case "$1" in
 "") echo "blank";;
 *) echo "set"
esac

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。

略读所有答案后，这同样有效：

if [[ -z $SOME_VAR ]]; then read -p "Enter a value for SOME_VAR: " SOME_VAR; fi
echo "SOME_VAR=$SOME_VAR"

如果您不使用SOME_VAR代替我的$SOME_VVAR，它会将其设置为空值；$这是工作所必需的。

要检查非空/非零字符串变量，即如果已设置，请使用

if [ -n "$1" ]

它与-z相反。我发现自己使用-n多于-z。

您可以像这样使用它：

if [ -n "$1" ]; then
  echo "You supplied the first parameter!"
else
  echo "First parameter not supplied."
fi

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。

为了明确回答OP关于如何确定变量是否已设置的问题，Lionel的回答是正确的：

if test "${name+x}"; then
    echo 'name is set'
else
    echo 'name is not set'
fi

这个问题已经有很多答案，但没有一个提供真正的布尔表达式来明确区分变量值。

以下是我得出的一些明确表达：

+-----------------------+-------------+---------+------------+
| Expression in script  | name='fish' | name='' | unset name |
+-----------------------+-------------+---------+------------+
| test "$name"          | TRUE        | f       | f          |
| test -n "$name"       | TRUE        | f       | f          |
| test ! -z "$name"     | TRUE        | f       | f          |
| test ! "${name-x}"    | f           | TRUE    | f          |
| test ! "${name+x}"    | f           | f       | TRUE       |
+-----------------------+-------------+---------+------------+

顺便说一下，这些表达式是等价的：测试＜表达式＞＜＝＞〔＜表达式＞〕

其他需谨慎使用的歧义表达：

+----------------------+-------------+---------+------------+
| Expression in script | name='fish' | name='' | unset name |
+----------------------+-------------+---------+------------+
| test "${name+x}"     | TRUE        | TRUE    | f          |
| test "${name-x}"     | TRUE        | f       | TRUE       |
| test -z "$name"      | f           | TRUE    | TRUE       |
| test ! "$name"       | f           | TRUE    | TRUE       |
| test ! -n "$name"    | f           | TRUE    | TRUE       |
| test "$name" = ''    | f           | TRUE    | TRUE       |
+----------------------+-------------+---------+------------+