ls        : display the content of the current directory
ls -l     : display the contents of the current directory, with info on file permissions
ls -l xxx : display the rights of file xxx
ls -al    : display the contents of the current directory, including hidden files
cat xxx   : display the content of file xxx
pwd       : current directory
cd xxx    : move to the xxx directory
cd .      : move to parent directory
id        : identifier of the current account and groups it belongs to
uname -a  : server information: which distribution and kernel version.