What if one of the programs to which you would like to connect with a pipe doesn’t work that way? For example, you can remove files with the rm command, specifing the files to be removed as parameters to the command:

$ rm my.java your.c their.*

but rm doesn’t read from standard input, so you can’t do something like:

find . -name '*.c' | rm

Since rm only takes its filenames as arguments or parameters on the command line, how can we get the output of a previously-run command (e.g., echo or ls) onto the command line?

Use the command substitution feature of bash:

$ rm $(find . -name '*.class')
$

The $() encloses a command that is run in a subshell. The output from that command is substituted in place of the $() phrase. Newlines in the output are replaced with a space character (actually it uses the first character of $IFS, which is a space by default, during word splitting), so several lines of output become several parameters on the command line.

The earlier shell syntax was to use back-quotes instead of $()for enclosing the sub-command. The $() syntax is preferred over the older backward quotes `` syntax because it easier to nest and arguably easier to read. However, you will probably see `` more often than $() especially in older scripts or from those who grew up with the original Bourne or C shells.

In our example, the output from find, typically a list of names, will become the arguments to the rm command.

Warning: be very careful when doing something like this because rm is very unforgiving. If your find command finds more than you expect, rm will remove it with no recourse. This is not Windows; you cannot recover deleted files from the trashcan. You can mitigate the danger with rm-i, which will prompt you to verify each delete. That’s OK on a small number of files, but interminable on a large set.

One way to use such a mechanism in bash with greater safety is to run that inner command first by itself. When you can see that you are getting the results that you want, only then do you use it in the command with back-quotes.

For example:

$ find . -name '*.class'
First.class
Other.class
$ rm $(find . -name '*.class')
$

We’ll see in an upcoming recipe how this can be made even more foolproof by using !! instead of retyping the find command (see Repeating the Last Command).