The best way

export VAR=value

The difference

Doing

VAR=value

only sets the variable for the duration of the script (.bashrc in this case). Child processes (if any) of the script won't have VAR defined, and once the script exits VAR is gone.

export VAR=value

explicitly adds VAR to the list of variables that are passed to child processes. Want to try it? Open a shell, do

PS1="foo > "
bash --norc

The new shell gets the default prompt. If instead you do something like

export PS1="foo > "
bash --norc

the new shell gets the prompt you just set.

Update: as Ian Kelling notes below variables set in .bashrc persist in the shell that sourced .bashrc. More generally whenever the shell sources a script (using the source scriptname command) variables set in the script persist for the life of the shell.

Both are a wrapper to the same fundamental function which is an unlink() system call.

To weigh up the differences between the userland utilies.

rm(1):

• More options.
• More feedback.
• Sanity checking.
• A bit slower for single calls as a result of the above.
• Can be called with multiple arguments at the same time.

unlink(1):

• Less sanity checking.
• Unable to delete directories.
• Unable to recurse.
• Can only take one argument at a time.
• Marginally leaner for single calls due to it's simplicity.
• Slower when compared with giving rm(1) multiple arguments.

You could demonstrate the difference with:

$ touch $(seq 1 100)
$ unlink $(seq 1 100)
unlink: extra operand `2'

$ touch $(seq 1 100)
$ time rm $(seq 1 100)

real    0m0.048s
user    0m0.004s
sys     0m0.008s

$ touch $(seq 1 100)
$ time for i in $(seq 1 100); do rm $i; done

real    0m0.207s
user    0m0.044s
sys     0m0.112s

$ touch $(seq 1 100)
$ time for i in $(seq 1 100); do unlink $i; done

real    0m0.167s
user    0m0.048s
sys     0m0.120s

If however we're talking about an unadulterated call to the system unlink(2) function, which I now realise is probably not what you're accounting for.

You can perform a system unlink() on directories and files alike. But if the directory is a parent to other directories and files, then the link to that parent would be removed, but the children would be left dangling. Which is less than ideal.

Edit:

Sorry, clarified the difference between unlink(1) and unlink(2). Semantics are still going to differ between platform.