SHELL

echo new line to file

echo $'username=user
password=pass' > ~/samba-credentials

echo to add line to file

touch sample.txt 
echo "this is line one" > sample.txt  #adds this as first line
echo "this is line two" >> sample.txt #appends the line to sample.txt

#if you do echo "this will remain alone" > sample.txt
#	it will rewrite the sample.txt file with the string passed above