mount hook dracut-pre-pivot.service loaded inactive dead dracut pre-pivot and cleanup hook dracut-pre-trigger.service loaded inactive dead dracut pre-trigger hook dracut-pre-udev.service loaded inactive dead dracut pre-udev hook ebtables.service loaded inactive dead Ethernet Bridge Filtering tables emergency.service loaded inactive dead Emergency Shell initrd-cleanup.service loaded inactive dead Cleaning Up and Shutting Down Daemons initrd-parse-etc.service loaded inactive dead Reload Configuration from the Real Root initrd-switch-root.service loaded inactive dead Switch Root initrd-udevadm-cleanup-db.service loaded inactive dead Cleanup udevd DB ● ip6tables.service not-found inactive dead ip6tables.service ● ipset.service not-found inactive dead ipset.service ● iptables.service not-found inactive dead iptables.service iscsi-onboot.service loaded inactive dead Special handling of early boot iSCSI sessions iscsi.service loaded inactive dead Login and scanning of iSCSI devices iscsid.service loaded inactive dead Open-iSCSI iscsiuio.service loaded inactive dead iSCSI UserSpace I/O driver ldconfig.service loaded inactive dead Rebuild Dynamic Linker Cache libvirt-guests.service loaded inactive dead Suspend/Resume Running libvirt Guests libvirtd.service loaded inactive dead Virtualization daemon loadmodules.service loaded inactive dead Load legacy module configuration ● lvm2-activation.service not-found inactive dead lvm2-activation.service lvm2-lvmpolld.service loaded inactive dead LVM2 poll daemon mdmonitor.service loaded inactive dead Software RAID monitoring and management microcode.service loaded inactive dead Load CPU microcode update ● network.service not-found inactive dead network.service nfs-blkmap.service loaded inactive dead pNFS block layout mapping daemon nfs-convert.service loaded inactive dead Preprocess NFS configuration convertion nfs-idmapd.service loaded inactive dead NFSv4 ID-name mapping service nfs-mountd.service loaded inactive dead NFS Mount Daemon nfs-server.service loaded inactive dead NFS server and services nfs-utils.service loaded inactive dead NFS server and client services nfsdcld.service loaded inactive dead NFSv4 Client Tracking Daemon nftables.service loaded inactive dead Netfilter Tables ● ntpd.service not-found inactive dead ntpd.service ● ntpdate.service not-found inactive dead ntpdate.service plymouth-switch-root.service loaded inactive dead Plymouth switch root service rc-local.service loaded inactive dead /etc/rc.d/rc.local Compatibility rescue.service loaded inactive dead Rescue Shell rpc-gssd.service loaded inactive dead RPC security service for NFS client and server --More--
The black dots above are white on the screen. These dots mean that the unit has not been found on the system. For example:
[root@centos8 ~]# systemctl status ntpd Unit ntpd.service could not be found.
To see unit status data, use the following command:
[root@centos8 ~]# systemctl list-unit-files | more UNIT FILE STATE proc-sys-fs-binfmt_misc.automount static -.mount generated boot.mount generated dev-hugepages.mount static dev-mqueue.mount static proc-fs-nfsd.mount static proc-sys-fs-binfmt_misc.mount static sys-fs-fuse-connections.mount static sys-kernel-config.mount static sys-kernel-debug.mount static tmp.mount disabled var-lib-machines.mount static var-lib-nfs-rpc_pipefs.mount static cups.path enabled systemd-ask-password-console.path static systemd-ask-password-plymouth.path static systemd-ask-password-wall.path static session-96.scope transient arp-ethers.service disabled atd.service enabled auditd.service enabled auth-rpcgss-module.service static autovt@.service enabled avahi-daemon.service enabled blk-availability.service disabled chrony-dnssrv@.service static chrony-wait.service disabled chronyd.service enabled cockpit-motd.service static cockpit-wsinstance-http-redirect.service static cockpit-wsinstance-http.service static cockpit-wsinstance-https-factory@.service static cockpit-wsinstance-https@.service static cockpit.service static console-getty.service disabled container-getty@.service static cpupower.service disabled crond.service enabled cups-browsed.service disabled cups.service enabled dbus-org.fedoraproject.FirewallD1.service enabled dbus-org.freedesktop.Avahi.service enabled dbus-org.freedesktop.hostname1.service static dbus-org.freedesktop.import1.service static dbus-org.freedesktop.locale1.service static dbus-org.freedesktop.login1.service static dbus-org.freedesktop.machine1.service static dbus-org.freedesktop.nm-dispatcher.service enabled dbus-org.freedesktop.portable1.service static dbus-org.freedesktop.timedate1.service enabled dbus.service static debug-shell.service disabled dm-event.service static dnf-makecache.service static dnsmasq.service disabled dracut-cmdline.service static --More--
To see units of a specific type, use the -t switch:
[root@centos8 ~]# systemctl list-unit-files -t mount UNIT FILE STATE -.mount generated boot.mount generated dev-hugepages.mount static dev-mqueue.mount static proc-fs-nfsd.mount static proc-sys-fs-binfmt_misc.mount static sys-fs-fuse-connections.mount static sys-kernel-config.mount static sys-kernel-debug.mount static tmp.mount disabled var-lib-machines.mount static var-lib-nfs-rpc_pipefs.mount static 12 unit files listed.
In the STATE column you can see the static and generated terms.
[root@centos8 ~]# ls -l /lib/systemd/system-generators/systemd-fstab-generator -rwxr-xr-x. 1 root root 46096 Apr 7 16:56 /lib/systemd/system-generators/systemd-fstab-generator
Other binaries exist to automatically generate other types of unit files:
[root@centos8 ~]# ls -l /lib/systemd/system-generators total 508 -rwxr-xr-x. 1 root root 504 Jan 4 11:25 kdump-dep-generator.sh -r-xr-xr-x. 1 root root 134976 Aug 17 2020 lvm2-activation-generator -rwxr-xr-x. 1 root root 67792 Jul 20 2020 nfs-server-generator -rwxr-xr-x. 1 root root 38216 Jul 20 2020 rpc-pipefs-generator -rwxr-xr-x. 1 root root 743 Apr 23 2020 selinux-autorelabel-generator.sh -rwxr-xr-x. 1 root root 33504 Apr 7 16:56 systemd-cryptsetup-generator -rwxr-xr-x. 1 root root 16648 Apr 7 16:56 systemd-debug-generator -rwxr-xr-x. 1 root root 46096 Apr 7 16:56 systemd-fstab-generator -rwxr-xr-x. 1 root root 17064 Apr 7 16:56 systemd-getty-generator -rwxr-xr-x. 1 root root 29432 Apr 7 16:56 systemd-gpt-auto-generator -rwxr-xr-x. 1 root root 12568 Apr 7 16:56 systemd-hibernate-resume-generator -rwxr-xr-x. 1 root root 12368 Apr 7 16:56 systemd-rc-local-generator -rwxr-xr-x. 1 root root 12408 Apr 7 16:56 systemd-system-update-generator -rwxr-xr-x. 1 root root 33544 Apr 7 16:56 systemd-sysv-generator -rwxr-xr-x. 1 root root 17024 Apr 7 16:56 systemd-veritysetup-generator
Command line switches of the systemctl command are as follows:
[root@centos8 ~]# systemctl --help
systemctl [OPTIONS...] {COMMAND} ...
