Substring search in dash [ _NOT_ bash ] shell

[Richard Owlett <rowlett@cloud85.net>]

The Installation Manual in _B.5.3. Chainloading preconfiguration 
files_ says in part:
# More flexibly, this runs a shell command and if it outputs the 
names of
# preconfiguration files, includes those files.
#d-i preseed/include_command \
#      string if [ "`hostname`" = bob ]; then echo bob.cfg; fi

I need for the case when evaluates to marybobsam .
I can find examples in bash shell, but I need dash.

Are there equivalents of
  https://www.gnu.org/software/bash/manual/bash.html
or

http://www.tldp.org/LDP/Bash-Beginners-Guide/html/Bash-Beginners-Guide.html
BUT for dash *NOT* bash?

TIA