RHEL 8 Linux Cheat-Sheet & Troubleshooting Guide

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.
journalctl -xb              # Logs from last boot
journalctl -k                # Kernel logs
dmesg  -T| less                  # Kernel ring buffer

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.
    • nmcli device status ; nmcli connection show

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:::
  • 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
  • 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
at

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