Handy guide to bash brackets, swiped from Stackoverflow.
Brackets
if [ CONDITION ] Test construct if [[ CONDITION ]] Extended test construct Array[1]=element1 Array initialization [a-z] Range of characters within a Regular Expression
Curly Brackets
${variable} Parameter substitution
${!variable} Indirect variable reference
{ command1; command2; . . . commandN; } Block of code
{string1,string2,string3,...} Brace expansion
{a..z} Extended brace expansion
{} Text replacement, after find and xargs
Parentheses
( command1; command2 ) Command group executed within a subshell Array=(element1 element2 element3) Array initialization result=$(COMMAND) Command substitution, new style >(COMMAND) Process substitution <(COMMAND) Process substitution
Double Parentheses
(( var = 78 )) Integer arithmetic var=$(( 20 + 5 )) Integer arithmetic, with variable assignment (( var++ )) C-style variable increment (( var-- )) C-style variable decrement (( var0 = var1<98?9:21 )) C-style trinary operation
This might be a bit of a niche subject, but I’m leaving it here for my own use.