Query or send control commands to the systemd manager.
-h --help Show this help
--version Show package version
--system Connect to system manager
--user Connect to user service manager
-H --host=[USER@]HOST
Operate on remote host
-M --machine=CONTAINER
Operate on local container
-t --type=TYPE List units of a particular type
--state=STATE List units with particular LOAD or SUB or ACTIVE state
-p --property=NAME Show only properties by this name
-a --all Show all properties/all units currently in memory,
including dead/empty ones. To list all units installed on
the system, use the 'list-unit-files' command instead.
--failed Same as --state=failed
-l --full Don't ellipsize unit names on output
-r --recursive Show unit list of host and local containers
--reverse Show reverse dependencies with 'list-dependencies'
--job-mode=MODE Specify how to deal with already queued jobs, when
queueing a new job
--show-types When showing sockets, explicitly show their type
--value When showing properties, only print the value
-i --ignore-inhibitors
When shutting down or sleeping, ignore inhibitors
--kill-who=WHO Who to send signal to
-s --signal=SIGNAL Which signal to send
--now Start or stop unit in addition to enabling or disabling it
--dry-run Only print what would be done
-q --quiet Suppress output
--wait For (re)start, wait until service stopped again
--no-block Do not wait until operation finished
--no-wall Don't send wall message before halt/power-off/reboot
--no-reload Don't reload daemon after en-/dis-abling unit files
--no-legend Do not print a legend (column headers and hints)
--no-pager Do not pipe output into a pager
--no-ask-password
Do not ask for system passwords
--global Enable/disable/mask unit files globally
--runtime Enable/disable/mask unit files temporarily until next
reboot
-f --force When enabling unit files, override existing symlinks
When shutting down, execute action immediately
--preset-mode= Apply only enable, only disable, or all presets
--root=PATH Enable/disable/mask unit files in the specified root
directory
-n --lines=INTEGER Number of journal entries to show
-o --output=STRING Change journal output mode (short, short-precise,
short-iso, short-iso-precise, short-full,
short-monotonic, short-unix,
verbose, export, json, json-pretty, json-sse, cat)
--firmware-setup Tell the firmware to show the setup menu on next boot
--plain Print unit dependencies as a list instead of a tree
lines 1-57
These files can be found in the /usr/lib/systemd/system directory:
[root@centos8 ~]# pkg-config systemd --variable=systemdsystemunitdir /usr/lib/systemd/system
[root@centos8 ~]# ls -l /usr/lib/systemd/system | more total 1464 -rw-r--r--. 1 root root 275 Apr 26 2020 arp-ethers.service -rw-r--r--. 1 root root 222 May 11 2019 atd.service -rw-r--r--. 1 root root 1512 Apr 23 2020 auditd.service -rw-r--r--. 1 root root 628 Jul 20 2020 auth-rpcgss-module.service lrwxrwxrwx. 1 root root 14 Apr 7 16:55 autovt@.service -> getty@.service -rw-r--r--. 1 root root 1044 Nov 16 2020 avahi-daemon.service -rw-r--r--. 1 root root 870 Nov 16 2020 avahi-daemon.socket -rw-r--r--. 1 root root 956 Apr 7 16:54 basic.target drwxr-xr-x. 2 root root 6 Apr 7 16:55 basic.target.wants -r--r--r--. 1 root root 408 Aug 17 2020 blk-availability.service -rw-r--r--. 1 root root 419 Jun 22 2018 bluetooth.target -rw-r--r--. 1 root root 455 Apr 7 16:54 boot-complete.target -rw-r--r--. 1 root root 209 Nov 19 2019 chrony-dnssrv@.service -rw-r--r--. 1 root root 138 Nov 19 2019 chrony-dnssrv@.timer -rw-r--r--. 1 root root 491 Nov 19 2019 chronyd.service -rw-r--r--. 1 root root 472 May 10 2019 chrony-wait.service -rw-r--r--. 1 root root 222 Aug 24 2020 cockpit-motd.service -rw-r--r--. 1 root root 835 Aug 24 2020 cockpit.service -rw-r--r--. 1 root root 373 Aug 24 2020 cockpit.socket -rw-r--r--. 1 root root 251 Aug 24 2020 cockpit-wsinstance-http-redirect.service -rw-r--r--. 1 root root 233 Aug 24 2020 cockpit-wsinstance-http-redirect.socket -rw-r--r--. 1 root root 221 Aug 24 2020 cockpit-wsinstance-http.service -rw-r--r--. 1 root root 165 Aug 24 2020 cockpit-wsinstance-https-factory@.service -rw-r--r--. 1 root root 244 Aug 24 2020 cockpit-wsinstance-https-factory.socket -rw-r--r--. 1 root root 215 Aug 24 2020 cockpit-wsinstance-http.socket -rw-r--r--. 1 root root 264 Aug 24 2020 cockpit-wsinstance-https@.service -rw-r--r--. 1 root root 478 Aug 24 2020 cockpit-wsinstance-https@.socket -rw-r--r--. 1 root root 1082 Apr 7 16:55 console-getty.service -rw-r--r--. 1 root root 1263 Apr 7 16:55 container-getty@.service -rw-r--r--. 1 root root 294 Apr 8 15:21 cpupower.service -rw-r--r--. 1 root root 356 Nov 8 2019 crond.service -rw-r--r--. 1 root root 465 Jun 22 2018 cryptsetup-pre.target -rw-r--r--. 1 root root 412 Jun 22 2018 cryptsetup.target lrwxrwxrwx. 1 root root 13 Apr 7 16:55 ctrl-alt-del.target -> reboot.target -rw-r--r--. 1 root root 234 Oct 4 2017 cups-browsed.service -r--r--r--. 1 root root 142 Jun 15 2020 cups.path -r--r--r--. 1 root root 248 Jun 15 2020 cups.service -r--r--r--. 1 root root 136 Jun 15 2020 cups.socket lrwxrwxrwx. 1 root root 25 Apr 7 16:55 dbus-org.freedesktop.hostname1.service -> systemd-hostnamed.service lrwxrwxrwx. 1 root root 23 Apr 7 16:55 dbus-org.freedesktop.import1.service -> systemd-importd.service lrwxrwxrwx. 1 root root 23 Apr 7 16:55 dbus-org.freedesktop.locale1.service -> systemd-localed.service lrwxrwxrwx. 1 root root 22 Apr 7 16:55 dbus-org.freedesktop.login1.service -> systemd-logind.service lrwxrwxrwx. 1 root root 24 Apr 7 16:55 dbus-org.freedesktop.machine1.service -> systemd-machined.service lrwxrwxrwx. 1 root root 25 Apr 7 16:55 dbus-org.freedesktop.portable1.service -> systemd-portabled.service lrwxrwxrwx. 1 root root 25 Apr 7 16:55 dbus-org.freedesktop.timedate1.service -> systemd-timedated.service -rw-r--r--. 1 root root 380 Apr 7 12:08 dbus.service -rw-r--r--. 1 root root 102 Apr 7 12:08 dbus.socket drwxr-xr-x. 2 root root 6 Apr 7 16:55 dbus.target.wants -rw-r--r--. 1 root root 1084 Apr 7 16:55 debug-shell.service lrwxrwxrwx. 1 root root 16 Apr 7 16:55 default.target -> graphical.target drwxr-xr-x. 2 root root 6 Apr 7 16:55 default.target.wants -rw-r--r--. 1 root root 750 Jun 22 2018 dev-hugepages.mount -rw-r--r--. 1 root root 665 Jun 22 2018 dev-mqueue.mount -r--r--r--. 1 root root 345 Aug 17 2020 dm-event.service -r--r--r--. 1 root root 248 Aug 17 2020 dm-event.socket -rw-r--r--. 1 root root 457 Jun 2 2020 dnf-makecache.service --More--
Certain configuration files are dynamically created and placed in the /run/systemd/system directory to then be deleted upon system reboot:
[root@centos8 ~]# ls -l /run/systemd/system/ total 0
User created unit files must be placed in the /usr/lib/systemd/user directory:
[root@centos8 ~]# pkg-config systemd --variable=systemduserunitdir /usr/lib/systemd/user
Important : This means that files in /usr/lib/systemd/user overload by files in /run/systemd/system which in turn overload files in /usr/lib/systemd/system.
