cat 命令

cat 命令用于连接文件并打印到标准输出设备上：

echo "Hello world" > hello.txt
read -p "Enter your current emotions:"
echo $REPLY >> hello.txt
cat -e hello.txt # put $ at ther end of line

从标准输入读取内容并写入文件：

echo -n "Enter current season, press CTR+D to finish input:"
cat > hello.txt
cat -e hello.txt # put $ at ther end of line

将两个文件合并到一个文件：

cat file1.txt file2.txt >> file3.txt
```<style>
  .scroll-to-top {
    font-size: 2.5rem;
    width: 3.2rem;
    height: 3.2rem;
    display: none;
    align-items: center;
    justify-content: center;
    position: fixed;
    padding: 0.75rem;
    bottom: 4rem;
    right: calc(1.25rem + 90px + var(--page-padding));
    z-index: 999;
    cursor: pointer;
    border: none;
    color: var(--bg);
    background: var(--fg);
    border-radius: 50%;
  }
  .scroll-to-top.hidden {
    display: none;
  }
  .scroll-to-top i {
    transform: translateY(-2px);
  }
  @media (min-width: 1080px) {
    .scroll-to-top {
      display: flex;
    }
  }
</style>
<button type="button" aria-label="scroll-to-top" class="scroll-to-top hidden" onclick="scrollToTop()">
  <i class="fa fa-angle-up"></i>
</button>
<script>
  const scrollToTop = () => window.scroll({ top: 0, behavior: "smooth" });
  window.addEventListener("scroll", () => {
    const button = document.querySelector(".scroll-to-top");
    button.classList.toggle("hidden", window.scrollY < 200);
  });
</script>