( 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.
Answers:
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Method 1
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.
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.
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