Take the case of the /usr/lib/systemd/system/sshd.service file:
[root@centos8 ~]# cat /usr/lib/systemd/system/sshd.service [Unit] Description=OpenSSH server daemon Documentation=man:sshd(8) man:sshd_config(5) After=network.target sshd-keygen.target Wants=sshd-keygen.target [Service] Type=notify EnvironmentFile=-/etc/crypto-policies/back-ends/opensshserver.config EnvironmentFile=-/etc/sysconfig/sshd ExecStart=/usr/sbin/sshd -D $OPTIONS $CRYPTO_POLICY ExecReload=/bin/kill -HUP $MAINPID KillMode=process Restart=on-failure RestartSec=42s [Install] WantedBy=multi-user.target
To see all of the values of the configuration directives, use systemctl show and specify the unit file:
[root@centos8 ~]# systemctl show sshd
Type=notify
Restart=on-failure
NotifyAccess=main
RestartUSec=42s
TimeoutStartUSec=1min 30s
TimeoutStopUSec=1min 30s
RuntimeMaxUSec=infinity
WatchdogUSec=0
WatchdogTimestamp=Thu 2021-06-03 15:09:54 EDT
WatchdogTimestampMonotonic=12502561
PermissionsStartOnly=no
RootDirectoryStartOnly=no
RemainAfterExit=no
GuessMainPID=yes
MainPID=902
ControlPID=0
FileDescriptorStoreMax=0
NFileDescriptorStore=0
StatusErrno=0
Result=success
UID=[not set]
GID=[not set]
NRestarts=0
ExecMainStartTimestamp=Thu 2021-06-03 15:09:54 EDT
ExecMainStartTimestampMonotonic=12446178
ExecMainExitTimestampMonotonic=0
ExecMainPID=902
ExecMainCode=0
ExecMainStatus=0
ExecStart={ path=/usr/sbin/sshd ; argv[]=/usr/sbin/sshd -D $OPTIONS $CRYPTO_POLICY ; ignore_errors=no ; start_time=[n/a] ; stop_time=[n/a] ; pid=0 ; code=(null) ; status=0/0 }
ExecReload={ path=/bin/kill ; argv[]=/bin/kill -HUP $MAINPID ; ignore_errors=no ; start_time=[n/a] ; stop_time=[n/a] ; pid=0 ; code=(null) ; status=0/0 }
Slice=system.slice
ControlGroup=/system.slice/sshd.service
MemoryCurrent=6270976
CPUUsageNSec=[not set]
EffectiveCPUs=
EffectiveMemoryNodes=
TasksCurrent=1
IPIngressBytes=18446744073709551615
IPIngressPackets=18446744073709551615
IPEgressBytes=18446744073709551615
IPEgressPackets=18446744073709551615
Delegate=no
CPUAccounting=no
CPUWeight=[not set]
StartupCPUWeight=[not set]
CPUShares=[not set]
StartupCPUShares=[not set]
CPUQuotaPerSecUSec=infinity
CPUQuotaPeriodUSec=infinity
AllowedCPUs=
AllowedMemoryNodes=
IOAccounting=no
IOWeight=[not set]
StartupIOWeight=[not set]
BlockIOAccounting=no
BlockIOWeight=[not set]
lines 1-57
To see unit dependencies, use the systemctl list-dependancies command and specify the unit:
[root@centos8 ~]# systemctl list-dependencies sshd.service sshd.service ● ├─system.slice ● ├─sshd-keygen.target ● │ ├─sshd-keygen@ecdsa.service ● │ ├─sshd-keygen@ed25519.service ● │ └─sshd-keygen@rsa.service ● └─sysinit.target ● ├─dev-hugepages.mount ● ├─dev-mqueue.mount ● ├─dracut-shutdown.service ● ├─import-state.service ● ├─iscsi-onboot.service ● ├─kmod-static-nodes.service ● ├─ldconfig.service ● ├─loadmodules.service ● ├─lvm2-lvmpolld.socket ● ├─lvm2-monitor.service ● ├─nis-domainname.service ● ├─plymouth-read-write.service ● ├─plymouth-start.service ● ├─proc-sys-fs-binfmt_misc.automount ● ├─rngd.service ● ├─selinux-autorelabel-mark.service ● ├─sys-fs-fuse-connections.mount ● ├─sys-kernel-config.mount ● ├─sys-kernel-debug.mount ● ├─systemd-ask-password-console.path ● ├─systemd-binfmt.service ● ├─systemd-firstboot.service ● ├─systemd-hwdb-update.service ● ├─systemd-journal-catalog-update.service ● ├─systemd-journal-flush.service ● ├─systemd-journald.service ● ├─systemd-machine-id-commit.service ● ├─systemd-modules-load.service ● ├─systemd-random-seed.service ● ├─systemd-sysctl.service ● ├─systemd-sysusers.service ● ├─systemd-tmpfiles-setup-dev.service ● ├─systemd-tmpfiles-setup.service ● ├─systemd-udev-trigger.service ● ├─systemd-udevd.service ● ├─systemd-update-done.service ● ├─systemd-update-utmp.service ● ├─cryptsetup.target ● ├─local-fs.target ● │ ├─-.mount ● │ ├─boot.mount ● │ └─systemd-remount-fs.service ● └─swap.target ● └─dev-disk-by\x2duuid-c8bb3f47\x2dd67f\x2d4b21\x2db781\x2d766899dc83d4.swap
Default configuration files can be overloaded by files in other directories:
[root@centos8 ~]# pkg-config systemd --variable=systemdsystemunitpath /etc/systemd/system:/etc/systemd/system:/run/systemd/system:/usr/local/lib/systemd/system:/usr/lib/systemd/system:/usr/lib/systemd/system:/lib/systemd/system
