With the first method you are not creating an alias, you are creating a symlink. Symlinks are short for symbolic links:
Symbolic links are files that act as
pointers to other files. [...] A
symbolic link is a special type of
file whose contents are a string that
is the pathname another file, the file
to which the link refers. In other
words, a symbolic link is a pointer to
another name, and not to an underlying
object.
Read more about symlinks here and here.
Only with the second method you are, in fact, creating an alias.
Aliases allow a string to be
substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases
that may be set and unset with the
alias and unalias builtin commands
(see SHELL BUILTIN COMMANDS below).
The first word of each simple
command, if unquoted, is checked to
see if it has an alias. If so, that
word is replaced by the text of the
alias.
You can define an alias anywhere where you can type a command and have the shell (bash in this case) interpret it, however in order for the alias to be available in other shells it needs to be defined in a file that's interpreted by the shell on startup (shell startup, not computer startup).
For bash these are /etc/bash.bashrc (system wide) and ~/.bashrc. These files are interpreted when the shell starts in interactive mode (like when using Terminal). I'm not going to mention the profile files because they serve a different purpose.
So, you want to add your aliases to ~/.bashrc to have them available in every interactive shell.
The .bash_aliases method accomplishes exactly the same thing as putting the aliases in ~/.bashrc but has the added benefit of being easier to be parsed and manipulated by programs.
The . ~/.bash_aliases means source (load) _~/.bash_aliases_ in the context of the currently running shell.
source and . are synonymous in Bash.
For anyone who might like to verify that the commands are simply synonyms and nothing more, look at the source code, say for version 4.3, and examine the file builtins/source.def. You will read that both of the built-in commands, source and ., use the very same function: source_builtin.
Best Answer
The 1st one sets the value to the variable. For instance you can do
and have it return the value but if you would issue that outside your shell you will have an empty string returned.
In comes
export.Export is an instruction to the shell. It tells the shell to make this environment variable available to other programs. Without the export, they are only available within the shell itself.
If you want the variable to be permanently available ssh reads
~/.ssh/environment,ssh2reads/etc/environmentand~/.ssh2/environment, and adds lines of the format VARNAME=value to the environment. From thesshman page.