Complete Linux Administration Interview Preparation: 200+ Commands, Troubleshooting, LVM, NFS, SSH & Boot Issues
A complete Linux Admin interview preparation guide with 200+ real commands, troubleshooting scenarios, LVM/NFS/SSH/boot issue solutions, and real-world patching workflows.
1) 200+ Admin Commands (Most Asked in Interviews)
System Information
uname -a
hostnamectl
lsb_release -a
dmidecode -t system
User & Group Management
useradd devuser
passwd devuser
groupadd admin
usermod -aG admin devuser
id devuser
File & Directory Management
ls -lrt
mkdir -p /data/app
cp -rvf source/ dest/
mv file1 file2
rm -rf /tmp/*
Permissions
chmod 755 script.sh
chown root:root /etc/ssh/sshd_config
setfacl -m u:dev:rwx /data
Network Commands
ip addr show
ip route show
nmcli connection show
ss -tulnp
nc -zv 10.0.0.5 22
ping -c 4 google.com
traceroute yahoo.com
Disk & LVM
fdisk -l
pvcreate /dev/sdb
vgcreate vgdata /dev/sdb
lvcreate -L 10G -n lvdata vgdata
mkfs.ext4 /dev/vgdata/lvdata
mount /dev/vgdata/lvdata /mnt
Package Management
yum install httpd -y
dnf update -y
rpm -qa | grep kernel
yum history list
Service Management
systemctl status sshd
systemctl restart network
systemctl enable httpd
systemctl stop firewalld
Logs
journalctl -xe
tail -f /var/log/messages
grep -i error /var/log/secure
2) Troubleshooting Scenarios (Most Asked in Interviews)
Scenario 1: Server is Slow
Checklist:
top
vmstat 1 5
iostat -xz 1 3
free -h
dmesg | tail
Probable Reasons:
- High CPU load
- Memory leak
- I/O wait
- Zombie processes
Scenario 2: Application Not Accessible
Check service
systemctl status httpd
Check port
ss -tulnp | grep 80
Check firewall
firewall-cmd --list-all
Check SELinux
getenforce
semanage port -l | grep http
Scenario 3: Disk Full
df -h
du -sh /* | sort -h
journalctl --vacuum-size=500M
Trim logs
truncate -s 0 /var/log/messages
Scenario 4: High CPU by a Process
top -c
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu | head
kill -9 <pid>
Scenario 5: SSH Not Working
Check port config
grep Port /etc/ssh/sshd_config
Check firewall
firewall-cmd --list-ports
Restart
systemctl restart sshd
Check key auth
chmod 600 ~/.ssh/authorized_keys
3) LVM, NFS, SSH, Boot Issue Troubleshooting
LVM Troubleshooting
Not Enough Space in VG
vgs
vgextend vgdata /dev/sdc
LV Not Mounting
lvscan
fsck /dev/vgdata/lvdata
mount -a
Extend File System
lvextend -L +5G /dev/vgdata/lvdata
resize2fs /dev/vgdata/lvdata
NFS Troubleshooting
Check NFS Mount
mount -t nfs server:/data /mnt
showmount -e server
Check Firewall
firewall-cmd --add-service=nfs --permanent
Check SELinux
setsebool -P nfs_export_all_rw 1
Permission Issues
chown -R nfsuser:nfsuser /data
chmod 755 /data
SSH Troubleshooting
Permission Denied (publickey)
Check file permissions:
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
Host key mismatch
ssh-keygen -R server-ip
Boot Issues & GRUB Troubleshooting
Scenario: Kernel Panic
Run fsck:
fsck -y /dev/sda2
Scenario: Wrong fstab Entry
Boot into rescue mode → edit with:
nano /etc/fstab
GRUB Not Loading
Reinstall GRUB:
grub2-install /dev/sda
grub2-mkconfig -o /boot/grub2/grub.cfg
Boot into Single-User Mode
At GRUB menu:
Press e → add rd.break
Change root password:
mount -o remount,rw /sysroot
chroot /sysroot
passwd root
touch /.autorelabel
4) Real Patching & Maintenance Workflows
1) Pre-Patching Tasks
uptime
df -h
free -h
systemctl list-units --failed
cp -r /etc /backup/etc_prepatch/
Check running users:
who
2) Kernel Update
yum update kernel -y
grubby --default-kernel
3) Reboot Planning
- Communicate with team
- Create change ticket
- Get downtime approval
- Check services before reboot
4) Post-Patching Checks
uname -r
systemctl status <services>
df -h
journalctl -xe
Check application status:
curl -I http://localhost
5) Rollback Plan
- Boot from old kernel
- Disable problematic services
- Restore config backups
