New to bash scripting. In the beginning of a bash script a variable is initialized
silent_install=true
Then this variable is used in the following 2 ways in the script
if [ -z "$silent_install" ]; then
and
if [ -z ${silent_install} ]; then
What is the difference between the 2 usages?
Short: the second one is wrong.
Wrong because an unquoted variable will be subject of word splitting. If the variable contains spaces the test will fail.
The {} does not make a difference, you can use them or not, with or without quotes. However, generally I would advice to use them. Especially if you interpolate them like "$foo_bar". What's the variable name here? "${foo}_bar" is much cleaner.
I would consequently use:
if [ -z "${silent_install}" ] ; then
[[ ]] is a bash syntax feature rather than an external command (or command builtin), word splitting would not happen in that case.
[[ ]], such as on the right-hand side of a string comparison. IMO double-quoting everywhere is easier than trying to keep track of where it matters & where they're safe to leave off.You should use the double quotes making it ONE string. With the value true you will see no difference, but you will get into trouble when silent_install has special characters or spaces (silent_install="I do not know").
Start getting used to {}. They do not matter here, but will matter when you have oher characters right after it ($SILENT_INSTALLandmore).
UPPERCASE characters are reserved for bash, so use if [ -z "${silent_install}" ]
In addition to the problems with word splitting the other answers point out, there's another problem with [ -z ${silent_install} ]. In one of the expected cases, where silent_install is unset or blank, the test command expands to [ -z ]. When the [ command gets an expression with just a single argument, it tests that argument to see if it's non-blank. In this case, "-z" is non-blank, so the test returns true. This is the expected result, but that's just a coincidence; it's actually testing something very different from what the script author assumed it was.
The more serious variant of this scripting problem is the reverse test, [ -n ${somevar} ] (-n means "is not blank", the opposite of -z). Because [ -n ] actually tests whether -n is non-blank, it returns true, which is the opposite of what you'd expect.
The moral of the story is "double-quote your variable references!" It prevents all sorts of weird bugs and unexpected behavior.
