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I want to add hashes to all the lines in a regular text file. I'm fine with both the use of terminal and GUI—I just need to get it done.

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up vote 35 down vote accepted

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.

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+1 This is the first time I encounter this backup method. I always did an inline replace directly: sed 's/^/##/' -i file. In this case, this would be preferred since it cannot go wrong. In other cases, this command can be combined with diff -u file.bak file to see the differences (if necessary, pipe it through less). If it's confirmed to work, the backup can be removed. Otherwise it could be restored with a simple mv file.bak file. – Lekensteyn Jul 30 '11 at 8:28
Elegant. I was thinking about something like sed 's/^\(.*\)$/##\1/', but this is much nicer. – arrange Jul 31 '11 at 8:54
Nice one.. I was thinking about suggesting Vim to do this... – Sathish Aug 6 '11 at 13:28
This helped me convert a block list file :) Thank you 1+ – Neil Nov 22 '15 at 4:25

Here is a solution to this problem using perl

perl -e 'while (<>) {print "##$_"}' < infile > outfile
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The -p switch is also useful: perl -pe 's/^/##/' infile > outfile. (There's also the -i[extension] switch for replacing the target file in-place.) – Jukka Matilainen Jul 31 '11 at 7:51

Here's a bash way:

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

(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.)

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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+.

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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
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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.

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You can use Vim in Ex mode:

ex -sc '%s/^/##/|x' file
  1. % select all lines

  2. s substitute

  3. x save and close

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