虽然这里所述的大多数技术都是正确的，但Bash 4.2支持对变量的存在进行实际测试（man-Bash），而不是测试变量的值。

[[ -v foo ]]; echo $?
# 1

foo=bar
[[ -v foo ]]; echo $?
# 0

foo=""
[[ -v foo ]]; echo $?
# 0

值得注意的是，与许多其他方法（如使用[-z）不同，这种方法在set-u/set-o nounset模式下用于检查未设置的变量时不会导致错误。

有许多方法可以做到这一点，以下是其中之一：

if [ -z "$1" ]

如果$1为空或未设置，则此操作成功。

以下是如何测试参数是否未设置、是否为空（“Null”）或是否设置了值：

+--------------------+----------------------+-----------------+-----------------+
|   Expression       |       parameter      |     parameter   |    parameter    |
|   in script:       |   Set and Not Null   |   Set But Null  |      Unset      |
+--------------------+----------------------+-----------------+-----------------+
| ${parameter:-word} | substitute parameter | substitute word | substitute word |
| ${parameter-word}  | substitute parameter | substitute null | substitute word |
| ${parameter:=word} | substitute parameter | assign word     | assign word     |
| ${parameter=word}  | substitute parameter | substitute null | assign word     |
| ${parameter:?word} | substitute parameter | error, exit     | error, exit     |
| ${parameter?word}  | substitute parameter | substitute null | error, exit     |
| ${parameter:+word} | substitute word      | substitute null | substitute null |
| ${parameter+word}  | substitute word      | substitute word | substitute null |
+--------------------+----------------------+-----------------+-----------------+

来源：POSIX：参数扩展：

在所有显示为“替换”的情况下，表达式将替换为显示的值。在所有显示为“assign”的情况下，参数都被指定该值，该值也会替换表达式。

要在操作中显示此内容，请执行以下操作：

+--------------------+----------------------+-----------------+-----------------+
|   Expression       |  When FOO="world"    |  When FOO=""    |    unset FOO    |
|   in script:       |  (Set and Not Null)  |  (Set But Null) |     (Unset)     |
+--------------------+----------------------+-----------------+-----------------+
| ${FOO:-hello}      | world                | hello           | hello           |
| ${FOO-hello}       | world                | ""              | hello           |
| ${FOO:=hello}      | world                | FOO=hello       | FOO=hello       |
| ${FOO=hello}       | world                | ""              | FOO=hello       |
| ${FOO:?hello}      | world                | error, exit     | error, exit     |
| ${FOO?hello}       | world                | ""              | error, exit     |
| ${FOO:+hello}      | hello                | ""              | ""              |
| ${FOO+hello}       | hello                | hello           | ""              |
+--------------------+----------------------+-----------------+-----------------+

if [[ ${!xx[@]} ]] ; then echo xx is defined; fi

这是我每天使用的：

#
# Check if a variable is set
#   param1  name of the variable
#
function is_set() { [[ $(eval echo "\${${1}+x}") ]]; }

这在Linux和Solaris以及Bash 3.0下运行良好。

$ myvar="TEST"
$ is_set myvar ; echo $?
0

$ myvar=
$ is_set myvar ; echo $?
0

$ unset myvar
$ is_set myvar ; echo $?
1

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。