What is the difference between echo $BASH_VERSION and echo ${BASH_VERSION}?

Question

I see a lot of ${VARIABLE} type syntax in scripts. What is the point of the braces?

Answer 1

It delimits the variable.

You might need $FOO as a variable, but need to concat it with other text. If so, this won't work:

echo "$FOObar"

This will complain that there's no variable $FOObar. To get around this, delimit the variable:

echo "${FOO}bar"

This will work, and print the value of $FOO with the text 'bar' concatenated.

It's one of those things that people often choose to do all the time, to avoid problems where it's really required. It might be a good habit to get into, as bash scripting is very unforgiving of syntax mistakes.

Answer 2

@mauvedeity is correct. The issue isn't just that some characters, when written after a variable reference are considered part of that reference — it's also that some characters are then considered to not be part of it. This breaks for example variable manipulating operations and use of arrays.

Arrays:

$ foo=( one two three )
$ echo $foo # implied first element with index 0
one
$ echo $foo[1] # this will not work, as [1] is not considered part of variable
one[1]
$ echo ${foo[1]} # this will work
two
$ echo ${foo[*]} # all elements
one two three
$ echo ${#foo[*]} # array length
3
$ echo ${#foo[2]} # length of third element (index 2)
5

Variable operations

$ file=filename.txt
$ echo $file
filename.txt
$ echo ${file%.txt} # remove last match of .txt in $file
filename

All of this would fail (as illustrated in the array example) if we didn't explicitly set the delimiters for the variable reference.