CentOS8 CheatSheet fuer System Administratoren

1 SOS Report (Log Bundle)

This collects a log bundle which can be used for later debugging and analyzing.
Start this BEFORE you try to repair or change something, as well do not forget to snapshot/backup System/Application before debugging.

dnf -y install sos
sosreport
cp -av /var/tmp/sosreport* /root/

1.1 Block Devices (Disks)

• Show block uage of mounted disks

df -hP

• Show inode uage of mounted disks

df -hiP

• show how blockdevices are configured in system

lsblk

• Show memory statistics

free -th

2 Services

• list all service units

systemctl list-unit-files

• show all services in failed state

systemctl --all --state=failed

• show all running services

systemctl --all --state=running

• show detailed service status

systemctl start <svc-name>

• service handing

 systemctl [start|stop|restart|enable|disable|mask] <svc-name>

• check if service is enabled to autostart after reboot

systemctl is-enabled <svc-name>

3 Events

3.1 Show last boots

journalctl --list-boots
last reboot

3.2 Journal

• Show log since last boot

journalctl -b

• Kernel messages (like dmesg)

journalctl -k

• Show latest log and wait for changes

journalctl -f

• Reverse output (newest first)

journalctl -r

• Show only errors and worse

journalctl -b -p err

• Filter on time (example)

journalctl --since=2014-06-00 --until="2014-06-07 12:00:00"

• Since yesterday

journalctl --since=yesterday

• Show only log of SERVICE

journalctl -u SERVICE

• Match executable, e.g. dhclient

journalctl /usr/sbin/dhclient

• Match device node, e.g. /dev/sda

journalctl /dev/sda

3.3 Log Files

• tail all currently open text files

lsof -F | sed '/^n\//!d;s/^n//;s/ .*//' | sort -u | xargs file | grep ' text$' | cut -d: -f1 | xargs tail -fn0

• tail all files under /var/log

tail -f -n0 /var/log/* /var/log/*/*

4 CentOS 8 Stream migration

CentOS8 will be discontinued by the Red Hat team. All the effort will go into CentOS Stream operating System which is a rolling release of CentOS.
You can directly migrate a CentOS8 into a CentOS8 Stream in place:

dnf install centos-release-stream
dnf distro-sync
cat /etc/centos-release
reboot