Re: quotes around a variable

[Robert E. Griffith]

Here are two rules that I follow that helped me...

1) even though an assignment will work even if unquoted, like...

   var1=$var2
       vs...
   var1="$var2"

even if var2 contains spaces, there is no reason that you need to use or 
remember that feature. If you always put quotes around "$var2" its 
simple and always does what you expect.

2) I remember that an eye opening moment for me in writing bash scripts 
was realizing that when you double quote parameters to a 
function/commend call, you are ensured that you are passing that many 
arguments even if some are empty. An example might make that more clear...

   function foo() {
        echo "first='$1'"
        echo "second='$2'"
   }
   var1=""
   var2="hello"

   foo "$var1" "$var2"

   foo $var1 $var2

With the double quotes around the arguments, 'hello' will be in the 
second position ($2) and the first position ($1) will be empty, but 
since var1 happens to be empty in this case, without quotes it wont take 
up a position and 'hello' will (unexpectedly) be in the first position.

The lesson for me is that I always use double quotes around 
function/command call positional arguments. Even if I am passing a 
literal string that can not be empty, I put it in double quotes...

   foo "blue" "$var2"  # its never wrong to quote every positional argument so 
its an easy rule to follow

For optional arguments I often purposely do not quote them...

   function foo() {
        [ "$1" == "-f" ] && { doSomething...; shift; }
        echo "first='$1'"
        echo "second='$2'"
   }

   forceFlag="-f"  # lets say that forceFlag comes from somewhere and could be empty or 
could contain "-f"

   foo $forceFlag "$var1" "$var2"

Now if forceFlag is empty, it wont take up an argument position and var1 
and var2 will be $1 and $2 as expected but if it contains "-f", it will 
take up a position but because function foo follows the convention that 
options are removed (shifted) before positional arguments are handled, 
var1 and var2 will still be in the correct and predictable positions.

One more note on passing arguments to functions/commands ...

   foo "${options[@]}" "position1" "position2"

Even though the "${options[@]}" term is double quoted, if the options 
array is empty, it will not add any arguments. The quoted [@] expansion 
will produce one token for each element in the array no matter what is 
in each array element. Therefore if there are no elements, it produces 
no output at all and it disappears.

--BobG

On 4/16/22 07:29, wilson wrote:
> when doing bash script, I am somewhat confused that, when I should use 
> Double quotes around the variable, when I should use single quotes 
> around the variable, and when I don't need the quotes. can you help 
> give a guide?
>
> Thank you.