The External Tools plugin comes with a built-in Run Command tool.  Suppose I want to use this tool to perform this task:

$ sed 's/a/ /'

If I invoke Run Command, and type this into the dialog box:

sed 's/a/ /'

then I get this error:

sed: -e expression #1, char 1: unknown command: `''

The problem is that the single quote is being passed literally to sed.

The user may need to use quotes to construct arguments which contain spaces in situations like this.  So we should use a shell to interpret the command to be run.