When do we need curly braces around shell variables?
In shell scripts, when do we use {} when expanding variables?
For example, I have seen the following:
var=10 # Declare variable
echo "${var}" # One use of the variable
echo "$var" # Another use of the variable
Is there a significant difference, or is it just style? Is one preferred over the other?
In this particular example, it makes no difference. However, the {} in ${} are useful if you want to expand the variable foo in the string
"${foo}bar"
since "$foobar" would instead expand foobar .
Curly braces are also unconditionally required when:
${array[42]} ${filename%.*} (remove extension) "$8 $9 ${10} ${11}" Doing this everywhere, instead of just in potentially ambiguous cases, can be considered good programming practice. This is both for consistency and to avoid surprises like $foo_$bar.jpg , where it's not visually obvious that the underscore becomes part of the variable name.
Variables are declared and assigned without $ and without {} . You have to use
var=10
to assign. In order to read from the variable (in other words, 'expand' the variable), you must use $ .
$var # use the variable
${var} # same as above
${var}bar # expand var, and append "bar" too
$varbar # same as ${varbar}, i.e expand a variable called varbar, if it exists.
This has confused me sometimes - in other languages we refer to the variable in the same way, regardless of whether it's on the left or right of an assignment. But shell-scripting is different, $var=10 doesn't do what you might think it does!
You use {} for grouping. The braces are required to dereference array elements. Example:
dir=(*) # store the contents of the directory into an array
echo "${dir[0]}" # get the first entry.
echo "$dir[0]" # incorrect
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