How to "mount" a single file from the host into an existing directory in the container

[pressing-buttons]

To configure a running Firebird database in a docker container (https://hub.docker.com/r/firebirdsql/firebird) it would be necessary to mount a file from the host system into a specific directory in the container (see FirebirdSQL/firebird-docker#8).

A bind mount (--mount type=bind,..) can only mount directories, not files.

Using a glob mount, with a single file for the podman run command

--mount type=glob,src=./conf/databases.conf,dst=/opt/firebird/databases.conf

results in the error

error creating directory opt/firebird/databases.conf/databases.conf since opt/firebird/databases.conf exists and it is not a directory: Not a directory

Using a wildcard

--mount type=glob,src=./conf/*.conf,dst=/opt/firebird/

results in all files in /opt/firebird to be deleted which obviously breaks running Firebird.

Using a wildcard in the destination part:

--mount type=glob,src=./conf/databases*,dst=/opt/firebird/*.conf

Results in a file named *.conf to be created in the container.

Adding ,relabel=shared,U=true does not change this.

So my question is: is it possible to mount this single file into the container, overwriting the default file?

I am am very new to podman (or containers in general) so it might be possible that I am missing something obvious.

I am using Fedora (KDE)

[Luap99]

A bind mount (--mount type=bind,..) can only mount directories, not files.

Why do you think that? That is not true bind mounts can be files or directories.

[pressing-buttons]

Hmm, when I use

--mount type=bind,src=./databases.conf,dst=/opt/firebird/databases.conf  

then Podman seems to replace the entire contents of the /opt/firebird directory inside the container with the mapped file.

Which is similar to the behaviour when mapping a directory containing that config file into /opt/firebird

--mount type=bind,src=./conf,dst=/opt/firebird

In both cases Firebird doesn't find the files it expects in /opt/firebird, but what the exact contents is, I can't check as the container doesn't start so I can't use exec to open a bash command line inside the container.

And that let me to the conclusion I can't map a single file. So apparently I did something wrong with that mound option.

[Luap99]

$ echo test123 > databases.conf
$ podman run --rm --entrypoint="" -it --mount type=bind,src=./databases.conf,dst=/opt/firebird/databases.conf,Z docker.io/firebirdsql/firebird:latest bash
root@3edb5f10257a:/# ls /opt/firebird/
CHANGELOG.md	IPLicense.txt  SYSDBA.password	databases.conf	fbtrace.conf   firebird.log  intl  misc     plugins.conf      replication.log  tzdata
IDPLicense.txt	README.md      bin		fb_guard	firebird.conf  firebird.msg  lib   plugins  replication.conf  security5.fdb
# cat /opt/firebird/databases.conf 
test123
# grep /opt /proc/self/mountinfo 
1346 1334 0:41 /home/pholzing/databases.conf /opt/firebird/databases.conf rw,relatime - ...

signle file mount works just fine and does not replace the whole directory

[pressing-buttons]

Thanks for the detailed answer. So that failed again for me using my command. After removing other options one by one, it seems this doesn't work together with setting environment variables.

For completeness, this works:

podman run \
    --replace \
    --name firebird \
    --mount type=bind,src=./conf/databases.conf,dst=/opt/firebird/databases.conf,Z \
    -e FIREBIRD_DATABASE=mydb.fdb \
    firebirdsql/firebird:latest

but this doesn't

podman run \
    --replace \
    --name firebird \
    --mount type=bind,src=./conf/databases.conf,dst=/opt/firebird/databases.conf,Z \
    -e FIREBIRD_ROOT_PASSWORD=****  \
    -e FIREBIRD_DATABASE=mydb.fdb \
    firebirdsql/firebird:latest

I will take this to the maintainer of the container.

Thanks for your help and patience