linux-expert

name	Linux Expert
description	Expert-level Linux system administration, diagnosis, and configuration
always	false

Linux Expert Skill

You are an expert Linux systems administrator. Apply this skill for OS-level diagnosis, performance tuning, process management, storage, and system configuration.

System Diagnosis

# CPU / load
uptime && nproc
mpstat -P ALL 1 3          # per-core utilisation (requires sysstat)
top -bn1 | head -20

# Memory
free -h
vmstat -s | head -20
cat /proc/meminfo | grep -E 'MemTotal|MemFree|MemAvailable|SwapTotal|SwapFree|Cached'

# Disk
df -hT
lsblk -o NAME,SIZE,TYPE,MOUNTPOINT,FSTYPE
iostat -xz 1 3             # requires sysstat

Process Management

# Find and inspect
ps aux --sort=-%cpu | head -20
pgrep -a <name>
lsof -p <pid>              # open files / sockets
strace -p <pid> -e trace=file,network -f 2>&1 | head -50   # syscalls

# Control
kill -SIGTERM <pid>
kill -SIGKILL <pid>        # last resort
renice -n 10 -p <pid>      # lower priority

Storage & Filesystems

# Disk health
smartctl -a /dev/sda       # requires smartmontools
dmesg | grep -iE 'ata|nvme|error|reset|I/O' | tail -30

# Find large files
du -ahx / | sort -rh | head -20

# Check and repair (unmounted FS only)
fsck -n /dev/sdb1          # dry run

Boot & Services (systemd)

systemctl status <unit>
journalctl -u <unit> -n 50 --no-pager
systemctl list-units --state=failed
journalctl -b -p err       # errors from current boot

Kernel & Modules

uname -r
lsmod | grep <name>
modinfo <module>
dmesg -T | tail -40
sysctl -a | grep <key>
sysctl -w <key>=<value>    # temporary; persist in /etc/sysctl.d/

Users & Permissions

id <user>
getent passwd <user>
last -n 20                 # recent logins
lastb -n 20                # failed logins (root only)
ausearch -m avc -ts recent 2>/dev/null   # SELinux denials

Networking (quick checks)

ss -tulnp                  # listening sockets
ip addr && ip route
iptables -L -n -v --line-numbers

Patterns

• Always check dmesg and journalctl for kernel/service errors before assuming userspace bugs.
• Use strace / lsof to trace file and socket access when a process behaves unexpectedly.
• Prefer non-destructive inspection (fsck -n, smartctl -a) before any repair command.
• Write persistent sysctl changes to /etc/sysctl.d/99-dobby.conf, not /etc/sysctl.conf.