Difference between braces {} and brackets () in shell scripting

Question

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?

Answer 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.

Answer 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