35
( du /etc; du /var; ) > tmp.txt

{ du /etc; du /var; } > tmp.txt

Is there a difference between the () and {}?

The output of tmp.txt seems exactly the same, and I was wondering whether i'm missing something here.

Gilles 'SO- stop being evil'
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Nelske
  • 461

1 Answers1

47

Parentheses cause the commands to be run in a subshell.

Braces cause the commands to be grouped together but not in a subshell.

Given that your example does not use side-effects, there is no real difference between both. If there were side-effects, e.g. setting or modifying shell variables, there is a difference as such side-effects applied to a sub-shell will be forgotten when this sub-shell ends.

To understand the "side-effect", see the following examples:

Using parentheses ():

v="test"; 
( echo $v; v="modified"; echo $v; ); 
echo $v;

output:


test


modified


test

Using curly braces {}:

v="test"; 
{ echo $v; v="modified"; echo $v; }; 
echo $v;

output:


test


modified


modified

If you however take a closer look and compare the behavior of different shell implementations, it becomes confusing:

The Bourne Shell e.g. runs grouped commands in a subshell in case there is an I/O redirection and ksh93 avoids subshells by implementing virtual subshell behavior that is done by creating a temporary copy of new parameters. Whether this is always 100% correct is not known, ksh93 Version M 1993-12-28 s+ from 2009 e.g. implements $(...) incorrectly and $(alias a=b) affects the main shell.

So in general: if you are interested in specific aspects, be careful and check your shell for it's actual behavior.

Amin.A
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schily
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    (I wouldn't have bothered mentioning it but since you're maintaining a Bourne shell ;)), ...not in a subshell, except for the Bourne shell that runs compound commands in subshells when redirected. – Stéphane Chazelas Mar 04 '16 at 17:11
  • Well parentheses grant the commands to be in a subshell, with braces, it still may happen ;-) but ksh93 does not even create a real subshell for parentheses, but rather emulates the effects of a subshell by using a temporary new copy of parameters. – schily Mar 04 '16 at 17:14
  • Yes, you'll notice that the POSIX spec is careful to talk only of subshell environment with no implication that it may involve a child process. And with many shells that implement subshells using a child process, (a;b) will not spawn more processes than {a;b;} (if b is not a builtin nor function nor compound command and there's no local trap) as b will be executed in that child subshell process. – Stéphane Chazelas Mar 04 '16 at 17:20
  • So if I understand correctly, the only difference is that: parentheses use a subshell and braces don't?

    And the output stays the same, regardless of the subshell (in this case).

    Thankyou, this helped!

     – Nelske Mar 04 '16 at 17:32
  • See my new paragraph about side-effects. In your example, there is no visible difference. – schily Mar 04 '16 at 17:36