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Search and replace (or add) using sed

· 3 min read
Christophe
Christophe
Markdown, WSL and Docker lover ~ PHP developer ~ Insatiable curious.

Search and replace (or add) using sed

Today, I was facing (once more) with the following need: I need to update a setting in a text file but if the variable is not yet present, I need to add it.

So, in short, I need to make a search and replace or insert new line.

Using sed it's quite easy to automate the search & replace but how to append?

This article will explains one-way to achieve this. You'll find a lot of other possibilities on Internet and some only using the sed instruction but ... can you read them?

I prefer to use a different approach, perhaps not the native one but, yeah, I can read it.

Search and replace​

Imagine a .env file with just one line, like f.i.:

echo 'APP_ENV = local' > .env

I can update APP_ENV f.i. using:

sed -i "s/APP_ENV =.*/APP_ENV = production/" .env

Easy no? The s in the command is for substitute (replace) and the used delimiter is /. So, sed will search for APP_ENV =.* and if found, will replace with APP_ENV = production. The -i flag means that the new content (after replacement) has to be rewritten in the file.

Don't replace but add if not found​

But what if APP_ENV is not present at all in the file?

Of course, by running sed -i "s/APP_ENV =.*/APP_ENV = production/" .env nothing will happens (you can verify with cat .env).

Before seeing how to do, run the following block and you'll get a NOT FOUND message.

echo 'APP_NAME = My application' > .env
grep -q "^APP_ENV =" .env && echo "FOUND" || echo "NOT FOUND"

So if grep -q is successful (we've retrieved APP_ENV in the file) then we continue (&&) and display FOUND otherwise (||) we'll display NOT FOUND.

&& means that the previous command is successful (i.e. has been retrieved by grep) and || means not successful (not retrieved).

So, the next example will now display FOUND.

echo 'APP_NAME = My application' > .env
echo 'APP_ENV = local' >> .env
grep -q "^APP_ENV =" .env && echo "FOUND" || echo "NOT FOUND"

Combine both​

Ok, first we can make our replace statement:

echo 'APP_NAME = My application' > .env
echo 'APP_ENV = local' >> .env

grep -q "^APP_ENV =" .env \
    && sed -i "s/APP_ENV =.*/APP_ENV = production/" .env \
    || echo "NOT FOUND"

By running cat .env, you will get, as expected, APP_ENV = production.

And the next block will still display NOT FOUND:

echo 'APP_NAME = My application' > .env

grep -q "^APP_ENV =" .env \
    && sed -i "s/APP_ENV =.*/APP_ENV = production/" .env \
    || echo "NOT FOUND"

The insert a new line command is this one: sed -i -e '$aAPP_ENV = production' .env. The -e argument allow to execute a script and it's quite strange but the script is $a. That command is for append line. And now you've understood that sed will here add a new line in the file.

The final instruction​

Here it's:

echo 'APP_NAME = My application' > .env

grep -q "^APP_ENV =" .env \
    && sed -i "s/APP_ENV =.*/APP_ENV = production/" .env \
    || sed -i -e '$aAPP_ENV = production' .env

Finally, by running cat .env we can see the result:

APP_NAME = My application
APP_ENV = production