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.

Most important difference is that > makes shell open a file or file-like object with O_WRONLY|O_CREAT|O_TRUNC flags - the file will be created or truncated if it exists, while >> opens file with O_WRONLY|O_CREAT|O_APPEND flags - file will be created or appended to if it exists. This is evident if you trace system calls, for example with

$ strace -e open,dup2 bash -c 'true >> /dev/null'
...
open("/dev/null", O_WRONLY|O_CREAT|O_APPEND, 0666) = 3
dup2(3, 1)                              = 1
dup2(10, 1)                             = 1

And with

$ strace -e open,dup2 bash -c 'true > /dev/null'
...
open("/dev/null", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
dup2(3, 1)                              = 1
dup2(10, 1)                             = 1
+++ exited with 0 +++

Notice that in both cases the file descriptor of the open file is duplicated onto file descriptor 1 ( stdout ) of the command, and that will be inherited by whatever command the shell forks.