Bash ${var} vs "$var"

Question

When you make a package when you can not be totally sure that the variable contents does not contain spaces you should use "$var" instead of $var or ${var}. Actually the ${var}  is only useful if need some replacement function you can read about in the man.
To see the difference in action try:
$ A="bye bye bye"; touch ${A}
and
$ A="bye bye bye"; touch "$A"
Otherwise try:
$ mkdir a\ a\ a
$ cd a\ a\ a
and inside create a PKGBUILD with this contents:
pkgname=a_a_a
pkgver=1
pkgrel=1
pkgdesc="ah ah ah"
arch=(i686 x86_64)
url=""
license=('WTFPL')
md5sums=()
build() {
cd /
cd ${startdir}/pkg
touch "testing spaces"
return 0 }
executing pkgbuild you'll see it does not work as intended. The second line does not override the first.
Instead using "$startdir" works flawlessly.
In fact the {} does nothing in this cases.
My advice is: if you are not really desperate for diskspace ALWAYS use "$VARIABLE" you can not go wrong. If you are desperate use "$VARIABLE" every time you can not be sure the variable will never contains spaces or other dividing characters (e.g. $pkgver).
Ok, flame war ON!
Edit: Little mistake fixed. But it does not change the whole meaning.
Last edited by ezzetabi (2008-09-27 18:18:34)

Answer 1

@ezzetabi: A valid use case for ${} without array indexing, replacement, and so on, is if you want to do:
MYVAR1="some words perhaps with space"
MYVAR2="a few words before ${MYVAR1}s but not so many"
You could find a way to do that using only "$MYVAR2" instead, but why bother?
Granted this kind of use case doesn't come up in PKGBUILDs. But I'd try to explain to people the difference between these, rather than tell them to always use one of them.
${VAR} is a way to separate the specification of your variable name from the surrounding text. For instance, you may want to say ${VAR}more_text. When the surrounding text isn't valid in a variable name (e.g., if it's spaces), then the ${} isn't needed. But it's a helpful habit to always use it anyway.
"$VAR" and "${VAR}" are ways to chunk together the values stored in VAR. If you're passing this value to a command or another variable, e.g. in:
ANOTHERVAR="$VAR"
rm -f "$VAR"
then if there's any chance that VAR has whitespace in it, you should use surrounding quotes. Otherwise here's what happens. Suppose VAR="first second". Then:
ANOTHERVAR=$VAR
rm -f $VAR
will be read as:
ANOTHERVAR=first second
rm -f first second
which means: set ANOTHERVAR to "first" and execute the command "second", and try to rm each of the files "first" and "second". There are times where that is what you want to do. But usually you'd want to do this instead:
ANOTHERVAR="first second"
rm -f "first second"
and to get that result you have to surround your $VAR (or ${VAR}) in double quotes.
Last edited by Profjim (2008-09-28 14:13:42)