It can't be over-emphasized how important it is to wrap expansions, such as $myvar, ${myvar[1]}, and ${myvar##*/}, in double quotes. Failing to do this will lead to bugs down the line when values contain characters that have special meaning to Bash, especially spaces. This is a common problem with code that has been cut and pasted from old documentation, or from bad advice posted on websites or in chat channels.

It only takes a few examples to realize how dangerous a failure to quote properly can be, especially in circumstances where files can be created by other users, and hence might include shell metacharacters. A file named *, for example, is legal, and if expanded incorrectly, can wreak havoc on your scripts:

$ cd ~/important
$ myfilename='*'
$ echo $myfilename
important-document  passwords-DO-NOT-DELETE  anniversary-plans
$ echo "$myfilename"
*

Imagine what might have happened if the first echo command here had been rm in a script instead!

In a few contexts in Bash, an expansion does not strictly require double quotes; none of these cases of myvar here require double quotes, due to their syntactic context:

newvar=$myvar
case $myvar in ...
[[ $myvar = "$othervar" ]]

However, adding double quotes in each of these cases does not do any harm, either:

newvar="$myvar"
case "$myvar" in ...
[[ "$myvar" = "$othervar" ]]

Similarly, watch for cases where you need more than one pair of nested quotes:

touch -- "$(basename -- "$filename")"

This might seem strange to those coming from other programming languages; it looks as if the two different sets of double quotes are mismatched. However, because one pair of quotes is inside a command substitution, the double quotes around it don't interfere with the expansion within it.

In summary, if you're in doubt about any given expansion: quote it!

You may occasionally run into objections of this sort: I don't allow files with spaces or asterisks in their names on my system, so I don't need to quote. Don't listen! Always use quoting to make your scripts safer and more robust.