使用“set -o nounset”测试是否在 Bash 中设置了变量

Question

以下代码退出并出现未绑定变量错误。如何在仍然使用 set -o nounset 选项的情况下解决此问题？

#!/bin/bash

set -o nounset

if [ ! -z ${WHATEVER} ];
 then echo "yo"
fi

echo "whatever"

Answer 1

#!/bin/bash

set -o nounset


VALUE=${WHATEVER:-}

if [ ! -z ${VALUE} ];
 then echo "yo"
fi

echo "whatever"

在这种情况下，如果未设置 WHATEVER，VALUE 最终将成为空字符串。我们使用的是 {parameter:-word} 扩展，您可以在“参数扩展”下的 man bash 中查找该扩展。

Answer 2

如果你想得到你期望的结果，你需要引用变量：

check() {
    if [ -n "${WHATEVER-}" ]
    then
        echo 'not empty'
    elif [ "${WHATEVER+defined}" = defined ]
    then
        echo 'empty but defined'
    else
        echo 'unset'
    fi
}

测试：

$ unset WHATEVER
$ check
unset
$ WHATEVER=
$ check
empty but defined
$ WHATEVER='   '
$ check
not empty

Answer 3

假设：

$ echo $SHELL

/bin/bash

$ /bin/bash --version | head -1

GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)

$ set -o nounset

如果您希望非交互式脚本在变量为空或未设置时打印错误并退出：

$ [[ "${HOME:?}" ]]

$ [[ "${IAMUNBOUND:?}" ]]

bash: IAMUNBOUND: parameter null or not set

$ IAMNULL=""
$ [[ "${IAMNULL:?}" ]]

bash: IAMNULL: parameter null or not set

如果您不希望脚本退出：

$ [[ "${HOME:-}" ]] || echo "Parameter null or not set."

$ [[ "${IAMUNBOUND:-}" ]] || echo "Parameter null or not set."

Parameter null or not set.

$ IAMNULL=""
$ [[ "${IAMUNNULL:-}" ]] || echo "Parameter null or not set."

Parameter null or not set.

您甚至可以使用 [ 和 < code>] 而不是上面的 [[ 和 ]]，但后者在 Bash 中更可取。

注意上面冒号的作用。来自文档：

换句话说，如果包含冒号，则运算符会测试两个参数是否存在以及其值是否不为空；如果省略冒号，则运算符仅测试是否存在。

显然不需要 -n 或 -z。

总而言之，我通常只使用 [[ "${VAR:?}" ]]。 根据示例，如果变量为 null，则会打印错误并退出或未设置。

Answer 4

使用 oneliner：

[ -z "${VAR:-}" ] && echo "VAR is not set or is empty" || echo "VAR is set to $VAR"

-z 检查空变量或未设置的变量

Answer 5

您可以使用

if [[ ${WHATEVER:+$WHATEVER} ]]; then

但

if [[ "${WHATEVER:+isset}" == "isset" ]]; then

可能更具可读性。

Answer 6

虽然这完全不是所要求的用例，但我发现如果您想使用nounset（或-u）默认行为是您想要的行为：以描述性消息退出非零值。

如果您只想在退出时回显其他内容或进行一些清理，则可以使用陷阱。

否则 :- 运算符可能就是您想要的。

Answer 7

对我来说，大多数答案充其量都是令人困惑的，不包括测试矩阵。它们通常也没有解决变量包含默认值的情况。

l0b0 的解决方案是唯一可读、可测试的（并且就实际问题而言是正确的），但尚不清楚如果反转/重新排序测试以简化逻辑会产生正确的结果。我缩小了他的解决方案

来自 Aleš 的（已经缩小的）对比解决方案暴露了已声明但未定义的变量的差异。其中之一可能适合您的场景。

#!/bin/bash -eu

check1() {
    if [[ -n "${WHATEVER-}" ]]; then
        echo 'something else: not empty'
    elif [[ "${WHATEVER+defined}" = defined ]]; then
        echo 'something else: declared but undefined'
    else
        echo 'unset'
    fi
}

check2() {
    if [[ "${WHATEVER+defined}" != "defined" ]]; then
        echo 'unset'
    else
        echo "something else"
    fi
}

check3() {
    if [[ "${WHATEVER-defined}" = defined ]]; then
        echo 'unset'
    else
        echo 'something else'
    fi
}

check4() {
    if [[ ! ${WHATEVER+$WHATEVER} ]]; then
        echo 'unset'
    else
        echo 'something else'
    fi
}

echo
echo "check1 from l0b0"

unset WHATEVER
check1
WHATEVER=
check1
WHATEVER='   '
check1
WHATEVER='defined'
check1

echo
echo "check2 prove simplification keeps semantics"

unset WHATEVER
check2
WHATEVER=
check2
WHATEVER='   '
check2
WHATEVER='defined'
check2

echo
echo "check3 other promising operator?"

