In the Bourne shell, very little string manipulation was possible without resorting to external commands. Strings could be concatenated by juxtaposition, they could be split by changing the value of IFS, and they could be searched with case, but anything else required an external command.
Even things that could be done entirely in the shell were often relegated to external commands, and that practice has continued to this day. In some Linux distributions, you can find the following snippet in /etc/profile. It checks whether a directory ...
