Question

I just saw an example on a site which piped the output of an awk command to the following command:

(echo "this is a header";cat -)

This effectively adds the header string at the top of the output..it seems like – represents stdout there. What is this construct though? With a parenthesis, and then cat -? How does this work, this seems quite useful but it’s the first time I see it..

Answer 1

The use of parentheses here is a “Grouping Construct” — see the details in the bash reference manual

A similar construct uses braces:

{ echo "this is a header"; cat -; }

This is different syntax that requires whitespace around the braces and the trailing semi-colon.

The difference between using braces and parentheses is this: command list in braces runs in your current shell, command list in parentheses runs in a subshell. This is important if you want the command list to alter your environment — when a subshell exits, environment changes will be lost.