If you got problems in remembering commands in linux, maybe you can find useful this quick reference about linux commands. I will updated it as soon as I found more common commands

Generics commands
df-h
      shows disk usage in human readable form
uname -a
     kernel release version
smbd -V
     samba version
httpd -v
     apache version
ps -aux
     outputs processes running
top -i
     shows only active processes
cat /proc/mdstat
     shows software raid
telinit 1
     changes to single user mode
grep -r "casesensitivesearch" *
     finds all documents containing the criteria in a dir
tail -f /var/log/<LOGFILE>
     realtime viewing of your log file
mdadm --detail /dev/mdx (where x is 0,1,2,etc)
     gives software raid info
tar -czvf foo.tar.gz foo
      creates a tar/zip file of a directory
tar -xvzf foo.tar.gz
     untar/unzip a tar/zip file
scp -P <ssh_portnumber> foo.tar.gz <other_server_ipaddress>:/opt
     transfers file to another server
sed -i s/foo/fee/g <FILENAMEORPATHTODIR>
     replaces foo with fee
mkfs.ext3 /dev/sda3
    formats /dev/sda3 disk into  a ext3 file system

RPM Commands
rpm -qa   
     shows all rpms installed
rpm -qa --last
     shows all rpms installed & installation date
rpm -q
     asks for rpm info
rpm -qi
     asks for detailed rpm info
rpm -ql <packagename>
     lists all files in a package
rpm -qf <filename>
     reports what package a file belongs to

DEB Installation
dpkg -i *. deb
     Installs all the *.deb packages in the current folder