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Previous: 13.7 The () Subshell Operators Chapter 13
Redirecting Input and Output
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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
$ echo $foo

$ { echo frep; foo=bar; }

$ echo $foo

- JP

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