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Previous: 13.7 The () Subshell Operators Chapter 13
Redirecting Input and Output
Next: 13.9 Send Output Two or More Places with tee
 

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

- JP


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