This guide is tailored for RHEL 8 users, providing updated tools, commands, and troubleshooting techniques. It covers a wide range of topics, including boot processes, advanced networking, user administration, storage management, and more.
1. RHEL 8 Boot Process
- MBR → GRUB (loads Kernel image → extracts initramfs) → Kernel (loads drivers from initrd → launches systemd) → SYSTEMD (reads /etc/systemd configs → follows /etc/systemd/system/default.target → reaches required target state).
2. Booting From the GRUB Prompt
- Use the grub> commands to troubleshoot boot issues:
grub> ls grub> ls (hd0,msdos2) grub> set root=(hd0,msdos1) grub> linux /vmlinuz-3.13.0-29-generic ro root=/dev/mapper/centos-root grub> initrd /initrd.img-3.13.0-29-generic grub> boot
3. Kernel-Panic Recovery
- Regenerate the Initrd image:
# mkinitrd /boot/initramfs-3.10.0-957.el7.x86_64.img 3.10.0-957.el7.x86_64 # dracut -f /boot/initramfs-3.10.0-957.el7.x86_64.img 3.10.0-957.el7.x86_64
4. Boot-Time Diagnostics
- Analyze boot performance:
- systemd-analyze → overall boot time.
- systemd-analyze blame → per-unit timing.
5. System Utilization & Diagnostics
- Core tools for monitoring: top, vmstat, iostat, dmesg, lsof, fuser, pmap, ps, sar, strace, watch, stress.
6. File Operations with find
- Delete files:
find / -type f -name tecmint.txt -exec rm -f {} ; find / -type f -perm 0777 -exec rm -f {} ; find / -type f -size +100M -exec rm -f {} ;
- Move files:
find /var/log -type f -name "*.log" -mtime +30 -exec mv {} /archive/logs/ ; find /tmp -type f -mtime +14 -exec mv -t /tmp/old {} +
7. Networking in RHEL 8
- NetworkManager is the default controller. Use nmcli, nmtui, and ip commands.
- Core operations:
- Show interfaces: ip addr, ip link show.
- Bring interface up/down: nmcli device disconnect ens33, nmcli device connect ens33.
- Create static address:
nmcli con add type ethernet con-name static-ens33 ifname ens33 ip4 192.168.1.50/24 gw4 192.168.1.1 nmcli con mod static-ens33 ipv4.dns "8.8.8.8 1.1.1.1" nmcli con up static-ens33
- View routes: ip route.
- Add temporary route: ip route add 10.10.0.0/24 via 192.168.1.254.
8. High-Availability with Pacemaker-Corosync
- Install packages:
yum install -y pcs pacemaker corosync resource-agents systemctl enable --now pcsd
- Authenticate and create cluster:
echo mypass | passwd --stdin hacluster pcs host auth node1 node2 -u hacluster -p mypass pcs cluster setup --name prod-cluster node1 node2 pcs cluster start --all pcs cluster enable --all
- Verify status: pcs status, pcs status corosync, pcs resource show.
9. User Administration
- Configuration Files:
- /etc/login.defs, /etc/default/useradd, /etc/skel/
- Updates /etc/passwd, /etc/group, /etc/shadow, /etc/gshadow.
- Generate shadow only:
/usr/sbin/pwconv
- Examples:
- /etc/passwd (7 fields):
student1:x:501:501::/home/student1:/bin/bash
- /etc/shadow (9 fields):
student1:$1$XSFMv2ru$lfTACjN.XxaxbHA0EkB4U0:13891:0:99999:7:::
- /etc/passwd (7 fields):
- Create user with full options:
useradd -u 9181361 -g 3100000 -G 3100004 -c "Vallabh Darole" -m a181361 ; echo pass@1234 | passwd --stdin a181361
10. Cron & Scheduling
- Cron:
- Example cron job:
* * * * * /bin/bash echo hello
- Example cron job:
- at Usage:
yum install at ; systemctl status atd at -f data.sh now at -f data.sh 04:18 atqu at -d <job#> at -f data.sh now + 1 hour
11. RPM Package Manager Quick Reference
- Commands:
rpm -qa rpm -qi nano rpm -qf /usr/bin/nano rpm -qpi <pkg>.rpm rpm -ql <pkg> rpm -ivh <pkg>.rpm rpm -Uvh <pkg>.rpm rpm -ev <pkg>.rpm rpm -ev --nodeps <pkg>.rpm
12. Disk & Storage Management
- Commands:
echo "1" > /sys/class/fc_host/host$i/issue_lip echo "- - -" > /sys/class/scsi_host/host$i/scan echo 1 > /sys/block/sdX/device/delete
- Restore deleted LV:
vgcfgrestore vg01 -f /etc/lvm/archive/ ; lvchange -a y /dev/vg01/lv01
- Storage migration:
pvmove -v /dev/sdb2 /dev/sdb3
- Move VG to new system:
vgchange -an design ; vgexport design pvscan ; vgimport design ; vgchange -ay design ; mount /dev/design/users /mnt/design/users
13. Multipathing
- Purpose: Provides multiple I/O paths between server ↔ storage for redundancy and load balancing.
- Linux Native Multipathd:
- Daemon: Consolidates device nodes under /dev/mapper/*.
- Config file: /etc/multipath.conf (blacklist, path selectors, policies, etc.).
- Common Commands:
multipath -ll # List multipath devices/paths multipath -f <device> # Flush/remove specific map
- Dell EMC PowerPath:
- Features: Tuned algorithms for EMC arrays, fast failover/failback, advanced load balancing.
- Common Commands:
powermt display dev=all powermt display paths
14. Sticky-Bit Permission
- Purpose: Inside a world-writable directory, only the file owner (or root) may delete/rename their files (e.g., /tmp).
- Permission String Example: drwxrwxrwt (note the trailing t).
- Commands:
chmod +t /dir # Set sticky bit chmod -t /dir # Remove sticky bit
15. NFS Quick Reference
- Export Example:
/etc/exports example /nfs1 192.168.75.10(rw)
- Client Mount:
mount -t nfs 192.168.75.199:/nfs1 /nfs1
- FSTAB Entry:
nfs-server:/export/share /mnt/share nfs rw,soft,timeo=10,retrans=3 0 0
16. Joining RHEL to Active Directory
- Installation:
yum install sssd realmd oddjob oddjob-mkhomedir adcli samba-common samba-common-tools krb5-workstation openldap-clients policycoreutils-python
- Join Command:
realm join --user=<domain_user> <domain_name> realm list cat /etc/sssd/sssd.conf
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