[root@centos8 ~]# ls -l /etc/systemd/system total 4 drwxr-xr-x. 2 root root 31 May 8 2020 basic.target.wants lrwxrwxrwx. 1 root root 41 May 8 2020 dbus-org.fedoraproject.FirewallD1.service -> /usr/lib/systemd/system/firewalld.service lrwxrwxrwx. 1 root root 44 Jun 3 14:02 dbus-org.freedesktop.Avahi.service -> /usr/lib/systemd/system/avahi-daemon.service lrwxrwxrwx. 1 root root 57 May 8 2020 dbus-org.freedesktop.nm-dispatcher.service -> /usr/lib/systemd/system/NetworkManager-dispatcher.service lrwxrwxrwx. 1 root root 41 May 8 2020 dbus-org.freedesktop.timedate1.service -> /usr/lib/systemd/system/timedatex.service lrwxrwxrwx. 1 root root 37 May 8 2020 default.target -> /lib/systemd/system/multi-user.target drwxr-xr-x. 2 root root 32 May 8 2020 getty.target.wants drwxr-xr-x. 2 root root 4096 Jun 3 14:02 multi-user.target.wants drwxr-xr-x. 2 root root 48 May 8 2020 network-online.target.wants drwxr-xr-x. 2 root root 33 Apr 19 12:07 nfs-blkmap.service.requires drwxr-xr-x. 2 root root 33 Apr 19 12:07 nfs-idmapd.service.requires drwxr-xr-x. 2 root root 33 Apr 19 12:07 nfs-mountd.service.requires drwxr-xr-x. 2 root root 33 Apr 19 12:07 nfs-server.service.requires drwxr-xr-x. 2 root root 26 Jun 3 14:02 printer.target.wants drwxr-xr-x. 2 root root 52 Apr 19 12:07 remote-fs.target.wants drwxr-xr-x. 2 root root 33 Apr 19 12:07 rpc-gssd.service.requires drwxr-xr-x. 2 root root 33 Apr 19 12:07 rpc-statd-notify.service.requires drwxr-xr-x. 2 root root 33 Apr 19 12:07 rpc-statd.service.requires drwxr-xr-x. 2 root root 260 Jun 3 14:02 sockets.target.wants drwxr-xr-x. 2 root root 235 Apr 19 12:07 sysinit.target.wants lrwxrwxrwx. 1 root root 39 May 8 2020 syslog.service -> /usr/lib/systemd/system/rsyslog.service lrwxrwxrwx. 1 root root 9 May 11 2019 systemd-timedated.service -> /dev/null drwxr-xr-x. 2 root root 34 May 8 2020 timers.target.wants
To see the startup duration, use the following command:
[root@centos8 ~]# systemd-analyze Startup finished in 1.665s (kernel) + 6.977s (initrd) + 8.458s (userspace) = 17.101s multi-user.target reached after 5.528s in userspace
THe blame sub-command is used to see which units are the slowest:
[root@centos8 ~]# systemd-analyze blame
4.080s dracut-initqueue.service
3.257s kdump.service
1.727s tuned.service
1.415s initrd-switch-root.service
1.393s NetworkManager-wait-online.service
1.116s systemd-machined.service
1.111s dracut-cmdline.service
850ms sssd.service
791ms vdo.service
713ms firewalld.service
622ms lvm2-monitor.service
605ms polkit.service
508ms chronyd.service
434ms avahi-daemon.service
426ms systemd-logind.service
417ms netcf-transaction.service
410ms dracut-pre-udev.service
295ms libvirtd.service
275ms dnf-makecache.service
243ms systemd-udevd.service
221ms systemd-journald.service
196ms systemd-tmpfiles-setup.service
151ms dracut-pre-pivot.service
139ms sysroot.mount
139ms systemd-update-utmp-runlevel.service
122ms systemd-vconsole-setup.service
110ms lvm2-pvscan@8:25.service
98ms systemd-udev-trigger.service
82ms gssproxy.service
81ms cups.service
79ms initrd-parse-etc.service
77ms NetworkManager.service
72ms lvm2-pvscan@8:23.service
69ms systemd-user-sessions.service
68ms lvm2-pvscan@8:22.service
67ms unbound-anchor.service
66ms rsyslog.service
62ms boot.mount
56ms sshd.service
54ms smartd.service
54ms systemd-fsck@dev-disk-by\x2duuid-2ae4c035\x2d9244\x2d458c\x2d82c5\x2da49ae169cdb6.service
53ms user@1000.service
52ms auditd.service
51ms plymouth-quit.service
49ms rngd-wake-threshold.service
46ms import-state.service
46ms systemd-tmpfiles-setup-dev.service
43ms ksmtuned.service
42ms plymouth-quit-wait.service
42ms var-lib-nfs-rpc_pipefs.mount
42ms rpc-statd-notify.service
38ms plymouth-switch-root.service
37ms systemd-remount-fs.service
37ms plymouth-start.service
33ms dev-disk-by\x2duuid-c8bb3f47\x2dd67f\x2d4b21\x2db781\x2d766899dc83d4.swap
33ms systemd-tmpfiles-clean.service
31ms dev-hugepages.mount
lines 1-57
The critical-chain sub-command shows the startup process of a specific unit:
[root@centos8 ~]# systemd-analyze critical-chain sshd.service
The time after the unit is active or started is printed after the "@" character.
The time the unit takes to start is printed after the "+" character.
sshd.service +56ms
└─network.target @3.799s
└─NetworkManager.service @3.719s +77ms
└─network-pre.target @3.718s
└─firewalld.service @3.004s +713ms
└─polkit.service @2.397s +605ms
└─basic.target @2.392s
└─sockets.target @2.392s
└─sssd-kcm.socket @2.391s
└─sysinit.target @2.379s
└─systemd-update-utmp.service @2.370s +8ms
└─auditd.service @2.317s +52ms
└─systemd-tmpfiles-setup.service @2.118s +196ms
└─import-state.service @2.070s +46ms
└─local-fs.target @2.069s
└─boot.mount @2.006s +62ms
└─systemd-fsck@dev-disk-by\x2duuid-2ae4c035\x2d9244\x2d458c\x2d82c5\x2da49ae169cdb6.service @1.943s +54ms
└─local-fs-pre.target @1.942s
└─lvm2-monitor.service @1.319s +622ms
└─dm-event.socket @1.317s
└─-.mount
└─system.slice
└─-.slice
Command line switches of the systemd-analyze command are as follows:
[root@centos7 ~]# systemd-analyze --help
systemd-analyze [OPTIONS...] {COMMAND} ...
