git-diff-sed

A script that applies a sed transformation to the "old" version of a file before running git diff, allowing you to ignore systematic changes when viewing differences.

Overview

This script is useful when you want to see what changed in a file while ignoring specific systematic transformations. It works by:

1. Retrieving the old version of the file from Git (HEAD)
2. Applying a sed command to transform the old content
3. Running git diff --no-index between the transformed old content and the current working file

This allows you to effectively "hide" systematic changes like variable renames, formatting changes, or other pattern-based modifications to focus on the meaningful differences.

Usage

git diff-sed <sed_command> <file_path> [<git_diff_options>...]

Parameters

• <sed_command>: The sed command to apply to the old version of the file
• <file_path>: Path to the file you want to diff
• <git_diff_options>...: Additional options to pass to git diff (optional)

Examples

# Hide variable rename from "oldVar" to "newVar"
git diff-sed 's/oldVar/newVar/g' src/main.js

# Ignore multiple systematic changes
git diff-sed 's/old-text/new-text/g; s/OldClass/NewClass/g' lib/module.py

# Use with word-diff to see changes more clearly
git diff-sed 's/legacy_function/new_function/g' utils.py --word-diff

# Hide whitespace normalization changes
git diff-sed 's/\t/    /g' config.json --word-diff=color

# Ignore case changes in function names
git diff-sed 's/myFunction/myfunction/g' script.js --color-words

Use Cases

Variable and Function Renaming

When you've systematically renamed variables or functions throughout a file, but want to see what other changes were made:

# Hide the variable rename to focus on logic changes
git diff-sed 's/user_data/userData/g' processor.py --word-diff

API Migration

When migrating from one API to another with systematic replacements:

# Hide jQuery to vanilla JS migration
git diff-sed 's/\$/document.querySelector/g' frontend.js

Formatting Changes

When you want to ignore specific formatting changes:

# Ignore tab to space conversion
git diff-sed 's/\t/    /g' source.py

# Ignore single to double quote changes
git diff-sed "s/'/\"/g" config.json

Legacy Code Updates

When updating legacy patterns to modern equivalents:

# Hide old-style imports
git diff-sed 's/var /let /g' legacy.js --color-words

Advanced Examples

# Multiple transformations with complex sed script
git diff-sed 's/old_prefix_/new_prefix_/g; s/LegacyClass/ModernClass/g' refactored.py

# Use extended regex for more complex patterns
git diff-sed 's/get([A-Z][a-z]+)/get\1Data/g' api.js --word-diff

# Combine with specific git diff options for better output
git diff-sed 's/oldLib/newLib/g' imports.py \
    --word-diff=color \
    --ignore-space-change \
    --ignore-blank-lines

How It Works

1. Retrieve old content: Uses git show HEAD:<file> to get the file content from the last commit
2. Transform: Applies the provided sed command to the old content
3. Compare: Uses git diff --no-index to compare the transformed old content with the current working file
4. Display: Shows the diff with any additional git diff options you specify

Requirements

• Git repository with committed changes
• sed command-line tool (standard on Unix-like systems)
• Bash shell environment

Important Notes

• File must exist in HEAD: The file must be present in the last commit for the script to work
• Sed syntax: Uses extended regex (-E flag), so patterns follow sed extended regex rules
• Working directory: Compares against the current working directory version, not staged changes
• No modification: The script doesn't modify any files; it only shows a transformed diff view
• Git diff options: All standard git diff options can be used (colors, word-diff, ignore-whitespace, etc.)

Error Handling

The script will exit with an error if:

• The sed command or file path is not provided
• The specified file doesn't exist in the HEAD commit
• Git operations fail

Tips

• Use --word-diff for better visibility of changes when hiding systematic replacements
• Test your sed command separately first: git show HEAD:file.txt | sed 's/old/new/g'
• Use single quotes around sed commands to avoid shell interpretation
• For complex transformations, consider using a sed script file: sed -f script.sed