Good Habit #6: Group your commands together in a list

Most shells have ways to group a set of commands together in a list so that you can pass their sum-total output down a pipeline or otherwise redirect any or all of its streams to the same place. You can generally do this by running a list of commands in a subshell or by running a list of commands in the current shell.

Run a list of commands in a subshell

Use parentheses to enclose a list of commands in a single group. Doing so runs the commands in a new subshell and allows you to redirect or otherwise collect the output of the whole, as in the following example:

Listing 1. Example of good habit #6: Running a list of commands in a subshell

~ $ ( cd tmp/a/b/c/ || mkdir -p tmp/a/b/c && \ > VAR=$PWD; cd ~; tar xvf -C $VAR archive.tar ) \ > | mailx admin -S "Archive contents"

In this example, the content of the archive is extracted in the tmp/a/b/c/ directory while the output of the grouped commands, including a list of extracted files, is mailed to the admin address.

The use of a subshell is preferable in cases when you are redefining environment variables in your list of commands and you do not want those definitions to apply to your current shell.

Run a list of commands in the current shell

Use curly braces ({}) to enclose a list of commands to run in the current shell. Make sure you include spaces between the braces and the actual commands, or the shell might not interpret the braces correctly. Also, make sure that the final command in your list ends with a semicolon, as in the following example:

Listing 2. Another example of good habit #6: Running a list of commands in the current shell

~ $ { cp ${VAR}a . && chown -R guest.guest a && \
> tar cvf newarchive.tar a; } | mailx admin -S "New archive"