Process systemd profiling information
-h --help Show this help
--version Show package version
--system Connect to system manager
--user Connect to user service manager
--order When generating a dependency graph, show only order
--require When generating a dependency graph, show only requirement
--from-pattern=GLOB, --to-pattern=GLOB
When generating a dependency graph, filter only origins
or destinations, respectively
--fuzz=TIMESPAN When printing the tree of the critical chain, print also
services, which finished TIMESPAN earlier, than the
latest in the branch. The unit of TIMESPAN is seconds
unless specified with a different unit, i.e. 50ms
--no-pager Do not pipe output into a pager
Commands:
time Print time spent in the kernel before reaching userspace
blame Print list of running units ordered by time to init
critical-chain Print a tree of the time critical chain of units
plot Output SVG graphic showing service initialization
dot Output dependency graph in dot(1) format
set-log-level LEVEL Set logging threshold for systemd
dump Output state serialization of service manager
Each Target has a configuration file:
[root@centos8 ~]# cat /usr/lib/systemd/system/graphical.target # SPDX-License-Identifier: LGPL-2.1+ # # This file is part of systemd. # # systemd is free software; you can redistribute it and/or modify it # under the terms of the GNU Lesser General Public License as published by # the Free Software Foundation; either version 2.1 of the License, or # (at your option) any later version. [Unit] Description=Graphical Interface Documentation=man:systemd.special(7) Requires=multi-user.target Wants=display-manager.service Conflicts=rescue.service rescue.target After=multi-user.target rescue.service rescue.target display-manager.service AllowIsolate=yes
Target dependencies are shown by the systemctl list-dependencies command:
[root@centos8 ~]# systemctl list-dependencies multi-user.target multi-user.target ● ├─atd.service ● ├─auditd.service ● ├─avahi-daemon.service ● ├─chronyd.service ● ├─crond.service ● ├─cups.path ● ├─cups.service ● ├─dbus.service ● ├─dnf-makecache.timer ● ├─firewalld.service ● ├─irqbalance.service ● ├─kdump.service ● ├─ksm.service ● ├─ksmtuned.service ● ├─libstoragemgmt.service ● ├─libvirtd.service ● ├─mcelog.service ● ├─mdmonitor.service ● ├─netcf-transaction.service ● ├─NetworkManager.service ● ├─plymouth-quit-wait.service ● ├─plymouth-quit.service ● ├─rpcbind.service ● ├─rsyslog.service ● ├─smartd.service ● ├─sshd.service ● ├─sssd.service ● ├─systemd-ask-password-wall.path ● ├─systemd-logind.service ● ├─systemd-update-utmp-runlevel.service ● ├─systemd-user-sessions.service ● ├─tuned.service ● ├─vdo.service ● ├─basic.target ● │ ├─-.mount ● │ ├─microcode.service ● │ ├─paths.target ● │ ├─slices.target ● │ │ ├─-.slice ● │ │ └─system.slice ● │ ├─sockets.target ● │ │ ├─avahi-daemon.socket ● │ │ ├─cups.socket ● │ │ ├─dbus.socket ● │ │ ├─dm-event.socket ● │ │ ├─iscsid.socket ● │ │ ├─iscsiuio.socket ● │ │ ├─libvirtd-ro.socket ● │ │ ├─libvirtd.socket ● │ │ ├─rpcbind.socket ● │ │ ├─sssd-kcm.socket ● │ │ ├─systemd-coredump.socket ● │ │ ├─systemd-initctl.socket ● │ │ ├─systemd-journald-dev-log.socket ● │ │ ├─systemd-journald.socket ● │ │ ├─systemd-udevd-control.socket lines 1-57
The black dots above are coloured on-screen:
To see failed units, use systemctl –failed :
[root@centos8 ~]# systemctl --failed 0 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use 'systemctl list-unit-files'.
Target dependencies are soft-links in /etc/systemd/system/multi-user.target.wants and /usr/lib/systemd/system/multi-user.target.wants:
[root@centos8 ~]# ls -l /etc/systemd/system/multi-user.target.wants total 0 lrwxrwxrwx. 1 root root 35 May 8 2020 atd.service -> /usr/lib/systemd/system/atd.service lrwxrwxrwx. 1 root root 38 May 8 2020 auditd.service -> /usr/lib/systemd/system/auditd.service lrwxrwxrwx. 1 root root 44 Jun 3 14:02 avahi-daemon.service -> /usr/lib/systemd/system/avahi-daemon.service lrwxrwxrwx. 1 root root 39 May 8 2020 chronyd.service -> /usr/lib/systemd/system/chronyd.service lrwxrwxrwx. 1 root root 37 May 8 2020 crond.service -> /usr/lib/systemd/system/crond.service lrwxrwxrwx. 1 root root 33 Jun 3 14:02 cups.path -> /usr/lib/systemd/system/cups.path lrwxrwxrwx. 1 root root 36 Jun 3 14:02 cups.service -> /usr/lib/systemd/system/cups.service lrwxrwxrwx. 1 root root 43 May 8 2020 dnf-makecache.timer -> /usr/lib/systemd/system/dnf-makecache.timer lrwxrwxrwx. 1 root root 41 May 8 2020 firewalld.service -> /usr/lib/systemd/system/firewalld.service lrwxrwxrwx. 1 root root 42 May 8 2020 irqbalance.service -> /usr/lib/systemd/system/irqbalance.service lrwxrwxrwx. 1 root root 37 May 8 2020 kdump.service -> /usr/lib/systemd/system/kdump.service lrwxrwxrwx. 1 root root 35 Apr 19 12:07 ksm.service -> /usr/lib/systemd/system/ksm.service lrwxrwxrwx. 1 root root 40 Apr 19 12:07 ksmtuned.service -> /usr/lib/systemd/system/ksmtuned.service lrwxrwxrwx. 1 root root 46 May 8 2020 libstoragemgmt.service -> /usr/lib/systemd/system/libstoragemgmt.service lrwxrwxrwx. 1 root root 40 Apr 19 12:07 libvirtd.service -> /usr/lib/systemd/system/libvirtd.service lrwxrwxrwx. 1 root root 38 May 8 2020 mcelog.service -> /usr/lib/systemd/system/mcelog.service lrwxrwxrwx. 1 root root 41 May 8 2020 mdmonitor.service -> /usr/lib/systemd/system/mdmonitor.service lrwxrwxrwx. 1 root root 49 Apr 19 12:07 netcf-transaction.service -> /usr/lib/systemd/system/netcf-transaction.service lrwxrwxrwx. 1 root root 46 May 8 2020 NetworkManager.service -> /usr/lib/systemd/system/NetworkManager.service lrwxrwxrwx. 1 root root 41 Apr 19 12:07 nfs-client.target -> /usr/lib/systemd/system/nfs-client.target lrwxrwxrwx. 1 root root 40 May 8 2020 remote-fs.target -> /usr/lib/systemd/system/remote-fs.target lrwxrwxrwx. 1 root root 39 Apr 19 12:07 rpcbind.service -> /usr/lib/systemd/system/rpcbind.service lrwxrwxrwx. 1 root root 39 May 8 2020 rsyslog.service -> /usr/lib/systemd/system/rsyslog.service lrwxrwxrwx. 1 root root 38 May 8 2020 smartd.service -> /usr/lib/systemd/system/smartd.service lrwxrwxrwx. 1 root root 36 May 8 2020 sshd.service -> /usr/lib/systemd/system/sshd.service lrwxrwxrwx. 1 root root 36 May 8 2020 sssd.service -> /usr/lib/systemd/system/sssd.service lrwxrwxrwx. 1 root root 37 May 8 2020 tuned.service -> /usr/lib/systemd/system/tuned.service lrwxrwxrwx. 1 root root 35 May 8 2020 vdo.service -> /usr/lib/systemd/system/vdo.service [root@centos8 ~]# ls -l /usr/lib/systemd/system/multi-user.target.wants total 0 lrwxrwxrwx. 1 root root 15 Apr 7 12:08 dbus.service -> ../dbus.service lrwxrwxrwx. 1 root root 15 Apr 7 16:55 getty.target -> ../getty.target lrwxrwxrwx. 1 root root 29 Oct 6 2020 plymouth-quit-wait.service -> ../plymouth-quit-wait.service lrwxrwxrwx. 1 root root 33 Apr 7 16:55 systemd-ask-password-wall.path -> ../systemd-ask-password-wall.path lrwxrwxrwx. 1 root root 25 Apr 7 16:55 systemd-logind.service -> ../systemd-logind.service lrwxrwxrwx. 1 root root 39 Apr 7 16:55 systemd-update-utmp-runlevel.service -> ../systemd-update-utmp-runlevel.service lrwxrwxrwx. 1 root root 32 Apr 7 16:55 systemd-user-sessions.service -> ../systemd-user-sessions.service
