13.8 Using {list} to Group Bourne Shell Commands

A lot of people know that you can group the output of a series of commands by using a subshell (13.7 ) . That is, instead of this:

$ date > log


$ who >> log


$ ls >> log

they start a subshell with parentheses:

>
 

$ (date


> who


> ls) > log

and only redirect once to log . But a subshell takes an extra process and takes time to start on a busy system. If all you need to do is redirect output (or input) of a set of commands, use the Bourne shell's list operators {} (curly braces):

$ { date


> who


> ls


> } > log

Notice the spaces and the extra RETURN at the end. Each command must be separated from others. You can also write (note the semicolon after the last command):

$ { date; who; ls; } > log

Here are two other differences between the subshell (parentheses) and list (curly braces) operators. A cd command in the subshell doesn't change the parent shell's current directory; it does in a list. Also, a variable set in a subshell isn't passed to the parent shell; from a list, the variable is passed out.

NOTE: Jonathan I. Kamens points out that some Bourne shells may run a list in a subshell anyway, especially if there's a pipe involved. If your Bourne shell works like the example shown here, it's using a subshell, too:

$ { echo frep; foo=bar; } | cat


frep
$ echo $foo



$ { echo frep; foo=bar; }



frep
$ echo $foo


bar