ローカルスコープと終了コードを使った関数設計

関数のスコープが重要な理由

グローバル変数の漏洩問題

#!/usr/bin/env bash

set_name() {
    result="Alice"   # No 'local' — this is GLOBAL
}

result="important data"
echo "Before: $result"

set_name

echo "After:  $result"   # Prints 'Alice', not 'important data'

'local' によるローカル変数の宣言

#!/usr/bin/env bash

greet() {
    local name="$1"          # local — safe
    local greeting="Hello, ${name}!"
    echo "$greeting"
}  # 'name' and 'greeting' vanish here

name="global value"
greet "Bob"
echo "name is still: $name"   # Prints 'global value'

戻り値としての終了コード

#!/usr/bin/env bash

is_even() {
    local -i n="$1"
    (( n % 2 == 0 ))   # arithmetic command: exits 0 if true, 1 if false
}

for num in 2 3 7 10; do
    if is_even "$num"; then
        echo "$num is even"
    else
        echo "$num is odd"
    fi
done

printf による文字列結果の伝達

#!/usr/bin/env bash

to_uppercase() {
    local input="$1"
    printf '%s' "${input^^}"   # Bash 4+ parameter expansion
}

word="hello"
upper=$(to_uppercase "$word")
echo "Original: $word"
echo "Upper:    $upper"

終了コードと stdout 出力の組み合わせ

#!/usr/bin/env bash

divide() {
    local -i numerator="$1"
    local -i denominator="$2"

    if (( denominator == 0 )); then
        printf 'Error: division by zero\n' >&2
        return 1
    fi

    printf '%d' $(( numerator / denominator ))
    return 0
}

if result=$(divide 20 4); then
    echo "20 / 4 = $result"
else
    echo "Division failed."
fi

if result=$(divide 10 0); then
    echo "10 / 0 = $result"
else
    echo "Division failed (caught the error)."
fi

'local' による再帰関数の保護

#!/usr/bin/env bash

factorial() {
    local -i n="$1"
    local -i sub

    if (( n <= 1 )); then
        printf '1'
        return 0
    fi

    sub=$(factorial $(( n - 1 )))
    printf '%d' $(( n * sub ))
}

for i in 1 2 3 4 5 6; do
    echo "${i}! = $(factorial $i)"
done

local -n（Nameref）によるサブシェルの落とし穴の回避

#!/usr/bin/env bash

# Fills caller's array by reference — no subshell needed
read_csv_line() {
    local -n _out="$1"    # nameref to caller's variable
    local line="$2"
    local IFS=','
    read -ra _out <<< "$line"
}

declare -a fields
read_csv_line fields "alice,30,engineer"

echo "Name:  ${fields[0]}"
echo "Age:   ${fields[1]}"
echo "Role:  ${fields[2]}"

小さな関数ライブラリの構築

#!/usr/bin/env bash
# lib/str.sh  — string utility library

[[ -n "${_LIB_STR_LOADED:-}" ]] && return 0
_LIB_STR_LOADED=1

str_trim() {
    local str="$1"
    str="${str#"${str%%[![:space:]]*}"}"
    str="${str%"${str##*[![:space:]]}"}" 
    printf '%s' "$str"
}

str_repeat() {
    local -i times="$2"
    local char="$1"
    local -i i
    for (( i = 0; i < times; i++ )); do
        printf '%s' "$char"
    done
}

str_contains() {
    local haystack="$1"
    local needle="$2"
    [[ "$haystack" == *"$needle"* ]]
}

# --- self-test when executed directly ---
if [[ "${BASH_SOURCE[0]}" == "$0" ]]; then
    trimmed=$(str_trim "   hello world   ")
    echo "Trimmed: '${trimmed}'"
    str_repeat '-' 20; echo
    if str_contains "bash scripting" "script"; then
        echo "Contains: yes"
    fi
fi

関数内での引数の検証

#!/usr/bin/env bash

file_line_count() {
    if (( $# != 1 )); then
        printf 'Usage: file_line_count <file>\n' >&2
        return 1
    fi

    local file="$1"

    if [[ ! -f "$file" ]]; then
        printf 'Error: not a file: %s\n' "$file" >&2
        return 2
    fi

    if [[ ! -r "$file" ]]; then
        printf 'Error: cannot read: %s\n' "$file" >&2
        return 3
    fi

    local -i count
    count=$(wc -l < "$file")
    printf '%d' "$count"
    return 0
}

# Test with /etc/hosts (exists on every Linux/macOS system)
if lines=$(file_line_count /etc/hosts); then
    echo "/etc/hosts has $lines lines"
else
    echo "Failed with exit code: $?"
fi

すべてを組み合わせる: 実践的な例

#!/usr/bin/env bash

# Parse a semver string into components via nameref
parse_version() {
    local -n _major="$2" _minor="$3" _patch="$4"
    local version="$1"
    local IFS='.'
    local -a parts
    read -ra parts <<< "$version"
    _major="${parts[0]:-0}"
    _minor="${parts[1]:-0}"
    _patch="${parts[2]:-0}"
}

# Return 0 if version $1 >= version $2
version_ge() {
    local -i maj_a min_a pat_a
    local -i maj_b min_b pat_b
    parse_version "$1" maj_a min_a pat_a
    parse_version "$2" maj_b min_b pat_b

    if   (( maj_a != maj_b )); then (( maj_a > maj_b ))
    elif (( min_a != min_b )); then (( min_a > min_b ))
    else                             (( pat_a >= pat_b ))
    fi
}

require_bash_version() {
    local required="$1"
    local actual="${BASH_VERSION%%(*}"
    if version_ge "$actual" "$required"; then
        printf 'Bash %s satisfies >= %s\n' "$actual" "$required"
        return 0
    else
        printf 'Error: need Bash >= %s, got %s\n' "$required" "$actual" >&2
        return 1
    fi
}

main() {
    require_bash_version "4.3" || return 1
    require_bash_version "99.0" || true   # demonstrates failure path
}

main

理解度チェック: ローカル変数と戻り値

transform() {
    local tmp="${1,,}"   # lowercase
    printf '%s' "$tmp"
    return 0
}

まとめ: ローカルスコープと戻りコードを持つ関数