clang hello.c compiling 汇编 source code to machine code
clang hello.c -lcs50 link in cs50
clang -o hello hello.c output hello.c to hello
./hello to run it
make hello = clang -o hello hello.c -lcs50
ls list 所有文件的列表
cd change directory 什么都不接的话跳回home directory
cd . to current directory
cd .. to parent directory (upper) cd ../.. 再返回一层
pwd present working directory
mkdir make directory 创建一个directory
cp hello.txt hi.txt copy hello.txt name as hi.txt
cp -r pset0 pset3 copy recursively directory pset0 and name the new pset 3 因为directory里还有别的文件，所以recursively
rm remove (delete it)
rm -f force remove
rm -r remove recursively a directory
rm -rf remove recursively and force directory
redir remove a directory
mv greddy.c greedy.c move (rename) greddy.c to greedy.c