What is the difference between &> and >& in bash?

The shell uses spaces to separate the command to run and its parameters.

In the first example, the command to run is . with a parameter of a.out. The . command is a shell shortcut for source, which takes the name of a file containing shell commands as its first parameter and runs those commands in the current shell. This command fails because a.out is a binary file, not a shell script.

In the second example, the command to run is ./a.out, which means run the file a.out residing in the current directory.

• ./program runs a file named program located in your current working directory (./) (in a new shell for a shell script).
• . is the same as source, which runs a shell script in your current shell. Unlike ./program, it can't be used to run binaries! As an example, you could use this command to run your .bashrc shell script, because you want this script to modify your current shell.