To check the defauylt target, use the systemctl get-default command:
[root@centos8 ~]# systemctl get-default multi-user.target
The default target is in fact a symbolic link:
[root@centos8 ~]# ls -l /etc/systemd/system/default.target lrwxrwxrwx. 1 root root 37 May 8 2020 /etc/systemd/system/default.target -> /lib/systemd/system/multi-user.target
To change the default traget for the next boot process, use the systemctl set-default command:
[root@centos8 ~]# systemctl set-default graphical.target Removed /etc/systemd/system/default.target. Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/graphical.target. [root@centos8 ~]# ls -l /etc/systemd/system/default.target lrwxrwxrwx. 1 root root 40 Jun 6 08:11 /etc/systemd/system/default.target -> /usr/lib/systemd/system/graphical.target [root@centos8 ~]# systemctl set-default multi-user.target Removed /etc/systemd/system/default.target. Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/multi-user.target. [root@centos8 ~]# ls -l /etc/systemd/system/default.target lrwxrwxrwx. 1 root root 41 Jun 6 08:11 /etc/systemd/system/default.target -> /usr/lib/systemd/system/multi-user.target
This can be done by using the systemctl isolate command:
[root@centos8 ~]# systemctl isolate rescue
[root@centos8 ~]# systemctl list-units --type target | egrep "eme|res|gra|mul" | head -1
rescue.target loaded active active Rescue Mode
[root@centos8 ~]# runlevel
3 1
[root@centos8 ~]# who -r
run-level 1 2021-06-15 04:22 last=3
[root@centos8 ~]# systemctl isolate multi-user
[root@centos8 ~]# systemctl list-units --type target | egrep "eme|res|gra|mul" | head -1
multi-user.target loaded active active Multi-User System
[root@centos8 ~]# runlevel
1 3
[root@centos8 ~]# who -r
run-level 3 2021-06-15 04:24 last=1
Start by installing httpd :
[root@centos8 ~]# dnf install httpd
To obtain information about a specific service, use the systemctl status command:
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: inactive (dead)
Docs: man:httpd.service(8)
In the above output you can see that the service is disabled. The status can have one of two values:
You can also check if a service is enabled with the systemctl is-enabled command:
[root@centos8 ~]# systemctl is-enabled httpd.service disabled
To configure the status as enabled, use the systemctl enable command:
[root@centos8 ~]# systemctl enable httpd.service
Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service → /usr/lib/systemd/system/httpd.service.
[root@centos8 ~]# systemctl is-enabled httpd.service
enabled
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: inactive (dead)
Docs: man:httpd.service(8)
In the above output, you can now see that the status is inactive (dead). The status can have one of 7 values:
You can also check if a service is active with the systemctl is-active command:
[root@centos8 ~]# systemctl is-active httpd.service inactive
To set the service to active(running), use the following command:
[root@centos8 ~]# systemctl start httpd.service
Now check the service status:
[root@centos8 ~]# systemctl is-active httpd.service
active
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: active (running) since Sun 2021-06-06 15:33:06 EDT; 14s ago
Docs: man:httpd.service(8)
Main PID: 34382 (httpd)
Status: "Running, listening on: port 80"
Tasks: 213 (limit: 23719)
Memory: 36.3M
CGroup: /system.slice/httpd.service
├─34382 /usr/sbin/httpd -DFOREGROUND
├─34383 /usr/sbin/httpd -DFOREGROUND
├─34384 /usr/sbin/httpd -DFOREGROUND
├─34385 /usr/sbin/httpd -DFOREGROUND
└─34386 /usr/sbin/httpd -DFOREGROUND
Jun 06 15:33:05 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 06 15:33:06 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 06 15:33:06 centos8.ittraining.loc httpd[34382]: Server configured, listening on: port 80
To stop a service use the following command:
[root@centos8 ~]# systemctl stop httpd.service
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: inactive (dead) since Sun 2021-06-06 23:58:04 EDT; 8s ago
Docs: man:httpd.service(8)
Process: 34382 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=0/SUCCESS)
Main PID: 34382 (code=exited, status=0/SUCCESS)
Status: "Running, listening on: port 80"
Jun 06 15:33:05 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 06 15:33:06 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 06 15:33:06 centos8.ittraining.loc httpd[34382]: Server configured, listening on: port 80
Jun 06 23:58:02 centos8.ittraining.loc systemd[1]: Stopping The Apache HTTP Server...
Jun 06 23:58:04 centos8.ittraining.loc systemd[1]: httpd.service: Succeeded.
Jun 06 23:58:04 centos8.ittraining.loc systemd[1]: Stopped The Apache HTTP Server.
To disable a service for the next boot sequence, use the disable sub-command:
[root@centos8 ~]# systemctl disable httpd.service
Removed /etc/systemd/system/multi-user.target.wants/httpd.service.
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: inactive (dead)
Docs: man:httpd.service(8)
Jun 06 15:33:05 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 06 15:33:06 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 06 15:33:06 centos8.ittraining.loc httpd[34382]: Server configured, listening on: port 80
Jun 06 23:58:02 centos8.ittraining.loc systemd[1]: Stopping The Apache HTTP Server...
Jun 06 23:58:04 centos8.ittraining.loc systemd[1]: httpd.service: Succeeded.
Jun 06 23:58:04 centos8.ittraining.loc systemd[1]: Stopped The Apache HTTP Server.
