The suggested way of adding all untracked files to the index in Git is to use the interactive add command:

$ git add -i
           staged     unstaged path

*** Commands ***
  1: status       2: update       3: revert       4: add untracked
  5: patch        6: diff         7: quit         8: help
What now> a
  1: testfile1.txt
Add untracked>> *
added one path
*** Commands ***
  1: status       2: update       3: revert       4: add untracked
  5: patch        6: diff         7: quit         8: help
What now> quit
Bye.

This can be fully automated as follows:

echo -e "a\n*\nq\n"|git add -i

The commands entered via standard input (a,*,q) will not be printed. For convenience, we can also define an alias in ~/.gitconfig:

[alias]
    add-untracked = !"echo \"a\\n*\\nq\\n\" | git add -i"

References

• [1] Mat’s and EAGER_STUDENT’s answer on StackOverflow