I needed to send a command (actually two separate) over ssh to check for the presence of a user (username) on a remote system. The command needed to be constructed entirely from variables. This is the solution I crafted (after much trial and error).

You will see that I send two ssh commands because I need to check for two styles of username (flast and first.last).  These functions are run inside a follower-script called by a leader-script responsible for gathering a host of data regarding departing users.

The leader passes a trio of variables into the follower.  The follower than crafts some variables of its own.  It is from all of this that the final command is crafted.

## 
#! /usr/bin/env bash 
# Title   :  termuserSub_Nagios.sh 
# Parent  :  termuser.sh 
# Author  :  JamesIsIn 20180719 Do something nice today. 

# Purpose :   
# SOP     :  https://some.url.lan/display/ops/NOC+Terminator+Script 
#         :  https://some.url.lan/display/ops/Termination+Procedure+-+NOC 
# 


############### 
#  Variables  # 

# # debugging 
# scriptUser_linux= 
# termuser_flast= 
# termuser_firstDotLast= 
# # 

# the original script included the following note:  
# using test nagios results in halved execution times (compared to prod); all configs in sync 
readonly const_nagiosHost="some.url.com" 
readonly const_nagios_cgi="/usr/local/nagios/etc/cgi.cfg" 
declare termuser_firstDotLast_grep 
declare termuser_flast_grep 

## 

############### 
#  Functions  # 

function func_sshCommand() { 
	local loc_sshArguments="-n -l ${scriptUser_linux} -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o LogLevel=quiet" 
	local loc_sshHostCommand="grep -c \"${1}\" ${const_nagios_cgi}" 
	# this is complex 
	# the export is for filling the passed parameter which is a variable name 
	# that passed variable name is then filled with the output from ssh 
	# where ssh is loaded from a trio of variables 
	export "$( printf '%s' "$( printf '%s' "${2}" )"="$( ssh $( printf '%s' " ${loc_sshArguments} ${const_nagiosHost} ${loc_sshHostCommand}" ) )" )" 
} 

function main() { 
	# printf '%s\n' "Your password will be required for two tests.  " 
	func_sshCommand "${termuser_firstDotLast}" termuser_firstDotLast_grep 
	func_sshCommand "${termuser_flast}" termuser_flast_grep 
	if [ ! "${termuser_firstDotLast_grep}" == "0" ] || [ ! "${termuser_flast_grep}" == "0" ] ; then 
		printf '%b\n' "" "Nagios user found.  Please remove the following user from the nagios cgi.cfg file.  " 
		if [ ! "${termuser_firstDotLast_grep}" == "0" ] ; then 
			printf '%s\n' "${termuser_firstDotLast} (${termuser_firstDotLast_grep}) " 
		fi 
		if [ ! "${termuser_flast_grep}" == "0" ] ; then 
			printf '%s\n' "${termuser_flast} (${termuser_flast_grep}) " 
		fi 
		printf '\n' 
		return 1 
	else 
		return 0 
	fi 
} 

## 

########## 
#  Main  # 

main 
exit $? 

## 

## 

The magic finally happens in the admittedly somewhat cryptic export line (hence the preceding lines of comment).

Clever? You be the judge.