Add a prefix string to beginning of each line

# If you want to edit the file in-place
sed -i -e 's/^/prefix/' file

# If you want to create a new file
sed -e 's/^/prefix/' file > file.new

If prefix contains /, you can use any other character not in prefix, or escape the /, so the sed command becomes

's#^#/opt/workdir#'
# or
's/^/\/opt\/workdir/'

awk '$0="prefix"$0' file > new_file

With Perl (in place replacement):

perl -pi 's/^/prefix/' file

You can use Vim in Ex mode:

ex -sc '%s/^/prefix/|x' file

1. % select all lines

2. s replace

3. x save and close

If your prefix is a bit complicated, just put it in a variable:

prefix=path/to/file/

Then, you pass that variable and let awk deal with it:

awk -v prefix="$prefix" '{print prefix $0}' input_file.txt