Systemd permits the use of unit configuration file templates. In this way, multiple service instances can co-exist. A template can be recognised by it's name which contains the @ character:
[root@centos8 ~]# cat /usr/lib/systemd/system/httpd@.service # This is a template for httpd instances. # See httpd@.service(8) for more information. [Unit] Description=The Apache HTTP Server After=network.target remote-fs.target nss-lookup.target Documentation=man:httpd@.service(8) [Service] Type=notify Environment=LANG=C Environment=HTTPD_INSTANCE=%i ExecStartPre=/bin/mkdir -m 710 -p /run/httpd/instance-%i ExecStartPre=/bin/chown root.apache /run/httpd/instance-%i ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND -f conf/%i.conf ExecReload=/usr/sbin/httpd $OPTIONS -k graceful -f conf/%i.conf # Send SIGWINCH for graceful stop KillSignal=SIGWINCH KillMode=mixed PrivateTmp=true [Install] WantedBy=multi-user.target
An instance created form this template must have a name of the following format:
httpd@<nom_instance>.service
In the configuration file you can see an %i which is called an identifier. Identifiers can be of two types - escaped where alphanumeric non-ASCII characters are replaced with C langauge escapes whilst the other type is non-escaped:
Now create two copies of the /usr/lib/systemd/system/httpd@.service file:
[root@centos8 ~]# cp /usr/lib/systemd/system/httpd@.service /usr/lib/systemd/system/httpd@instance01.service [root@centos8 ~]# cp /usr/lib/systemd/system/httpd@.service /usr/lib/systemd/system/httpd@instance02.service
Create two copies of the /etc/httpd/conf/httpd.conf file:
[root@centos8 ~]# cp /etc/httpd/conf/httpd.conf /etc/httpd/conf/instance01.conf [root@centos8 ~]# cp /etc/httpd/conf/httpd.conf /etc/httpd/conf/instance02.conf
Change the value of the Listen directive and add* the PidFile directive in the /etc/httpd/conf/instance01.conf file: <code> [root@centos8 ~]# vi /etc/httpd/conf/instance01.conf [root@centos8 ~]# more /etc/httpd/conf/instance01.conf # # This is the main Apache HTTP server configuration file. It contains the # configuration directives that give the server its instructions. # See <URL:http://httpd.apache.org/docs/2.4/> for detailed information. # In particular, see # <URL:http://httpd.apache.org/docs/2.4/mod/directives.html> # for a discussion of each configuration directive. # # See the httpd.conf(5) man page for more information on this configuration, # and httpd.service(8) on using and configuring the httpd service. # # Do NOT simply read the instructions in here without understanding # what they do. They're here only as hints or reminders. If you are unsure # consult the online docs. You have been warned. # # Configuration and logfile names: If the filenames you specify for many # of the server's control files begin with “/” (or “drive:/” for Win32), the # server will use that explicit path. If the filenames do *not* begin # with “/”, the value of ServerRoot is prepended – so 'log/access_log' # with ServerRoot set to '/www' will be interpreted by the # server as '/www/log/access_log', where as '/log/access_log' will be # interpreted as '/log/access_log'. # # ServerRoot: The top of the directory tree under which the server's # configuration, error, and log files are kept. # # Do not add a slash at the end of the directory path. If you point # ServerRoot at a non-local disk, be sure to specify a local disk on the # Mutex directive, if file-based mutexes are used. If you wish to share the # same ServerRoot for multiple httpd daemons, you will need to change at # least PidFile. # ServerRoot “/etc/httpd” # # Listen: Allows you to bind Apache to specific IP addresses and/or # ports, instead of the default. See also the <VirtualHost> # directive. # # Change this to Listen on specific IP addresses as shown below to # prevent Apache from glomming onto all bound IP addresses. # #Listen 12.34.56.78:80 Listen 8008 PidFile /var/run/httpd/instance01.pid # # Dynamic Shared Object (DSO) Support # # To be able to use the functionality of a module which was built as a DSO you # have to place corresponding `LoadModule' lines at this location so the # directives contained in it are actually available _before_ they are used. # Statically compiled modules (those listed by `httpd -l') do not need # to be loaded here. # # Example: # LoadModule foo_module modules/mod_foo.so –More–(19%) </code> Change the value of the Listen directive and add* the PidFile directive in the /etc/httpd/conf/instance02.conf file:
[root@centos8 ~]# vi /etc/httpd/conf/instance02.conf [root@centos8 ~]# more /etc/httpd/conf/instance02.conf # # This is the main Apache HTTP server configuration file. It contains the # configuration directives that give the server its instructions. # See <URL:http://httpd.apache.org/docs/2.4/> for detailed information. # In particular, see # <URL:http://httpd.apache.org/docs/2.4/mod/directives.html> # for a discussion of each configuration directive. # # See the httpd.conf(5) man page for more information on this configuration, # and httpd.service(8) on using and configuring the httpd service. # # Do NOT simply read the instructions in here without understanding # what they do. They're here only as hints or reminders. If you are unsure # consult the online docs. You have been warned. # # Configuration and logfile names: If the filenames you specify for many # of the server's control files begin with "/" (or "drive:/" for Win32), the # server will use that explicit path. If the filenames do *not* begin # with "/", the value of ServerRoot is prepended -- so 'log/access_log' # with ServerRoot set to '/www' will be interpreted by the # server as '/www/log/access_log', where as '/log/access_log' will be # interpreted as '/log/access_log'. # # ServerRoot: The top of the directory tree under which the server's # configuration, error, and log files are kept. # # Do not add a slash at the end of the directory path. If you point # ServerRoot at a non-local disk, be sure to specify a local disk on the # Mutex directive, if file-based mutexes are used. If you wish to share the # same ServerRoot for multiple httpd daemons, you will need to change at # least PidFile. # ServerRoot "/etc/httpd" # # Listen: Allows you to bind Apache to specific IP addresses and/or # ports, instead of the default. See also the <VirtualHost> # directive. # # Change this to Listen on specific IP addresses as shown below to # prevent Apache from glomming onto all bound IP addresses. # #Listen 12.34.56.78:80 Listen 8009 PidFile /var/run/httpd/instance02.pid # # Dynamic Shared Object (DSO) Support # # To be able to use the functionality of a module which was built as a DSO you # have to place corresponding `LoadModule' lines at this location so the # directives contained in it are actually available _before_ they are used. # Statically compiled modules (those listed by `httpd -l') do not need # to be loaded here. # # Example: # LoadModule foo_module modules/mod_foo.so --More--(19%)
Now start both services:
[root@centos8 ~]# systemctl start httpd@instance01.service
[root@centos8 ~]# systemctl status httpd@instance01.service
● httpd@instance01.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd@instance01.service; disabled; vendor preset: disabled)
Active: active (running) since Mon 2021-06-07 01:40:43 EDT; 7s ago
Docs: man:httpd@.service(8)
Process: 43854 ExecStartPre=/bin/chown root.apache /run/httpd/instance-instance01 (code=exited, status=0/SUCCESS)
Process: 43852 ExecStartPre=/bin/mkdir -m 710 -p /run/httpd/instance-instance01 (code=exited, status=0/SUCCESS)
Main PID: 43856 (httpd)
Status: "Started, listening on: port 8008"
Tasks: 213 (limit: 23719)
Memory: 43.6M
CGroup: /system.slice/system-httpd.slice/httpd@instance01.service
├─43856 /usr/sbin/httpd -DFOREGROUND -f conf/instance01.conf
├─43857 /usr/sbin/httpd -DFOREGROUND -f conf/instance01.conf
├─43858 /usr/sbin/httpd -DFOREGROUND -f conf/instance01.conf
├─43859 /usr/sbin/httpd -DFOREGROUND -f conf/instance01.conf
└─43860 /usr/sbin/httpd -DFOREGROUND -f conf/instance01.conf
Jun 07 01:40:43 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 07 01:40:43 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 07 01:40:43 centos8.ittraining.loc httpd[43856]: Server configured, listening on: port 8008
[root@centos8 ~]# systemctl start httpd@instance02.service
[root@centos8 ~]# systemctl status httpd@instance02.service
● httpd@instance02.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd@instance02.service; disabled; vendor preset: disabled)
Active: active (running) since Mon 2021-06-07 01:36:45 EDT; 4min 24s ago
Docs: man:httpd@.service(8)
Process: 43568 ExecStartPre=/bin/chown root.apache /run/httpd/instance-instance02 (code=exited, status=0/SUCCESS)
Process: 43566 ExecStartPre=/bin/mkdir -m 710 -p /run/httpd/instance-instance02 (code=exited, status=0/SUCCESS)
Main PID: 43569 (httpd)
Status: "Running, listening on: port 8009"
Tasks: 213 (limit: 23719)
Memory: 43.6M
CGroup: /system.slice/system-httpd.slice/httpd@instance02.service
├─43569 /usr/sbin/httpd -DFOREGROUND -f conf/instance02.conf
├─43571 /usr/sbin/httpd -DFOREGROUND -f conf/instance02.conf
├─43572 /usr/sbin/httpd -DFOREGROUND -f conf/instance02.conf
├─43573 /usr/sbin/httpd -DFOREGROUND -f conf/instance02.conf
└─43574 /usr/sbin/httpd -DFOREGROUND -f conf/instance02.conf
Jun 07 01:36:45 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 07 01:36:45 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 07 01:36:45 centos8.ittraining.loc httpd[43569]: Server configured, listening on: port 8009
It is possible to disallow modifications to a service status by using the systemctl maskcommand:
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: inactive (dead)
Docs: man:httpd.service(8)
Jun 07 18:27:25 centos8.ittraining.loc httpd[58535]: Server configured, listening on: port 80
Jun 07 18:27:29 centos8.ittraining.loc systemd[1]: Stopping The Apache HTTP Server...
