Overlayfs example

mount-propagation-showcast.sh

#!/bin/bash

more="$@"
saved="$EUID"

if [ "$EUID" = "0" ]; then
    echo "don't run as root"
    exit
fi

m="/tmp/mount-point"

exec sudo unshare -m /bin/bash <<UNSHARED
mkdir -p "$m"
mount -t tmpfs tmpfs "$m"
cd "$m"
mkdir top merge work

mount -t overlay overlay -o lowerdir=/,upperdir=top,workdir=work merge

#bash < /dev/tty
chroot merge sh -c 'sudo -u "#$saved" sh -c "id; ls /proc -lR"'
UNSHARED

overlayfs-showcast.zsh

#!/bin/zsh

set -eu

grep -q . /etc/gshadow # assert root

dir=/tmp/overlayfstest

create-dirs() {
    (find "$dir" -type d | xargs umount 2> /dev/null) 2> /dev/null || :
    rm -rf $dir
    mkdir -p $dir/{main/{upper,lower,work,merged},tmp}
    cd $dir/main
}

 # seperate dirs, single shared file, one being on top
create-data-a() {
    touch \
        upper/u$RANDOM \
        lower/l$RANDOM \

        nr=$RANDOM
    echo upper > upper/both-$nr
    echo lower > lower/both-$nr
}

# touch upper/upper/upper, lower/lower/lower.
# Where will touch lower/lower/new
create-data-b() {
    mkdir upper/upper lower/lower
    touch upper/upper/upper lower/lower/lower
}

create-data-c() {
    mkdir lower/directory
    touch lower/directory/deletable
}

showcast() {
    # data-a
    ls merged

    touch merged/new
    touch lower

    # data-b
    touch lower/lower/should-be-lower

    # data-c
    find -type c | sort > $dir/tmp/1
    rm merged/directory/deletable
    rm merged/lower/lower
    find -type c | sort > $dir/tmp/2
    diff $dir/tmp/{1,2} || : # TWO `rm`S, BUT ONLY ONE MARK OF IT!
    ls -l upper/directory/deletable

    # show the results
    tree -a; echo; cat merged/both-$nr
}

create-dirs
create-data-a # basic expose
create-data-b # find out where will files go when written to lowerdirs
create-data-c # touch a file to delete

mount -t overlay overlay -o lowerdir=lower,upperdir=upper,workdir=work merged
(set -x; showcast) # outputs `tree'(1).
umount merged

exit

"
code/gists/overlayfs-showcast.sh master % sudo ./overlayfs-showcast.zsh
+./overlayfs-showcast.zsh:67> showcast
+showcast:2> ls merged
b24170  directory  l17395  lower  u1629  upper
+showcast:4> touch merged/new
+showcast:5> touch lower
+showcast:8> touch lower/lower/should-be-lower
+showcast:11> find -type c
+showcast:11> sort
+showcast:12> rm merged/directory/deletable
+showcast:13> rm merged/lower/lower
+showcast:14> find -type c
+showcast:14> sort
+showcast:15> diff /tmp/overlayfstest/tmp/1 /tmp/overlayfstest/tmp/2
0a1,2
> ./upper/directory/deletable
> ./upper/lower/lower
+showcast:15> :
+showcast:16> ls -l upper/directory/deletable
c--------- 1 root root 0, 0 Mar 19 16:54 upper/directory/deletable
+showcast:19> tree -a
.
├── lower
│   ├── b24170
│   ├── directory
│   │   └── deletable
│   ├── l17395
│   └── lower
│       ├── lower
│       └── should-be-lower
├── merged
│   ├── b24170
│   ├── directory
│   ├── l17395
│   ├── lower
│   │   └── should-be-lower
│   ├── new
│   ├── u1629
│   └── upper
│       └── upper
├── upper
│   ├── b24170
│   ├── directory
│   │   └── deletable
│   ├── lower
│   │   └── lower
│   ├── new
│   ├── u1629
│   └── upper
│       └── upper
└── work
    └── work

13 directories, 17 files
+showcast:19> echo

+showcast:19> cat merged/b24170
upper