How to add a prefix string at the beginning of each line in Bash shell script on Linux?

How to add a prefix string at the beginning of each line in Bash shell script on Linux?

For example, assume we have a file a.txt:

line 1
line 2

I want to have,

pre line 1
pre line 2

You may use sed, the stream editor for filtering and transforming text:

sed -e 's/^/pre /'

For example,

$ echo "line 1
line 2" | sed -e 's/^/pre /'
pre line 1
pre line 2

You may also do prefix adding to lines of certain patterns. But this is out of this question.