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Mount

The mount command is used to mount a file system to a directory.

Table of Contents

What Mount does

Without args, mount will dump every file system that's mounted, in the order they were mounted.

  • mount reads from /etc/mtab when it does this.
    • Never edit /etc/mtab in real-time yourself.

Syntax

mount -t type /dev/device /dir
This tells the kernel to attach the filesystem found on /dev/device (which is of type type) at the directory /dir.

  • mount -t xfs: Tells mount that the -type of file system is xfs.

Making a New File System and Mounting It

To make a new file system, use the mkfs command. 

mkfs.xfs /dev/sda3
This creates a new file system, with the xfs format, on the block device /dev/sda3.

This can now be mounted directly to a directory.

sudo mkdir /new_mountpoint       # Make a new directory to mount to.  
mount /dev/sda3 /new_mountpoint  # Mount the file system to the directory.
Everything stored in /new_mountpoint will be stored on the new filesystem.

Weird things about mount

mount can be used in a lot of weird, complex ways.

  • The same filesystem can be mounted more than once.
  • In some cases (e.g., network filesystems) the same filesystem can be mounted on the same mountpoint multiple times.

mount and fstab

By default, mount will use /etc/fstab if either device or directory are omitted.

  • The /etc/fstab (file system tables) file contains info about the file systems and their mount points.
  • mount uses this file to determine how to mount certain filesystems automatically, when the user doesn't specify exactly how to mount them.
  • If you want to override mount options from /etc/fstab, use the -o option:
    mount /device/or/directory -o options
  • The mount options from the command line will be appended to the list of options from /etc/fstab.
  • The mount program does not read the /etc/fstab file if both device and directory are given.

Mounting a File System by Editing /etc/fstab

You can add an entry to /etc/fstab and then running mount -a to mount a file system.

  • Open /etc/fstab to edit: 

    vi /etc/fstab

  • Then add: 

    /dev/mapper/VolGroup-my_lv /space ext4 defaults 0 0

  • After that, run: 

    mount -a

  • Check if the filesystem was mounted: 

    df -h

Bind Mounts

A bind mount is when you take an existing directory or file on the Linux filesystem and mount it again somewhere else in the filesystem tree.

Unlike a typical mount (like mountaing a USB stick), bind mounts don't change devices. They just provice a second access point to the same file or directory.

Bind mounts let you reuse existing directories or files in multiple places without duplication.

For example, if you wanted to reuse a directory in a chrooted environment, you could use a bind mount: 

mkdir /mnt/real_data
mkdir /var/chroot/mnt_data
mount --bind /mnt/real_data /var/chroot/mnt_data

Now /mnt/real_data and var/chroot/mnt_data point to the same exact data.

You can also bind mount a single file if you want.

mkdir /var/chroot/etc
mount --bind /etc/hosts /var/chroot/etc/hosts

Making a Bind Mount Read-Only

You can remount a bind mount as read-only.

This is great for security, especially in jailed environments where you might want to give the user access to certain files (like /etc/hosts, /bin/bash, /usr/bin/ssh), but not allow them to change those files.

For example, mounting /bin into a chroot jail as read-only: 

mkdir -p /var/chroot/bin
mount --bind /bin/var/chroot/bin
mount -o remount,bind,ro /bin /var/chroot/bin

Note: You must do the --bind first, then a second -o remount,bind,ro to make it readonly. Linux doesn't allow --bind and ro together in the same step.

This is really useful for when mounting single files that are not meant to be changed, like /etc/hosts: 

mkdir /var/chroot/etc
mount --bind /etc/hosts /var/chroot/etc/hosts
mount -o remount,bind,ro /etc/hosts /var/chroot/etc/hosts