linux command

sudo:superuser do
su:substitute user
touch:give the specified file's current date and time;if the specified file doesn't exist,then create an empty file;
free:display information about memory usage;add -m command option to see output in megabytes;
file:display useful information about the specified file;
find: eg. find /home/zyj -name 'a.txt';
whereis:locate where programs are stored;