0 
 $n1='foo' a=1; echo n$a# n1
 $n1='foo' a=1; eval echo \$n$a #foo

1 Basic Evaluation:
--
command="ls -l"
eval $command

2 Variable Expansion:
v="HOME"
eval value=\$$v

3 Command Construction:
cmd="echo"
arg="hello world"
eval $cmd $arg

4 Combining with Loops:
 for i in 1 2 3
 do
  eval echo \$i
 done

safer way
--
var="ls -l"
$var

cases with command example
--
cmd1="echo 'Command 1'"
cmd2="echo 'Command 2'"
cmd3="echo 'Command 3'"

for cmd in "$cmd1" "$cmd2" "$cmd3"; do
    eval $cmd
done