System Monitoring Tools

I/O: iostat (under *BSD), vmstat (Linux)
Memory: vmstat, free (Linux)
Network: netstat

Example: virtual memory statistics while executing the find command

$ vmstat 1
   procs                      memory    swap          io     system         cpu
 r  b  w   swpd   free   buff  cache  si  so    bi    bo   in    cs  us  sy  id
 1  0  0      0  52296   3476  50092   0   0   340     0  189   190   0   6  94
 0  1  0      0  51736   3588  50428   0   0   448     0  214   243   0   5  95
 1  0  0      0  51072   3716  50724   0   0   424     0  210   218   0   5  95
 0  1  0      0  50596   3848  50968   0   0   376     0  196   204   0   4  96
 1  0  0      0  49952   3976  51304   0   0   464     0  220   247   0   6  94
 0  1  0      0  49556   4068  51512   0   0   300     0  177   172   2   2  96
 1  0  0      0  49024   4132  51816   0   0   368     0  196   195   0   7  93
 1  0  0      0  48228   4200  52416   0   0   668     0  270   307   0   9  91
 1  0  0      0  47668   4352  52712   0   0   448     0  215   227   0   5  95
 1  0  0      0  47048   4412  53092   0   0   440     0  213   244   1   5  94
 0  1  0      0  44912   4668  54080   0   0   432     0  211   225   0   4  96