Insert a string on a new line after a pattern

I want to replace

\usepackage[french]{babel}

by

\usepackage[french]{babel}
\addto\captionsfrench{\def\tablename{Tableau}}
                  

so far, I came to:

 #!/bin/bash                                                                                                              
 newString='\\addto\\captionsfrench{\\def\\tablename{Tableau}}'                                                                                                                                              
 sed -i "/babel.*/a $newString"  $1    

which almost does the job
The problem I encounter is that if I write newString=’\addto\captionsfrench{\def\tablename{Tableau}}’ the backslash before \addto is absent, and if I escape the backslash \addto I obtain ^Gaddto
~

>Solution :

This sed should work for you with a sed command:

cat file

\usepackage[french]{babel}

newString='\\addto\\captionsfrench{\\def\\tablename{Tableau}}'
sed -i.bak "/{babel}/a\\
$newString
" file

cat file

\usepackage[french]{babel}
\addto\captionsfrench{\def\tablename{Tableau}}