How to git ignore file specific changes without ignoring the file

Introduction

When working with git, there are cases when you want to track a file but you don’t want to override a part of its content,when you git commit, neither you want this part of its content to be overridden when you git pull or git checkout .

Gitignore is not an option

Since you want to track this file you can’t add it to the .gitignore file.

Git filters is the answer

Let’s assume that I have a file src/main/resources/config/application-config.properties, this file contains a configuration of my local application configuration. Let’s call this the property that I don’t want override assets_path

assets_path=my_local_assets_path
dummy_property=dummy_value

The property value is my_local_assets_path but I don’t want to commit it, I want to keep it on the file locally, what I want to commit instead is this value path_to_your_assets_folder.

Adding filters

The first step is to add a filter to the file .git/config on your local repository. It is also possible to add the file globally.

[filter "my_filter_name"]
    clean = sed 's/assets_path=.*/assets_path=path_to_your_assets_folder/g'
    smudge = sed 's/assets_path=.*/assets_path=my_local_assets_path/g'

• Git will run the command defined in the clean property when staging your changes.
• And runs the command defined in the smudge property when checking out the remote changes.

The sed command

The sed command is a linux command that allows searching and replacing text in files. So basically what we are doing here is:

• Cleaning the staged filtered file when staging changes by replacing assets_path= and all the content after it in the same line by assets_path=path_to_your_assets_folder.
• Smudging the filtered file when checking it out by replacing assets_path=path_to_your_assets_folder and all the content after it in the same line by assets_path=my_local_assets_path.

This way whenever you commit the committed value will be path_to_your_assets_folder and whenever you checkout the value will be my_local_assets_path.

Using the filters

Now that we have our filter you need to tell git what are the files you need to filter, just add a file .gitattributes in the root folder of your repository. and add this content to it.

src/main/resources/config/application-config.properties filter=my_filter_name

Let’s test that

Now we have everything in place, let’s test it, doing the following commands in a row.

#Init the repository
mkdir -p ~/projects/blog/git-filtering
cd ~/projects/blog/git-filtering
git init

#Creating the file src/main/resources/config/application-config.properties
mkdir -p src/main/resources/config/
echo 'assets_path=my_local_assets_path\ndummy_property=dummy_value\n' >> src/main/resources/config/application-config.properties

#Adding the filter to .git/config
echo "\n[filter \"my_filter_name\"]\n\tclean = sed 's/assets_path=.*/assets_path=path_to_your_assets_folder/g'\n\tsmudge = sed 's/assets_path=.*/assets_path=my_local_assets_path/g'\n" >> .git/config
#Adding the filter to .gitattributes
echo  "\nsrc/main/resources/config/application-config.properties filter=my_filter_name\n" >> .gitattributes

# Track and stage the changes of the properties file
git add src

Now we have everything in place, if you check the content of your local properties file you should see:

cat src/main/resources/config/application-config.properties

It displays

assets_path=my_local_assets_path
dummy_property=dummy_value

Let’s check what will be committed

git diff --staged

You should see something like:

diff --git a/src/main/resources/config/application-config.properties b/src/main/resources/config/application-config.properties
new file mode 100644
index 0000000..aec222b
--- /dev/null
+++ b/src/main/resources/config/application-config.properties
@@ -0,0 +1,3 @@
+assets_path=path_to_your_assets_folder
+dummy_property=dummy_value
+

Congratulations! As you can see, the content of the local file is different from the staged changes.

Don’t use this with sensitive data

At first sight it looks like a good idea to use the same technique to commit a file without committing its sensitive data, like passwords and api keys, and it actually works, however I don’t recommend that, because it’s not meant to be used for such a thing.

Actually I already used it for API KEYS and when I changed the code formatter the regular expression used in the sed command stopped working, and the filter didn’t work.

If you’re working with sensitive data please consider using environment variables or other alternatives.

Further reading

• Git attributes and filters documentation
• Sed command documentation