我总是使用这个，因为任何第一次看到代码的人都很容易理解：

if [ "$variable" = "" ]
    then
    echo "Variable X is empty"
fi

如果要检查是否为空；

if [ ! "$variable" = "" ]
    then
    echo "Variable X is not empty"
fi

就是这样。

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

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

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 [ "$1" != "" ]; then
  echo \$1 is set
else
  echo \$1 is not set
fi

尽管对于参数，通常最好测试$#，我认为这是参数的数量。

if [ $# -gt 0 ]; then
  echo \$1 is set
else
  echo \$1 is not set
fi

要查看变量是否为非空，我使用

if [[ $var ]]; then ...       # `$var' expands to a nonempty string

如果变量未设置或为空，则相反：

if [[ ! $var ]]; then ...     # `$var' expands to the empty string (set or not)

要查看变量是否已设置（空或非空），我使用

if [[ ${var+x} ]]; then ...   # `var' exists (empty or nonempty)
if [[ ${1+x} ]]; then ...     # Parameter 1 exists (empty or nonempty)

如果变量未设置，则进行相反的测试：

if [[ ! ${var+x} ]]; then ... # `var' is not set at all
if [[ ! ${1+x} ]]; then ...   # We were called with no arguments