change password:
  $ passwd 
  $ passwd s987654321   # if your username is longer than 8 chars

file system:
  $ df -h           # display file system info

  $ mount /dev/sda1   /mnt/usb   # mount /dev/sda1 on /mnt/usb
                                 # /mnt/usb should exist and be a directory
                                 # the original contents of /mnt/usb is hidden

  $ mount /mnt/usb               # If /etc/fstab contains a line for /mnt/usb,
                                 # the device name can be omitted.

  $ umount /mnt/usb              # umount /mnt/usb, original contents is back

  $ mount fs:/export/ghibili  /movies/ghibili   
      'fs' is the NFS server which exports his /export/ghibili
      and this command mounts this directory as our /movies/ghibili.

file:
  viewing:  cat, more, less, head, tail
  remove(delete):   rm 
  move(rename):     mv
  copy:             cp
  link:             ln -s
  permission:       chmod
  others:           wc, file, grep, find

directory:
  viewing:     ls
  change dir:  cd 
  create:      mkdir
  delete:      rmdir

relative pathname:
  cd ..       # go to parent directory
  cd ~        # go to home directory
  cd          #  ... the same ...
  cd ~/sp-981 # go to my sp-981 
  cd ~user    # go to user's home directory
  cd ~klim    # go to klim's direcotory to find .....
  
  cp ../zoo/zebra bala  # cp ../zoo/zebra to current directory and name it bala
  cp ../zoo/zebra .     # cp ../zoo/zebra to current directory (the same name)
  cp ~klim/SIC/.... .   # copy something from klim's SIC direcory 

  mv bala barara        # move bala to barara in current directory 
                        #   same dir ==> change the filename
  mv ../zoo/lion .      # move ../zoo/lion to current direcotory
                        #   different dir ==> move a file
                        #   the ../zoo/lion has gone to where you are