Unix: How to check the existence of a tag at the beginning of each file before insertion?

Question

The question is related to the question "Unix: Add a Tag to the beginning of each file?".

How can I put the tag only to the files that do not contain the tag, such as <?php, already?

Answer 1

head -n1 file | grep -q "^<?php" || sed -i "1s/^/<?php /" file

Answer 2

Suitably modified version of my answer to the original question:

perl -0777 -p -i -e 's/^/<?php/g unless m/^<\?php/' testfile

Running this command with -i.bak instead of -i will preserve a backup of the original files with a .bak extension.

Edit: Updated the match from m/<?php/ to m/^<\?php/ to make it match only the beginning of the file, and to escape the ? character.

Answer 3

You want || (OR). If the left hand is true, it will not bother with the right hand. e.g.

test "$(grep PATTERN FILE)" || sed COMMAND FILE