Aqui está um exemplo de como o loop FOR pode ser aplicado em um script bash para resolver essa tarefa:
#!/bin/bash
# Execute do-done inner part for each "item" under "folder/subjects/"
for subject in folder/subjects/*
do
# Check whether the source file exists
if [[ -f ${subject}/mri/norm.mgz ]]
then
# ${subject##*/} will cut the parent path from the value of the variable '$subject'
echo "Processing: ${subject##*/}"
# Create the new subject directory
mkdir -p "folder/investigation_folder/fs_norms/${subject##*/}"
# Copy the .mgz file into the new directory
cp "${subject}/mri/norm.mgz" "folder/investigation_folder/fs_norms/${subject##*/}/"
# Do the conversion to .nii (I'm not sure this is the correct syntax of mri_convert that you are using)
mri_convert "folder/investigation_folder/fs_norms/${subject##*/}/norm.mgz" "folder/investigation_folder/fs_norms/${subject##*/}/norm.nii"
fi
done