uname -a   # lists name and version of operating system, architecture (CPU), IPC
arch      # architecture of system
hostname   # name of system    -i shows IP address
dmesg     # boot messages.  Linux.  hardware detected and drivers.
df     # filesystem(s) size.  -i shows inodes. -h human -T type
free      # how much memory 
top       # shows RAM
/proc/version
/proc/cpuinfo      # CPU type and current speed, cache size, BogoMips
/proc/interrupts   # IRQs and # of interrupts
/proc/partitions
/proc/swaps
/proc/ioports
/proc/devices    #files that have IRQs and other device parameters

GUI admin tools:
control-panel  Linux configuration: users, networking, printer, filesystem
linuxconf      admin configuration tool
webmin         web-based admin
system-config-*

Users.
who -i    # who's logged on, idle or active.  -l shows from where.
w      # who's logged on.  Login time, from what remote host, idle time, running what command now, how much CPU time its taken (PCPU), how much total CPU time has used (JCPU).
last      # previous logins of users
lastlog   #last login of every user
/etc/passwd    #system users
write username      send msg 
talk username       chat


System state.
times     #bash builtin.  amount of CPU time used by shell and its processes
ps aux # processes in system: all users, in user format, also processes without terminal x
        $ -ef on SVR4

ifconfig      #network packets statistics
iwconfig      #wireless network packets statistics
netstat       #network connections.  -l listening ports. -t tcp statuses.
netstat -l    #ports being listened on
netstat -t    #active TCP ports
nmap          #port scanner

top        # Load average (average number of processes, i.e multitasking)
        # States of all processes: running, sleeping, stopped, undead
        # CPU usage: %user, %system, %idle
        # Memory: total , free, used, shared, buffers usage
        # Swap: total, free, used
        # per process information like in ps:  
        SIZE: virtual memory in KB
        RSS: % of physical memory
        %CPU
        WCHAN: system call sleeping on
        PAGEIN: #page faults