if [[ ${1:+isset} ]]
then echo "It was set and not null." >&2
else echo "It was not set or it was null." >&2
fi

if [[ ${1+isset} ]]
then echo "It was set but might be null." >&2
else echo "It was was not set." >&2
fi

对于那些希望在使用set-u的脚本中检查未设置或空的脚本：

if [ -z "${var-}" ]; then
   echo "Must provide var environment variable. Exiting...."
   exit 1
fi

常规[-z“$var”]检查将因var而失败；未绑定变量如果设置-u但[-z“${var-}”]如果var未设置而不失败，则扩展为空字符串。

阅读bash手册页的“参数扩展”部分。参数扩展并不能为正在设置的变量提供常规测试，但如果未设置参数，可以对其执行一些操作。

例如：

function a {
    first_arg=${1-foo}
    # rest of the function
}

如果分配了firstarg，则将其设置为等于$1，否则将使用值“foo”。如果绝对必须采用单个参数，并且不存在良好的默认值，则可以在未给定参数时退出并显示错误消息：

function a {
    : ${1?a must take a single argument}
    # rest of the function
}

（注意：作为null命令的用法，它只是扩展其参数的值。在本例中，我们不想对$1做任何操作，如果未设置，只需退出）

当启用Bash选项集-u时，上面的答案不起作用。此外，它们不是动态的，例如，如何测试是否定义了名为“dummy”的变量？试试看：

is_var_defined()
{
    if [ $# -ne 1 ]
    then
        echo "Expected exactly one argument: variable name as string, e.g., 'my_var'"
        exit 1
    fi
    # Tricky.  Since Bash option 'set -u' may be enabled, we cannot directly test if a variable
    # is defined with this construct: [ ! -z "$var" ].  Instead, we must use default value
    # substitution with this construct: [ ! -z "${var:-}" ].  Normally, a default value follows the
    # operator ':-', but here we leave it blank for empty (null) string.  Finally, we need to
    # substitute the text from $1 as 'var'.  This is not allowed directly in Bash with this
    # construct: [ ! -z "${$1:-}" ].  We need to use indirection with eval operator.
    # Example: $1="var"
    # Expansion for eval operator: "[ ! -z \${$1:-} ]" -> "[ ! -z \${var:-} ]"
    # Code  execute: [ ! -z ${var:-} ]
    eval "[ ! -z \${$1:-} ]"
    return $?  # Pedantic.
}

相关：在Bash中，如何测试变量是否以“-u”模式定义

我很惊讶没有人尝试编写一个shell脚本来以编程方式生成这个臭名昭著的难以摸索的表。既然我们在这里试图学习编码技术，为什么不用代码表达答案？：）这是我的看法（应该在任何POSIX shell中都适用）：

H="+-%s-+-%s----+-%s----+-%s--+\n"       # table divider printf format
R="| %-10s | %-10s | %-10s | %-10s |\n"  # table row printf format

S='V'     # S is a variable that is set-and-not-null
N=''      # N is a variable that is set-but-null (empty "")
unset U   # U is a variable that is unset

printf "$H" "----------" "-------" "-------" "---------";
printf "$R" "expression" "FOO='V'" "FOO='' " "unset FOO";
printf "$H" "----------" "-------" "-------" "---------";
printf "$R" "\${FOO:-x}" "${S:-x}" "${N:-x}" "${U:-x}  "; S='V';N='';unset U
printf "$R" "\${FOO-x} " "${S-x} " "${N-x} " "${U-x}   "; S='V';N='';unset U
printf "$R" "\${FOO:=x}" "${S:=x}" "${N:=x}" "${U:=x}  "; S='V';N='';unset U
printf "$R" "\${FOO=x} " "${S=x} " "${N=x} " "${U=x}   "; S='V';N='';unset U
#                                  "${N:?x}" "${U:?x}  "
printf "$R" "\${FOO:?x}" "${S:?x}" "<error>" "<error>  "; S='V';N='';unset U
#                                            "${U?x}   "
printf "$R" "\${FOO?x} " "${S?x} " "${N?x} " "<error>  "; S='V';N='';unset U
printf "$R" "\${FOO:+x}" "${S:+x}" "${N:+x}" "${U:+x}  "; S='V';N='';unset U
printf "$R" "\${FOO+x} " "${S+x} " "${N+x} " "${U+x}   "; S='V';N='';unset U
printf "$H" "----------" "-------" "-------" "---------";

以及运行脚本的输出：

+------------+------------+------------+------------+
| expression | FOO='V'    | FOO=''     | unset FOO  |
+------------+------------+------------+------------+
| ${FOO:-x}  | V          | x          | x          |
| ${FOO-x}   | V          |            | x          |
| ${FOO:=x}  | V          | x          | x          |
| ${FOO=x}   | V          |            | x          |
| ${FOO:?x}  | V          | <error>    | <error>    |
| ${FOO?x}   | V          |            | <error>    |
| ${FOO:+x}  | x          |            |            |
| ${FOO+x}   | x          | x          |            |
+------------+------------+------------+------------+

该脚本缺少了一些功能，比如在发生（或不发生）副作用分配时显示，但也许其他更有野心的人希望以此为出发点，并以此为出发。

使用[[-z“$var”]]是了解变量是否已设置的最简单方法，但选项-z无法区分未设置的变量和设置为空字符串的变量：

$ set=''
$ [[ -z "$set" ]] && echo "Set" || echo "Unset" 
Unset
$ [[ -z "$unset" ]] && echo "Set" || echo "Unset"
Unset

最好根据变量的类型进行检查：env变量、参数或常规变量。

对于env变量：

[[ $(env | grep "varname=" | wc -l) -eq 1 ]] && echo "Set" || echo "Unset"

对于参数（例如，检查参数$5的存在）：

[[ $# -ge 5 ]] && echo "Set" || echo "Unset"

对于正则变量（使用辅助函数，以优雅的方式实现）：

function declare_var {
   declare -p "$1" &> /dev/null
}
declare_var "var_name" && echo "Set" || echo "Unset"

笔记：

$#：提供位置参数的数量。declare-p：提供作为参数传递的变量的定义。如果它存在，则返回0，如果不存在，返回1并打印错误消息。&>/dev/null:在不影响其返回代码的情况下抑制declare-p的输出。