Unix Power Tools
by Tim O'Reilly and Jerry Peek
The () Subshell Operators
A useful shell trick is to use parentheses,
(), to group commands.
Combining Several Commands
The output of the entire group can be passed together into a single pipeline. For example:
$ (cat file1; echo .bp; cat file2) | nroff
This will interpose the nroff
.bp (break page)
request between two files to be formatted. *
Parentheses are also very useful in the Bourne shell if you want to put an entire sequence of commands separated by semicolons into the background. In the C shell, the command line below will go immediately into the background.
$ nroff -ms file1; nroff -ms file2 &
But in the Bourne shell, the background request (
&) will only apply to the second command, forcing you
to wait for completion of the first job before you get back the system prompt.
To get right back to work, you can type:
$ (nroff -ms file1; nroff -ms file2) &
Temporary Change of Directory and Environment
The parentheses start a subshell. Commands that run between the parentheses won't affect the parent shell's environment. For instance, to run a command in another directory without changing your active shell's current directory :
% (cd somewhere-else; nroff -ms file1 > file.out) &
The file file.out will be created in the somewhere-else directory.
* If you're using only cat and a single echo, you can use this command instead:
$ echo .bp | cat file1 - file2 | nroff
The cat - option tells cat to read its standard input (in this case, from the pipe and the echo) at that point. nroff gets exactly the same input.