Simple way to diff a file in local SFDX project against Production version with SFDX

After installing the sfdx:source:diff tool from @Mohith Shrivastava's response, you can use the --json flag to return the locations of the remote and local files that VS Code uses to diff. You can then pass those files to git diff to see the diff in the command line.

Here is the whole command:

SFDX_DIFF_JSON=$(sfdx force:source:diff -p force-app/main/default/classes/MyController.cls -u MyProductionOrg --json)
git diff $(echo ${SFDX_DIFF_JSON} | jq '.result.remote' -r) $(echo ${SFDX_DIFF_JSON} | jq '.result.local' -r)

Have you tried using the Salesforce official Diff Feature on the Extension Pack for Salesforce

https://developer.salesforce.com/tools/vscode/en/user-guide/source-diff/

From the terminal, run sfdx plugins:install @salesforce/sfdx-diff.

After the installation is complete, @salesforce/sfdx-diff appears in the list of installed plugins when running sfdx plugins.

Use the vscode menu to take the diff

This requires enabling the preview flag from your vscode settings

VS Code has an option to detect conflicts on deployment for non-source-tracked orgs. Go to Settings > Salesforce Feature Previews > Salesforcedx-vscode-core: Detect Conflicts At Sync. Note that this is still a limited feature, so some caution is advised.

In the interim, if I want to check for differences, I usually do:

git checkout -b temp-branch
git add force-app
git commit -m "temp commit for compare"
sfdx force:source:retrieve -p force-app
git diff

This gives me a line-by-line analysis of everything. I can also use --name-only just to get a feel for which files are affected.