Remove all '#' characters from commit messages in a Git repository history.

git-cleanup-number-signs.sh

#!/bin/bash
# Copyright (C) 2025 陳鈞, licensed under GPL-3.0-or-later
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
# ==================================================================
#
# Automated Git Commit Message '#' Cleaner Script
# Purpose: Remove all '#' characters from commit messages in a Git repository history.
# This script is fully automated and requires no manual editing of commit messages.
#
# Usage:
#   ./git-cleanup-number-signs.sh [options] [start-commit]
#
# Options:
#   -h, --help     Show this help message and exit
#   -n, --dry-run  Preview mode, show changes without modifying history
#   -f, --force    Force execution without confirmation prompt
#
# Arguments:
#   start-commit   The commit hash to start processing from (exclusive). If omitted, all commits are processed.
#
# Example:
#   ./git-cleanup-number-signs.sh d61eef44d0e57bacf9417131a764cd5dd219f069
#   ./git-cleanup-number-signs.sh --dry-run HEAD~10
#   ./git-cleanup-number-signs.sh --force
#
# Features:
#   - Scans commit history for messages containing '#'
#   - Optionally previews changes before applying
#   - Backs up the current branch before rewriting history
#   - Uses git filter-branch to rewrite commit messages
#   - Cleans up filter-branch backup refs and temporary files
#   - Displays a summary and sample of modified commits

set -euo pipefail

# Color definitions for log output
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
BLUE='\033[0;34m'
NC='\033[0m'

# Logging functions for different message types
log_info() {
    echo -e "${BLUE}ℹ️  $1${NC}"
}

log_success() {
    echo -e "${GREEN}✅ $1${NC}"
}

log_warning() {
    echo -e "${YELLOW}⚠️  $1${NC}"
}

log_error() {
    echo -e "${RED}❌ $1${NC}"
}

# Print usage instructions
usage() {
    cat << EOF
Usage: $0 [options] [start-commit]

Options:
  -h, --help     Show this help message
  -n, --dry-run  Preview changes only, do not modify history
  -f, --force    Force execution without confirmation

Arguments:
  start-commit   The commit hash to start processing from (exclusive)
                If not specified, all commits will be processed

Examples:
  $0 d61eef44d0e57bacf9417131a764cd5dd219f069
  $0 --dry-run HEAD~10
  $0 --force
EOF
}

# Default values for script options
START_COMMIT=""
DRY_RUN=false
FORCE=false

# Parse command-line arguments
while [[ $# -gt 0 ]]; do
    case $1 in
        -h|--help)
            usage
            exit 0
            ;;
        -n|--dry-run)
            DRY_RUN=true
            shift
            ;;
        -f|--force)
            FORCE=true
            shift
            ;;
        -* )
            log_error "Unknown option: $1"
            usage
            exit 1
            ;;
        * )
            if [[ -z "$START_COMMIT" ]]; then
                START_COMMIT="$1"
            else
                log_error "Too many arguments"
                usage
                exit 1
            fi
            shift
            ;;
    esac
done

# Ensure script is run inside a Git repository
if ! git rev-parse --git-dir > /dev/null 2>&1; then
    log_error "Not inside a Git repository"
    exit 1
fi

# Ensure working directory is clean before rewriting history
if ! git diff-index --quiet HEAD --; then
    log_error "Working directory has uncommitted changes. Please commit or stash first."
    exit 1
fi

# Validate the start commit hash if provided
if [[ -n "$START_COMMIT" ]] && ! git rev-parse --verify "$START_COMMIT" > /dev/null 2>&1; then
    log_error "Invalid commit hash: $START_COMMIT"
    exit 1
fi

# Get the current branch name
CURRENT_BRANCH=$(git branch --show-current)
if [[ -z "$CURRENT_BRANCH" ]]; then
    log_error "Unable to determine current branch"
    exit 1
fi

# Scan for commits that need modification (contain '#')
log_info "Scanning for commits to modify..."
COMMITS_TO_MODIFY=()
COMMITS_INFO=() # Store commit details for reporting

if [[ -n "$START_COMMIT" ]]; then
    # Process commits after the specified start commit
    while IFS= read -r commit; do
        full_msg=$(git log --format=%B -n 1 "$commit")
        if [[ "$full_msg" == *"#"* ]]; then
            COMMITS_TO_MODIFY+=("$commit")
            author=$(git log --format=%an -n 1 "$commit")
            date=$(git log --format=%aI -n 1 "$commit")
            msg=$(git log --format=%B -n 1 "$commit")
            COMMITS_INFO+=("$commit|$author|$date|$msg")
        fi
    done < <(git rev-list --reverse "${START_COMMIT}..HEAD")
else
    # Process all commits in the repository
    while IFS= read -r commit; do
        full_msg=$(git log --format=%B -n 1 "$commit")
        if [[ "$full_msg" == *"#"* ]]; then
            COMMITS_TO_MODIFY+=("$commit")
            author=$(git log --format=%an -n 1 "$commit")
            date=$(git log --format=%aI -n 1 "$commit")
            msg=$(git log --format=%B -n 1 "$commit")
            COMMITS_INFO+=("$commit|$author|$date|$msg")
        fi
    done < <(git rev-list --reverse HEAD)
fi

