测试返回值：找不到命令

Question

我编写了一个函数来将是或否的答案转换为真或假（0 或 1）。但是，每次运行脚本时都会出现“找不到命令”错误。请帮我解决问题

get_boolean(){
    #==============================================================================================
    # Returns false if the first argument is NO and returns true if it is YES.
    # If the first argument is not a valid YES or NO,
    # then the return value depends on the default specified by argument 2 (Default value)
    #==============================================================================================

    if [ "$1" == 'NO' ] || [ "$1" == 'no' ] || [ "$1" == 'No' ] || [ "$1" == 'N' ] || [ "$1" == 'n' ]; then
            return 1;
    elif [ "$1" == 'YES' ] || [ "$1" == 'yes' ] || [ "$1" == 'Yes' ] || [ "$1" == 'Y' ] || [ "$1" == 'y' ]; then
            return 0;
    elif [ "$2" == 'NO' ] || [ "$2" == 'no' ] || [ "$2" == 'No' ] || [ "$2" == 'N' ] || [ "$2" == 'n' ]; then
            return 1;
    elif [ "$2" == 'YES' ] || [ "$2" == 'yes' ] || [ "$2" == 'Yes' ] || [ "$2" == 'Y' ] || [ "$2" == 'y' ]; then
            return 0;
    fi
}

read -p 'Do you want to drop the table of invalids? [n]:' DROP_TABLE_OF_INVALIDS
echo "After read: $DROP_TABLE_OF_INVALIDS"
DROP_TABLE_OF_INVALIDS=get_boolean "$DROP_TABLE_OF_INVALIDS" 'n'
echo "After assignment: $DROP_TABLE_OF_INVALIDS"
if $DROP_TABLE_OF_INVALIDS; then
        echo "Hello. I will drop the table"
fi

当我运行脚本时，我得到了这些错误：

bash-3.2$ sh test.sh
Do you want to drop the table of invalids? [n]:y
After read: y
test.sh: line 24: y: command not found
After assignment: y
test.sh: line 26: y: command not found
bash-3.2$ sh test.sh
Do you want to drop the table of invalids? [n]:n
After read: n
test.sh: line 24: n: command not found
After assignment: n
test.sh: line 26: n: command not found

更新：下面的代码有效（感谢 Barmar！）

get_boolean(){
        #==============================================================================================
        # Outputs false if the first argument is NO and outputs true if it is YES.
        # If the first argument is not a valid YES or NO,
        # then the output value depends on the default specified by argument 2 (Default value)
        #==============================================================================================

        if [ "$1" == 'NO' ] || [ "$1" == 'no' ] || [ "$1" == 'No' ] || [ "$1" == 'N' ] || [ "$1" == 'n' ]; then
                        echo false;
        elif [ "$1" == 'YES' ] || [ "$1" == 'yes' ] || [ "$1" == 'Yes' ] || [ "$1" == 'Y' ] || [ "$1" == 'y' ]; then
                        echo true;
        elif [ "$2" == 'NO' ] || [ "$2" == 'no' ] || [ "$2" == 'No' ] || [ "$2" == 'N' ] || [ "$2" == 'n' ]; then
                        echo false;
        elif [ "$2" == 'YES' ] || [ "$2" == 'yes' ] || [ "$2" == 'Yes' ] || [ "$2" == 'Y' ] || [ "$2" == 'y' ]; then
                        echo true;
        fi
}

read -p 'Do you want to drop the table of invalids? [n]:' DROP_TABLE_OF_INVALIDS
echo "After read: $DROP_TABLE_OF_INVALIDS"
DROP_TABLE_OF_INVALIDS=$(get_boolean "$DROP_TABLE_OF_INVALIDS" 'n')
echo "After assignment: $DROP_TABLE_OF_INVALIDS"
if $DROP_TABLE_OF_INVALIDS; then
                echo "Hello. I will drop the table"
fi

以下是使它起作用的编辑：

• 函数“回显”而不是返回。
• 输出值为 bash true 或 false（用于在 if 中进行测试，否则会得到 0：未找到命令或 1：未找到命令）
• 函数调用包含在'$('和')'中

Answer 1

有几个问题。线

DROP_TABLE_OF_INVALIDS=get_boolean "$DROP_TABLE_OF_INVALIDS" 'n'

不调用get_boolean。它尝试在修改后的环境中运行DROP_TABLE_OF_INVALIDS 命名的命令。你会想要：

DROP_TABLE_OF_INVALIDS=$(get_boolean "$DROP_TABLE_OF_INVALIDS" 'n')

这就引出了第二个问题，即DROP_TABLE_OF_INVALIDS包含get_boolean的标准输出，但是你使用的是返回值。试着这样称呼它：

if get_boolean "$DROP_TABLE_OF_INVALIDS" 'n'; then
    ...
fi

这里的返回值是测试目录，而不是捕获一个字符串来测试。

第三个问题是你试图返回一个空字符串。 return 语句只能返回一个数值；它不像其他语言中返回任意值的函数。如果第一个参数为空，则需要判断它是真还是假； false 似乎是一个很好的默认值，因此返回 0。或者，您可以忽略它，将其视为使用 $2 的值的情况。

---

一个更简单的版本是

get_boolean(){
    #==================================================================================
    # Returns false if the first argument is NO and returns true if it is YES.
    # If the first argument is not a valid YES or NO,
    # then the return value depends on the default specified by argument 2 (Default value)
    #==================================================================================
    case $1 in
        NO|No|no|N|n|"" ) return 1 ;;
        YES|yes|Yes|Y|y) return 0;;
        * ) case $2 in
            YES|yes|Yes|Y|y) return 0;;
            * ) return 1 ;;
            esac ;;
    esac
}

Answer 2

语法：

DROP_TABLE_OF_INVALIDS=get_boolean "$DROP_TABLE_OF_INVALIDS" 'n'

表示在执行命令`"$DROP_TABLE_OF_INVALIDS" 'n'"时将环境变量DROP_TABLE_OF_INVALIDS设置为字符串"get_boolean"。

用函数的输出赋值变量的方法是：

DROP_TABLE_OF_INVALIDS=$(get_boolean "$DROP_TABLE_OF_INVALIDS" 'n')

另外，您需要将函数更改为使用echo 而不是return。 return 设置退出状态，而不是函数的输出。