A better pre-commit hook

gistfile1.txt

#!/bin/bash

# This script is meant to be symlinked into your project's .git/hooks
# or global ~/.githooks directory within projects you are working
# with.

# Note: linters should be configured by project-local config files, so
# we can avoid changing this script. It's fine to change it, but,
# remember if you do that, you're on your own. Python and JavaScript linters
# will ideally run from project-specific folders so they are aware of the
# context the files should be analyzed with.

# Set up the environment so that we exit (-e) immediately as soon as a
# command exits with a non-zero return value and (-o pipefail) any
# pipeline with internal non-zero returns will cause the script to
# error out. Do not set -u yet.
set -eo pipefail

# ANSI sequences, if we are running in a terminal. If not, leave empty.
if [ -t 1 ]; then
    YELLOW="\033[33m"
    RESET="\033[0m"
else
    YELLOW=""
    RESET=""
fi

# If there is a branch named "main", we assume it's the
# parent. Otherwise we assume the parent is "master".
if git branch --list | grep '^main$'; then
    # There is a branch called main, so let's use it.
    PARENT_BRANCH="main"
else
    # We didn't find a main, so we assume base is master.
    PARENT_BRANCH="master"
fi
echo "Using $PARENT_BRANCH as parent branch"

# Any unset (-u) variable from now on causes an error. We didn't do
# this before because we may want to manually override the parent
# branch from the command line. This override logic needs to be 
# implemented
set -u

function lint_shell() {
    # Reformat and ling a shell file. Since we do the same for added and
    # modified files, we ignore $2.
    shfmt -i 4 -w "$1"
    shellcheck "$1"
}

function lint_python() {
    # The two paths exist so that we can leave already existing files
    # undisturbed while we enforce much stricter standards for new
    # ones.
    if [[ "$2" == "A" ]]; then # A indicates an added file.
        # If this is a new file, we check and reformat the file more
        # rigorously than we do existing files.
        isort -faas "$1"
        black -v "$1"
        flake8 "$1"
    else
        # If the file is not being added in this branch, we just run a
        # Flake 8 with default behavior.
        flake8 "$1"
    fi
}

function lint_go() {
    # Lint a Go file. Since we do the same for added and mofified
    # files, we ignore $2.
    go fmt "$1"
}

function lint_markdown() {
    # Lint a markdown file. Since we do the same for added and
    # mofified files, we ignore $2.
    lint-md "$1"
}

function lint_web() {
    if [[ "$2" == "A" ]]; then # A indicates an added file.
        # If this is a new file, we check and reformat the file more
        # rigorously than we do existing files.
        prettier --write "$1"
    else
        # If the file is not being added in this branch, we just check
        # it with prettier.
        prettier --check "$1"
    fi
}

function lint_tf() {
    # Always reformat Terraform files.
    terraform fmt -check=true "$1"
}

function show_unprocessed() {
    # Show a message in case a file wasn't processed.
    echo -e "${YELLOW}File $1 ($2) was not processed.${RESET}"
}

function lint_file() {
    # $1: the file we want to lint
    # $2: "A" for added, any other value for modified

    # Get the file extension.
    extension=${1##*\.}

    case $extension in
    "sh")
        lint_shell "$1" "$2"
        ;;
    "py")
        lint_python "$1" "$2"
        ;;
    "go")
        lint_go "$1" "$2"
        ;;
    "md")
        lint_markdown "$1" "$2"
        ;;
    "js" | "css" | "html" | "ts" | "tsx")
        lint_web "$1" "$2"
        ;;
    "tf" | "tfvars")
        lint_tf "$1" "$2"
        ;;
    *)
        # Extension is not a known one, but we can use file magic
        type=$(file --mime-type -b "$1")
        case $type in
        "text/x-shellscript")
            lint_shell "$1" "$2"
            ;;
        *)
            show_unprocessed "$1" "$type"
            ;;
        esac
        ;;
    esac
}

# Iterate over the files added on this branch (we computer the
# difference between the current branch and the parent branch we
# identified.
for file in $(git diff --name-only --diff-filter=Ad "$PARENT_BRANCH"); do
    lint_file "$file" "A"
done

# Iterate over the files modified on this commit
for file in $(git diff --name-only --diff-filter=Md HEAD); do
    lint_file "$file" "M"
done