bash脚本，调用函数作为条件测试

有人能告诉我为什么第7行（if语句）会生成错误：

test.sh: line 7: [: command_exists: unary operator expected

谢谢

#!/usr/bin/env bash

command_exists () {
  command -v "$1" &> /dev/null ;
}

if [ ! command_exists ruby ]; then  # test.sh: line 7: [: command_exists: unary operator expected
  echo 'found ruby'
else
  echo 'ruby not found'
fi

在Bash中，

if

执行命令并对其返回值进行操作<代码>[恰好是一个命令

if ! command_exists ruby; then