Jun 07 18:27:30 centos8.ittraining.loc systemd[1]: httpd.service: Succeeded.
Jun 07 18:27:30 centos8.ittraining.loc systemd[1]: Stopped The Apache HTTP Server.
Jun 07 18:27:32 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 07 18:27:32 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 07 18:27:32 centos8.ittraining.loc httpd[58760]: Server configured, listening on: port 80
Jun 07 18:27:34 centos8.ittraining.loc systemd[1]: Stopping The Apache HTTP Server...
Jun 07 18:27:36 centos8.ittraining.loc systemd[1]: httpd.service: Succeeded.
Jun 07 18:27:36 centos8.ittraining.loc systemd[1]: Stopped The Apache HTTP Server.
[root@centos8 ~]# systemctl mask httpd.service
Created symlink /etc/systemd/system/httpd.service → /dev/null.
[root@centos8 ~]# systemctl enable httpd.service
Failed to enable unit: Unit file /etc/systemd/system/httpd.service is masked.
[root@centos8 ~]# systemctl start httpd.service
Failed to start httpd.service: Unit httpd.service is masked.
To once again allow modifications, use the systemctl unmask command:
[root@centos8 ~]# systemctl unmask httpd.service
Removed /etc/systemd/system/httpd.service.
[root@centos8 ~]# systemctl enable httpd.service
Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service → /usr/lib/systemd/system/httpd.service.
[root@centos8 ~]# systemctl start httpd.service
[root@centos8 ~]# systemctl status httpd.service
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: active (running) since Mon 2021-06-07 18:30:59 EDT; 5s ago
Docs: man:httpd.service(8)
Main PID: 59101 (httpd)
Status: "Started, listening on: port 80"
Tasks: 213 (limit: 23719)
Memory: 39.4M
CGroup: /system.slice/httpd.service
├─59101 /usr/sbin/httpd -DFOREGROUND
├─59102 /usr/sbin/httpd -DFOREGROUND
├─59103 /usr/sbin/httpd -DFOREGROUND
├─59104 /usr/sbin/httpd -DFOREGROUND
└─59105 /usr/sbin/httpd -DFOREGROUND
Jun 07 18:30:59 centos8.ittraining.loc systemd[1]: Starting The Apache HTTP Server...
Jun 07 18:30:59 centos8.ittraining.loc systemd[1]: Started The Apache HTTP Server.
Jun 07 18:31:00 centos8.ittraining.loc httpd[59101]: Server configured, listening on: port 80
Using RHEL / CentOS 8 the halt, poweroff, reboot and shutdown commands are all soft links pointing to /bin/systemctl :
[root@centos8 ~]# ls -l /usr/sbin/shutdown /usr/sbin/halt /usr/sbin/poweroff /usr/sbin/reboot lrwxrwxrwx. 1 root root 16 Apr 7 16:55 /usr/sbin/halt -> ../bin/systemctl lrwxrwxrwx. 1 root root 16 Apr 7 16:55 /usr/sbin/poweroff -> ../bin/systemctl lrwxrwxrwx. 1 root root 16 Apr 7 16:55 /usr/sbin/reboot -> ../bin/systemctl lrwxrwxrwx. 1 root root 16 Apr 7 16:55 /usr/sbin/shutdown -> ../bin/systemctl
The correct way to use these four commands is now:
However you can still use the halt, poweroff, reboot and shutdown commands without specifying systemctl.
The shutdown command's procedure includes :
shutdown's command line switches are as follows:
[root@centos8 ~]# shutdown --help
shutdown [OPTIONS...] [TIME] [WALL...]
Shut down the system.
--help Show this help
-H --halt Halt the machine
-P --poweroff Power-off the machine
-r --reboot Reboot the machine
-h Equivalent to --poweroff, overridden by --halt
-k Don't halt/power-off/reboot, just send warnings
--no-wall Don't send wall message before halt/power-off/reboot
-c Cancel a pending shutdown
The time argument can take several values:
| Value | Description |
|---|---|
| hh:mm | The time at which to shutdown |
| +m | Shutdown the system in m minutes |
| now | Shutdown immediately |
Important : If a shutdown is programmed for less than 5 minutes in the future any future connections are rejected, including those for root.
This command calls the shutdown -r command.
reboot's command line switches are as follows:
[root@centos8 ~]# reboot --help
reboot [OPTIONS...] [ARG]
Reboot the system.
--help Show this help
--halt Halt the machine
-p --poweroff Switch off the machine
--reboot Reboot the machine
-f --force Force immediate halt/power-off/reboot
-w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
-d --no-wtmp Don't write wtmp record
--no-wall Don't send wall message before halt/power-off/reboot
This command calls the shutdown -h command.
halt's command line switches are as follows:
[root@centos8 ~]# halt --help
halt [OPTIONS...]
Halt the system.
--help Show this help
--halt Halt the machine
-p --poweroff Switch off the machine
--reboot Reboot the machine
-f --force Force immediate halt/power-off/reboot
-w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
-d --no-wtmp Don't write wtmp record
--no-wall Don't send wall message before halt/power-off/reboot
This command calls the shutdown -hP command.
halt's command line switches are as follows:
[root@centos8 ~]# poweroff --help
poweroff [OPTIONS...]
Power off the system.
--help Show this help
--halt Halt the machine
-p --poweroff Switch off the machine
--reboot Reboot the machine
-f --force Force immediate halt/power-off/reboot
-w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
-d --no-wtmp Don't write wtmp record
--no-wall Don't send wall message before halt/power-off/reboot
<html> <div align=“center”> Copyright © 2021 Hugh Norris. </html>