Linux losetup命令用于设置循环设备。
循环设备可把文件虚拟成区块设备，籍以模拟整个文件系统，让用户得以将其视为硬盘驱动器，光驱或软驱等设备，并挂入当作目录来使用。
语法
<span class="pln">losetup </span><span class="pun">[-</span><span class="pln">d</span><span class="pun">][-</span><span class="pln">e </span><span class="pun"><加密方式>][-</span><span class="pln">o </span><span class="pun"><平移数目>][循环设备代号][文件]</span>
参数：
- -d 卸除设备。
- -e<加密方式> 启动加密编码。
- -o<平移数目> 设置数据平移的数目。
实例
（1）创建空的磁盘镜像文件，这里创建一个1.44M的软盘
<span class="pln">$ dd </span><span class="kwd">if</span><span class="pun">=</span><span class="str">/dev/</span><span class="pln">zero of</span><span class="pun">=</span><span class="pln">floppy</span><span class="pun">.</span><span class="pln">img bs</span><span class="pun">=</span><span class="lit">512</span><span class="pln"> count</span><span class="pun">=</span><span class="lit">2880</span>
（2）使用 losetup将磁盘镜像文件虚拟成快设备
<span class="pln">$ losetup </span><span class="pun">/</span><span class="pln">dev</span><span class="pun">/</span><span class="pln">loop1 floppy</span><span class="pun">.</span><span class="pln">img</span>
（3）挂载块设备
<span class="pln">$ mount </span><span class="pun">/</span><span class="pln">dev</span><span class="pun">/</span><span class="pln">loop0 </span><span class="pun">/</span><span class="pln">tmp</span>
经过上面的三步之后，我们就可以通过/tmp目录，像访问真实快设备一样来访问磁盘镜像文件floppy.img。
（4） 卸载loop设备
<span class="pln">$ umount </span><span class="pun">/</span><span class="pln">tmp $ losetup </span><span class="pun">-</span><span class="pln">d </span><span class="pun">/</span><span class="pln">dev</span><span class="pun">/</span><span class="pln">loop1</span>
一个完整测试实例
1. 首先创建一个 1G 大小的空文件：
<span class="com"># dd if=/dev/zero of=loopfile.img bs=1G count=1</span> <span class="lit">1</span><span class="pun">+</span><span class="lit">0</span><span class="pln"> records </span><span class="kwd">in</span> <span class="lit">1</span><span class="pun">+</span><span class="lit">0</span><span class="pln"> records </span><span class="kwd">out</span> <span class="lit">1073741824</span><span class="pln"> bytes </span><span class="pun">(</span><span class="lit">1.1</span><span class="pln"> GB</span><span class="pun">)</span><span class="pln"> copied</span><span class="pun">,</span> <span class="lit">69.3471</span><span class="pln"> s</span><span class="pun">,</span> <span class="lit">15.5</span><span class="pln"> MB</span><span class="pun">/</span><span class="pln">s</span>
2. 对该文件格式化为 ext4 格式：
<span class="com"># mkfs.ext4 loopfile.img</span> <span class="pun">。。。。</span>
3. 用 file 命令查看下格式化后的文件类型：
<span class="com"># file loopfile.img</span><span class="pln"> loopfile</span><span class="pun">.</span><span class="pln">img</span><span class="pun">:</span> <span class="typ">Linux</span><span class="pln"> rev </span><span class="lit">1.0</span><span class="pln"> ext4 filesystem data</span><span class="pun">,</span><span class="pln"> UUID</span><span class="pun">=</span><span class="pln">a9dfb4a0</span><span class="pun">-</span><span class="lit">6653</span><span class="pun">-</span><span class="lit">4407</span><span class="pun">-</span><span class="pln">ae05</span><span class="pun">-</span><span class="lit">7044d92c1159</span> <span class="pun">(</span><span class="pln">extents</span><span class="pun">)</span> <span class="pun">(</span><span class="pln">large files</span><span class="pun">)</span> <span class="pun">(</span><span class="pln">huge files</span><span class="pun">)</span>
4. 准备将上面的文件挂载起来：
<span class="com"># mkdir /mnt/loopback</span> <span class="com"># mount -o loop loopfile.img /mnt/loopback</span>
mount 命令的 -o loop 选项可以将任意一个 loopback 文件系统挂载。
上面的 mount 命令实际等价于下面两条命令：
<span class="com"># losetup /dev/loop0 loopfile.img</span> <span class="com"># mount /dev/loop0 /mnt/loopback</span>
因此实际上，mount -o loop 在内部已经默认的将文件和 /dev/loop0 挂载起来了。
然而对于第一种方法(mount -o loop)并不能适用于所有的场景。比如，我们想创建一个硬盘文件，然后对该文件进行分区，接着挂载其中一个子分区，这时就不能用 -o loop 这种方法了。因此必须如下做：
<span class="com"># losetup /dev/loop1 loopfile.img</span> <span class="com"># fdisk /dev/loop1</span>
6. 卸载挂载点：
欢迎转载本文：磁盘维护Linux losetup命令！
