Handling Lists of Parameters with Blanks in bash

OK, you have quotes around your variable as the previous recipe recommended.

It has to do with the $* in the script, used in the for loop.

For this case we need to use a different but related shell variable, $@.

When it is quoted, the resulting list has quotes around each argument separately.

The shell script should be written as follows:

The parameter $* expands to the list of arguments supplied to the shell script. If you invoke your script like this:

then $* refers to the three arguments these are args. And when used in a for loop, such as:

then the first time through the loop, $FN is assigned the first word (these) and the second time, the second word (are), etc.

If the arguments are filenames and they are put on the command line by pattern matching, as when you invoke the script this way:

then the shell will match all the files in the current directory whose names end with the four characters .mp3, and they will be passed to the script.

So consider an example where there are three MP3 files whose names are:

The second song title has a blank in the filename between cool and music. When you invoke the script with:

you’ll get, in effect:

If your script contains the line:

that will expand to:

which has four words in its list, not three.

The second song title has a blank as the fifth character (cool music.mp3), and the blank causes the shell to see that as two separate words (cool and music.mp3), so $FN will be cool on the second iteration through the for loop.

On the third iteration, $FN will have the value music.mp3 but that, too, is not the name of your file.

You’ll get file-not-found error messages.
It might seem logical to try quoting the $* but

will expand to:

and you will end up with a single value for $FN equal to the entire list. You’ll get an error message like this:

Instead you need to use the shell variable $@ and quote it. Unquoted, $* and $@ give you the same thing. But when quoted, bash treats them differently.

A reference to $* inside of quotes gives the entire list inside one set of quotes, as we just saw.

But a reference to $@ inside of quotes returns not one string but a list of quoted strings, one for each argument.

In our example using the MP3 filenames:

will expand to:

and you can see that the second filename is now quoted so that its blank will be kept as part of its name and not considered a separator between two words.
The second time through this loop, $FN will be assigned the value cool music.mp3, which has an embedded blank.

So be careful how you refer to $FN—you’ll probably want to put it in quotes too, so that the space in the filename is kept as part of that string and not used as a separator.

That is, you’ll want to use “$FN” as in:

Shouldn’t you always use “$@” in your for loop?

Well, it’s a lot harder to type, so for quick-and-dirty scripts, when you know your filenames don’t have blanks, it’s probably OK to keep using the old-fashioned $* syntax.

For more robust scripting though, we recommend “$@” as the safer way to go.

We’ll probably use them interchangeably throughout this book, because even though we know better, old habits die
hard—and some of us never use blanks in our filenames! (Famous last words.)