What's is the difference between ">" and ">>" in shell command?

> is used to overwrite (“clobber”) a file and >> is used to append to a file.

Thus, when you use ps aux > file, the output of ps aux will be written to file and if a file named file was already present, its contents will be overwritten.

And if you use ps aux >> file, the output of ps aux will be written to file and if the file named file was already present, the file will now contain its previous contents and also the contents of ps aux, written after its older contents of file.

if you write in terminal

ps aux > log

It will put the output of ps aux to log named file.

then if you put

ps aux >> log

then the next output will be appended below the first. if you put only one > it will overwrite the previous file.

Yes, >> appends, > always overwrites/destroys the previous content.

ps -aux > log

is the same as

rm log 2>/dev/null
ps -aux >> log

On Wintel it is the same for .bat, .cmd and .ps1 scripts too; common heritage, common sense.