Create child directories from parent directory

A parent directory has many directories, for example:

cd parent
ls    
ALA_31_C   ALA_31_D   ALA_31_G  ALA_31_L

I want to create the same name directory within each child directory, something like this:

cd  parent
ls
ALA_31_C   ALA_31_D   ALA_31_G  ALA_31_L

cd ALA_31_C
ls
ALA_31_C

Similarly, I need to create it for all remaining directories: ALA_31_D, ALA_31_G and ALA_31_L from the parent directory.

A simple Bourne shell script will do it:

#!/bin/sh
  
for dir_name in */ ; do
    echo "$dir_name"
    mkdir "$dir_name/$dir_name"
done        

Loop through only the directories (*/), and for each directory, make a child with the same name as the parent (mkdir "$dir_name/$dir_name").

This only works for one level - it is not recursive - which is probably what you want anyway.

For more examples, see How do I loop through only directories in bash?