Expanding Commands

With command substitution, you essentially let the shell handle a command's output rather than the command itself. This nifty trick allows one command's output to be passed as an argument to another command. You've got two ways to do this:

1. $(command)

2. `command`

Just a heads up, those are backticks on the latter, not your usual single quotes.

In this approach, the command can have all the bells and whistles: options, metacharacters, and arguments. Here's an example of command substitution in action:

vi $(find /home | grep xyzzy)

In this example, the command substitution is done before the vi command is run. It runs as follows:

1. First, the find command starts at the /home directory and prints out all of the files and directories below that point in the filesystem

2. The output is piped to the grep command, which filters out all files except for those that include the string xyzzy in the filename

3. Finally, the vi command opens all filenames for editing (one at a time) that include xyzzy.

If you run the above steps and are not familiar with vi, you can type :q! to exit the file.

This particular example is useful if you want to edit a file for which you know the name, but not the location. As long as the string is uncommon, you can find and open every instance of a filename existing beneath a point you choose in the filesystem.

In other words, don’t use grep from the root filesystem or you’ll match and try to edit several thousand files.

Expanding Arithmetic Expressions

Occasionally, you might fancy passing some maths results over to a command. There are a couple of ways you can expand a mathematical expression and hand it over to the shell:

$[expression]

Arithmetic expression is interpreted first, and passed to echo for printing to the terminal

or

 $(expression)

The result of the commands, once complete, is passed to echo for printing to ther

Expanding Variables

Variables that store information within the shell can be expanded using the dollar sign $ metacharacter.

When you expand an environment variable on a command line, what you'll see printed is the variable's value, not its name, like this:

ls -l $HOME

Using $HOME as an argument to ls -l causes a long listing of the home directory to be printed