Search and replace (or add) using sed

Today, I was facing (once more) 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?

The one-liner​

Here it is — update APP_ENV if the key is there, append it if it isn't:

$ 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▋

The .env file only contained APP_NAME, so the key has been appended. Let's read the file back:

$ cat .env
APP_NAME = My application
APP_ENV = production▋

Run the very same block again — or against a file where APP_ENV already exists — and the line is substituted instead of duplicated. That's the whole idea: the command is idempotent, you can run it in a deployment script without checking anything first.

Why it works​

• grep -q tests without printing. -q means quiet: it produces no output, only an exit code, which is exactly what a conditional needs.
• && is the "found" branch. The key exists → substitute the value in place.
• || is the "missing" branch. The key doesn't exist → append a new line at the end of the file.

You'll find a lot of other possibilities on the Internet, some using only the sed instruction, but ... can you read them? I prefer this approach, perhaps not the native one but, yeah, I can read it.

The rest of this article builds the command piece by piece, if you want to understand each half before using it.

Search and replace​

Imagine a .env file with just one line (which your Bash script can then load as real environment variables), 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 happen (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"▋

The insert a new line command is this one: sed -i -e '$aAPP_ENV = production' .env. The -e argument allows you 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.

Combine both​

Ok, first we can plug the replace statement into the && branch:

$ 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"
▋

APP_ENV was there, so it has been substituted: running cat .env gives, as expected, APP_ENV = production.

And the next block will still display NOT FOUND, because this time the key is missing:

$ echo 'APP_NAME = My application' > .env
$ grep -q "^APP_ENV =" .env 

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

|| echo "NOT FOUND"▋

Replace that echo "NOT FOUND" with the append command seen above and you get the one-liner shown at the top of this article. Both halves are now in place.

Conclusion​

grep -q for the test, && for substitute, || for append: three pieces you can read out loud, in a single command that behaves the same whether the key is already there or not. It's the kind of line you end up pasting into every deployment script.

This pattern is the building block of Batch edit of environment file, where the same logic becomes a reusable function that reports UPDATED or ADDED for every key it touches.