faverity
df
ping
top,htop – system and per-process interval summary
ps – process status listing
iostat – block i/o (disk) stats. output is since boot
iotop – block device i/o (disk) by process
nicstat – network interface stats, iostat-like output
nmon
ss,iptraf – more socket statistics
etc
vmstat – virtual memory statistic
mpstat – multi-processor statistics, per-CPU
free – main memory usage
strace – system call tracer
netstat – various network protocol statistics
pidstat – very useful process stats. eg, by-thread, disk i/o
swapon – show swap device usage
sar – system activity reporter. many stats
collectl – sar-like multitool
slabtop – kernel slab allocator memory usage
iosnoop – block i/o(disk) events with latency
iolatency – block i/o(disk) latency distributions
opensnoop – trace open() syscalls showing filenames
funcgraph – trace a graph of kernel code flow