sed

10 Jun 2013 in Unix Toolbox

This week, I've been using sed quite a lot. sed is short for stream editor, and is designed to work with large amounts of streaming text.

The most common use case that most people have for sed is text substitution. As it's a stream editor, by default it looks at one line of input at a time.

So for example, given the following input:

the number one is number one then comes number two and number three comes third

Running the following command would replace "number" with "NUMBER".

bash
sed 's/number/NUMBER/'

Just like perl, substitution only affects the first match - you need to be explicit to make it replace all occurrences. In this case, that's by adding the g flag.

bash
sed 's/number/NUMBER/g'

So, if we run:

bash
echo "the number one is number one\nthen comes number two\nand number three comes third" | sed 's/number/NUMBER/g'

We should see:

the NUMBER one is NUMBER one then comes NUMBER two and NUMBER three comes third

Applying this to an existing set of text can be done by feeding filenames into sed and using the -i flag, saying "change in place".

So for example, to replace all occurrences of the word "Dog" with the word "Cat":

bash
find . -type f | xargs sed -i 's/Dog/Cat/g'