A script to mount a host directory into a running Docker container

docker-remount.sh

#!/bin/bash

# A script for mounting a host directory into a running Docker container

if [ "$#" -ne 3 ]; then
    echo "Usage: $0 <container> <host path> <target path>"
    exit 1
fi

CONTAINER=$1
SOURCE_DIR=$2
DEST_DIR=$3

REMOUNTS_DIR=/remounts

# Get the PID of the container into which we will mount
CONTAINER_PID=$(docker inspect --format {{.State.Pid}} $CONTAINER)
ENTER_ARGS="--target $CONTAINER_PID --mount --uts --ipc --net --pid --"

# Get the canonical location of the source
REAL_SOURCE=$(readlink -f $SOURCE_DIR)

# Find the device on which it resides, and where it is mounted
read SOURCE_DEV HOST_MOUNT_POINT <<< $(df --output=source,target $REAL_SOURCE | sed -e 1d)
MOUNT_TAIL=${REAL_SOURCE#*$HOST_MOUNT_POINT}

# Find the device major and minor numbers
read DEV_MAJOR DEV_MINOR <<< $(lsblk -n -o "MAJ:MIN" $SOURCE_DEV | tr ":" " ")

# Make the device if it doesn't already exist
nsenter $ENTER_ARGS test -b $SOURCE_DEV || nsenter $ENTER_ARGS mknod --mode 0600 $SOURCE_DEV b $DEV_MAJOR $DEV_MINOR

# Make sure we have somewhere to mount things and make a mountpoint
nsenter $ENTER_ARGS test -d $REMOUNTS_DIR || nsenter $ENTER_ARGS mkdir $REMOUNTS_DIR
DEST_MOUNT_PATH=$(nsenter $ENTER_ARGS mktemp --directory --tmpdir=$REMOUNTS_DIR)

# Mount the actual device
nsenter $ENTER_ARGS mount $SOURCE_DEV $DEST_MOUNT_PATH

# Bind-mount the requested path to the desired location
nsenter $ENTER_ARGS test -d $DEST_DIR || nsenter $ENTER_ARGS mkdir $DEST_DIR
nsenter $ENTER_ARGS mount --bind $DEST_MOUNT_PATH$MOUNT_TAIL $DEST_DIR