BASH: Single-quotes inside of single-quoted strings

Using single quotes in BASH ensures that the shell doesn't expand the contents of the quoted string and this is useful most of the time. However if you want to use single quotes within a single quoted string things don't work out as you might expect.

For example using the following simple PHP snippet:

<?php
echo '<p>this is a string</p>';
?>

If I want to use sed to match some text with a single quotes in it, I will run into trouble if I run:

sed -e s/'<p>/'<p class="test">/g

This will output:

/bin/bash: line 1: p: No such file or directory

This is because the unquoted string will be expanded and BASH will think that < is a redirection.

Alternatively if I run (on the same snippet):

sed -e 's/\'<p>/\'<p class="test">/g'

I will get:

/bin/bash: -c: line 1: unexpected EOF while looking for matching `''
/bin/bash: -c: line 2: syntax error: unexpected end of file

This doesn't work because the escaped single-quotes (\') are not expanded and are therefore treated literally.

To single quotes work you need to break out of the single quoted string then escape your single quote. Like so:

sed -e 's/'\''<p>/'\''<p class="test">/g'

Because \' is not inside of single quotes the single-quote is properly escaped and the output is as we'd expect:

<?php
echo '<p class="test">this is a string</p>';
?>

In conclusion, the title of this post is a bit of a misnomer. You actually can't put single quotes inside of a single-quoted string. However breaking out allows us to get to where we want to be.

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