Prepend ## to every line in a text file

You can use sed to do that:

sed -i.bak 's/^/##/' file

This replaces the start of the line (^) with ##.

With the -i.bak switch, sed edits the file in-place, but creates a backup copy with extension.bak.

Here is a solution to this problem using perl

perl -e 'while (<>) {print "##$_"}' < infile > outfile

While we are at it:

gawk -i inplace '{print "##"$0}' infile

This uses the (comparatively new) inplace editing plugin for GNU awk 4.1.0+.

Here's a bash way:

while read -r; do printf '##%s\n' "$REPLY"; done < infile > outfile

(In the bash shell, running read -r with no other arguments works like IFS= read -r REPLY.)

This is stylistically inspired by beav_35's perl solution, which I admit probably runs much faster for huge files, since perl may be expected to be more efficient than a shell when it comes to text processing.

sed -i is not POSIX-standard, so if you are a purist you will want to use ed:

printf ",s/^/##/\nw\nq" | ed -s file.txt

Here's an easier perl way than presented elsewhere:

perl -pi -e 'print "##"' YOURFILEHERE

This (ab)uses the fact that perl -p prints the line after executing the command given in -e.

You can use Vim in Ex mode:

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

1. % select all lines

2. s substitute

3. x save and close

Can be done with python's mapping function and redirecting stdin:

$ cat input.txt                                                                                                          
lorem ipsum
quick brown fox
hello world
$ python -c 'import sys;print "".join(map(lambda x: "##"+x,sys.stdin.readlines()))'  < input.txt                         
##lorem ipsum
##quick brown fox
##hello world

Save the output to new file and use it instead of original