# Exit if no commits require modification
if [[ ${#COMMITS_TO_MODIFY[@]} -eq 0 ]]; then
    log_success "No commit messages containing '#' found."
    exit 0
fi

# List commits that will be modified
log_info "Found ${#COMMITS_TO_MODIFY[@]} commit(s) to modify:"
for commit in "${COMMITS_TO_MODIFY[@]}"; do
    title=$(git log --format=%s -n 1 "$commit")
    full_msg=$(git log --format=%B -n 1 "$commit")
    body=$(git log --format=%b -n 1 "$commit")
    if [[ "$title" == *"#"* ]]; then
        echo "  ${commit:0:8}: $title [# in subject]"
    elif [[ "$body" == *"#"* ]]; then
        echo "  ${commit:0:8}: $title [# in body]"
    fi
done

# Preview mode: show before/after commit messages without modifying history
if [[ "$DRY_RUN" == true ]]; then
    log_info "Preview of commit message changes:"
    for commit in "${COMMITS_TO_MODIFY[@]}"; do
        echo "--- Commit ${commit:0:8} ---"
        echo "Before:"
        git log --format=%B -n 1 "$commit" | sed 's/^/  /'
        echo "After:"
        git log --format=%B -n 1 "$commit" | sed 's/#//g' | sed 's/^/  /'
        echo
    done
    exit 0
fi

# Confirm before proceeding unless --force is specified
if [[ "$FORCE" != true ]]; then
    log_warning "This operation will rewrite Git history!"
    read -p "Are you sure you want to continue? (y/N): " -n 1 -r
    echo
    if [[ ! $REPLY =~ ^[Yy]$ ]]; then
        log_info "Operation cancelled."
        exit 0
    fi
fi

# Create a backup branch before rewriting history
BACKUP_BRANCH="backup-${CURRENT_BRANCH}-$(date +%Y%m%d-%H%M%S)"
git branch "$BACKUP_BRANCH"
log_info "Backup branch created: $BACKUP_BRANCH"

# Prepare to rewrite commit messages using git filter-branch
log_info "Starting automated commit message rewrite..."

# Create a temporary script for the message filter
TEMP_SCRIPT=$(mktemp)
cat > "$TEMP_SCRIPT" << 'EOF'
#!/bin/bash
# Remove all '#' characters from commit message
sed 's/#//g'
EOF
chmod +x "$TEMP_SCRIPT"

# Run git filter-branch to rewrite commit messages
if [[ -n "$START_COMMIT" ]]; then
    # Only process commits after the specified start commit
    log_info "Processing range: ${START_COMMIT}..HEAD"
    git filter-branch -f --msg-filter "$TEMP_SCRIPT" "${START_COMMIT}..HEAD"
else
    # Process all commits in the repository
    log_info "Processing all commits"
    git filter-branch -f --msg-filter "$TEMP_SCRIPT" -- --all
fi

# Remove the temporary message filter script
rm -f "$TEMP_SCRIPT"

# Clean up filter-branch backup refs and run garbage collection
if [[ -d .git/refs/original ]]; then
    log_info "Cleaning up filter-branch backup refs..."
    git for-each-ref --format="%(refname)" refs/original/ | xargs -n 1 git update-ref -d
    git reflog expire --expire=now --all
    git gc --prune=now --quiet
fi

log_success "All '#' characters have been removed from commit messages."
log_info "Backup branch: $BACKUP_BRANCH"
log_warning "If you are satisfied with the result, you may delete the backup branch: git branch -D $BACKUP_BRANCH"
log_warning "You must force-push to remote: git push --force-with-lease"

# Display summary statistics
log_info "Modification summary:"
log_success "Successfully processed ${#COMMITS_TO_MODIFY[@]} commit(s)."

# Show up to 3 examples of modified commit messages
log_info "Sample of modified commits (showing up to 3):"
count=0
for info in "${COMMITS_INFO[@]}"; do
    if [[ $count -ge 3 ]]; then
        break
    fi
    IFS='|' read -r orig_hash orig_author orig_date orig_msg <<< "$info"
    # After filter-branch, '#' has been removed from the message
    new_msg=$(echo "$orig_msg" | sed 's/#//g')
    # Attempt to find the new commit in rewritten history by author, date, and message
    new_commit=$(git log --all --format="%H" --author="$orig_author" --since="$orig_date" --until="$orig_date" --grep="$(echo "$new_msg" | head -n1 | sed 's/[][.*^$(){}?+|/]/\\&/g')" -n 1 2>/dev/null || echo "")
    if [[ -n "$new_commit" ]]; then
        echo "  Modified commit ${new_commit:0:8}:"
        git log --format=%B -n 1 "$new_commit" | head -3 | sed 's/^/    /'
        echo
    else
        echo "  Could not find corresponding new commit (author/date/message may not be unique)"
    fi
    ((count++))
done

One of my forked projects accidentally backlinked the upstream project's issues and PRs in the commit messages. I originally intended to refer to another internal backlog numbering system of my own. This script will remove all # symbols from historical commits. After processing is complete, force push; GitHub should remove the backlinks after a while (this behavior is based on community experiments rather than official documentation and may stop working in the future).

Note: I have only run it on a single-line git log branch. I haven't tried it, but merged commits are likely to fail.