How to apply shell command to each line of a command output?

It's probably easiest to use xargs. In your case:

ls -1 | xargs -L1 echo

The -L flag ensures the input is read properly. From the man page of xargs:

-L number
    Call utility for every number non-empty lines read. 
    A line ending with a space continues to the next non-empty line. [...]

You can use a for loop:

for file in * ; do
   echo "$file"
done

Note that if the command in question accepts multiple arguments, then using xargs is almost always more efficient as it only has to spawn the utility in question once instead of multiple times.

You can use a basic prepend operation on each line:

ls -1 | while read line ; do echo $line ; done

Or you can pipe the output to sed for more complex operations:

ls -1 | sed 's/^\(.*\)$/echo \1/'