问题
We use braces {} for variable expression like
NAME="test"
FILE_NAME=${NAME}file
But I don't understand in which scenarios we use brackets ()
Say nslookup $(hostname) works only with () brackets.
Can someone explain?
回答1:
Minor nitpick first:
- Brackets
[] - Parentheses
() - Braces
{} - (Double) Quotation marks
"" - (Single) Quotation marks (apostrophes)
'' - Backticks
``(Same as the tilde ~ key)
Braces are used in BASh scripts for complex variable expansion. Consider string concatenation:
STR="hello"
STR2=$STR
STR2 evaluates to "hello". What if you wanted to make it something like "helloWorld". Doing something like STR2="$STR2World" won't work, so you use braces, ie: STR2="${STR}World".
As for brackets, they are used, similar to the backtick, `, which expands the text between them as the text output from a command.
What if you wanted to store the current time as a string?
STR2=$(date)
Now STR2 stores the string "Thu May 7 09:32:06 PDT 2015".
Additionally, you can use parentheses to execute something in a subshell, which will potentially affect your environment, PID, etc. Very useful for cases where you want a "throwaway" environment with having to track/restore environment variables, directories via pushd/popd instead of cd, etc.
回答2:
Using parentheses ( executes something. There happens to be a program named hostname - so $(hostname) will execute it.
try which hostname to see where that program resides
来源：https://stackoverflow.com/questions/30106758/difference-between-braces-and-brackets-in-shell-scripting