The use of parentheses and quotes in Bash can be quite confusing. Here is a quick guide, which comes largely from https://www.tldp.org, in the public domain.
Square 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=$(( 20 + 5 )) | Integer arithmetic, with variable assignment |
| (( var++ )) | C-style variable increment |
| (( var– )) | C-style variable decrement |
| (( var0 = var1=98?9:21 )) | C-style ternary operation |
Quoting
| “$variable” | “Weak” quoting |
| ‘string’ | ‘Strong’ quoting |
Back Quotes
| result=`COMMAND` | Command substitution, classic style |