unset WHATEVER
check3
WHATEVER=
check3
WHATEVER='   '
check3
WHATEVER='defined'
check3

echo
echo "check4 other interesting suggestion, from aleš"

unset WHATEVER
check4
WHATEVER=
check4
WHATEVER='   '
check4
WHATEVER='defined'
check4

• Check1 和 Check2 的行为相同
• Check3 显然是错误的
• Check4：正确，取决于您认为声明/定义的内容多变的。

check1 from l0b0
unset
something else: declared but undefined
something else: not empty
something else: not empty

check2 prove simplification keeps semantics
unset
something else
something else
something else

check3 other promising operator?
unset
something else
something else
unset

check4 other interesting suggestion, from aleš
unset
unset
something else
something else

Answer 8

这是我的贡献，当我开始使用数组/关联数组时，我需要一种简单的方法来获得答案。我的问题是：对于 0 个元素数组，这将返回错误：

if [ ${#my_array[@]} -eq 0 ]; then #this will error if nounset is enabled!!
    echo "return error due to empty array"
    return 1
fi

var_stat() {
    #Determines if a variable (or array) is unset, blank (or no keys) or populated (has key(s))
    #Input: single parameter to be text of a variable name or an array or an associative array
    #stdout depends on whether the input represents a variable (or array) that is:
        # -1: unset
        #  0: set and blank (or set with 0 keys)
        #  1: set and not blank (or set and has key(s))
    local input_var_name="${1:-"__NA"}"
    if [[ "${input_var_name:-"__NA"}" = "__NA" ]]; then
        echo -1
        return 0
    fi
    #evaluate results of declare -p
    case "$( declare -p ${input_var_name} 2>/dev/null || echo "__NA" )" in
        #if begins with __NA then above command failed and variable is unset
        __NA*)
            echo -1
            ;;
        *\))
            #if ending with ) (escaped as '\)') then it is an array and populated
            echo 1
            ;;
        declare\ -[aA]*)
            #if it begins with declare -a or declare -A it is an empty array (bc it failed the prior test)
            echo 0
            ;;
        #otherwise, it is a regular variable; z test is safe
        *)
            if [ -z "${!input_var_name}" ]; then  #"${!input_var_name}" is indirect variable usage
                #it is empty
                echo 0
            else
                #it is not empty
                echo 1
            fi
    esac
}

单元测试：

#Usage - tested with both errexit/nounset on/off
#first, define a bunch of variables, some blank, some keyless, etc.
    mysettext="settext"
    myblanktext=""
    declare -A myemptyAA
    declare -a myemptya
    declare -A myhaskeyAA
        myhaskeyAA[oneElement]="associative array with one key"
    declare -a myhaskeya
        myhaskeya+=("array with one value")
    declare -i myemptyinteger
    declare -i mysetinteger
        mysetinteger=3

#regular array to store the variable names of our tests
unset var_stat_test_array
declare -a var_stat_test_array
    var_stat_test_array+=('mysettext')
    var_stat_test_array+=('myblanktext')
    var_stat_test_array+=('myemptyAA')
    var_stat_test_array+=('myemptya')
    var_stat_test_array+=('myhaskeyAA')
    var_stat_test_array+=('myhaskeya')
    var_stat_test_array+=('myemptyinteger')
    var_stat_test_array+=('mysetinteger')
    var_stat_test_array+=('myUNSET') #this is not set per above

#cycle through the test array, run declare -p then the var_stat function etc.
    for eachElement in "${var_stat_test_array[@]}"; do
        declare -p ${eachElement} || true #guarantee we succeed even if unset
        var_stat "${eachElement}" | sed "s/^/    var_stat returned:/"
        # echo "    $? varstat outcome" #uncomment to verify 100% successes
        echo "" #just formatting output
    done

输出：

declare -- mysettext="settext"
    var_stat returned:1

declare -- myblanktext=""
    var_stat returned:0

declare -A myemptyAA
    var_stat returned:0

declare -a myemptya
    var_stat returned:0

declare -A myhaskeyAA=([oneElement]="associative array with one key" )
    var_stat returned:1

declare -a myhaskeya=([0]="array with one value")
    var_stat returned:1

declare -i myemptyinteger
    var_stat returned:0

declare -i mysetinteger="3"
    var_stat returned:1

-bash: declare: myUNSET: not found
    var_stat returned:-1

实际示例：

if [[ $( var_stat my_unset_var ) -eq -1 ]]; then
    echo 'exit with error if this is a required variable; else safe to succeed'
    return 1
fi

declare -a my_empty_array  #or declare -A my_empty_array 
if [[ $( var_stat my_empty_array )  -le 0 ]]; then
    echo 'exit with error if an array (associative or not) must have at least one key/element; else safe to succeed'
    return 1
fi