How to mount a single file in a volume

Question

I am trying to dockerize a PHP application  In the dockerfile  I download the archive  extract it  etc  Everything works fine  However  if a new version gets released and I update the dockerfile  I have to reinstall the application  because the config php gets overwritten  So I thought I can mount the file as a volume  like I do with the database  I tried it two ways  with a volume and a direct path  docker-compose  version   2  services    app      build  src     ports        -  quot 8080 80 quot      depends on        - mysql     volumes        -  app-conf  var www html upload       -  app-conf  var www html config php     environment        DB TYPE  mysql       DB MANAGER  MysqlManager    mysql      image  mysql 5 6     container name  mysql     volumes        - mysqldata  var lib mysql     ports        - 3306 3306     environment        MYSQL ROOT PASSWORD        MYSQL DATABASE        MYSQL USER        MYSQL PASSWORD   volumes    mysqldata    app-conf   Which results in the error  And I tried it with a given path  as a mounted volume   src docker myapp upload  var www html upload  src docker myapp upload  var www html config php  However  both ways are not working  With the mounted volume  I see that upload gets created  But then it fails with    var www html config php  quot  caused   quot not a directory  quot  quot  quot   If I try it with  src docker myapp upload config php  var www html config php  Docker creates the upload folder and then a config php folder  Not a file  Or is there another way to persist the config

User · Answer

For those who use Docker Desktop for Mac: If the file is present in your local filesystem but it's mounted as a directory inside the container, probably, you didn't share the file/directory with Docker Desktop. You need to check Docker Desktop file-sharing settings:

Go to "Preferences" -> "Resources" -> "File sharing".
If the directory with the desired file is missing, add a path to the directory containing your file.

Note! Do not add your root directory or any system directory to the file-sharing settings as it will load your CPU. The issue is described in Github, and this comment gives a workaround.

User · Answer

TL DR Notice       If you experience a directory being created in place of the file you are trying to mount  you have probably failed to supply a valid and absolute path  This is a common mistake with a silent and confusing failure mode    File volumes are done this way in docker  absolute path example  can use env variables   and you need to mention the file name         volumes        -  src docker myapp upload  var www html upload       -  src docker myapp upload config php  var www html config php   You can also do       volumes        -   PWD  upload  var www html upload       -   PWD  upload config php  var www html config php   If you fire the docker-compose from  src docker myapp folder

User · Answer

I had the same issue  docker-compose was creating a directory instead of a file  then crashing mid-way  what i did    run the container without mapping the file  copy the config file to the host location   docker cp containername  var www html config php   config php  remove the container  docker-compose down   put the mapping back and remount up the container   docker compose will find the config file  and will map that instead of trying to create a directory

User · Answer

In windows   if you need the a   PWD  env variable in your docker-compose yml you can creat a  env file in the same directory as your docker-compose yml file then insert manualy the location of your folder   CMD  pwd var bat      echo PWD  cd   gt  gt   env   Powershell  pwd var ps1      PSDefaultParameterValues  Out-File Encoding      utf8   echo  PWD   get-location  path   gt  gt   env   There is more good features hear for docker-compose  env variables  https   docs docker com compose reference envvars  especially for the COMPOSE CONVERT WINDOWS PATHS env variable that allow docker compose to accept windows path with baskslash       When you want to share a file on windows  the file must exist before sharing it with the container

User · Answer

As of docker-compose file version 3 2  you can specify a volume mount of type  quot bind quot   instead of the default type  quot volume quot   that allows you to mount a single file into the container  Search for  quot bind mount quot  in the docker-compose volume docs  https   docs docker com compose compose-file  volumes In my case  I was trying to mount a single  quot  secrets quot  file into my application that contained secrets for local development and testing only  In production  my application fetches these secrets from AWS instead  If I mounted this file as a volume using the shorthand syntax  volumes   -    secrets  data app  secrets  Docker would create a  quot  secrets quot  directory inside the container instead of mapping to the file outside of the container  My code would then raise an error like  quot IsADirectoryError   Errno 21  Is a directory    secrets  quot   I fixed this by using the long-hand syntax instead  specifying my secrets file using a read-only  quot bind quot  volume mount  volumes   - type  bind    source     secrets    target   data app  secrets    read only  true  Now Docker correctly mounts my  secrets file into the container  creating a file inside the container instead of a directory

User · Answer

I had the same issue on Windows  Docker 18 06 1-ce-win73  19507    Removing and re-adding the shared drive via the Docker settings panel and everything worked again

User · Answer

For anyone using Windows container like me  know that you CANNOT bind or mount single files using windows container   The following examples will fail when using Windows-based containers  as the destination of a volume or bind mount inside the container must be one of  a non-existing or empty directory  or a drive other than C   Further  the source of a bind mount must be a local directory  not a file  net use z    remotemachine share docker run -v z  foo c  dest     docker run -v   uncpath to directory c  dest     docker run -v c  foo somefile txt c  dest     docker run -v c  foo c      docker run -v c  foo c  existing-directory-with-contents      It s hard to spot but it s there Link to the Github issue regarding mapping files into Windows container

User · Answer

For me  the issue was that I had a broken symbolic link on the file I was trying to mount into the container

User · Answer

Maybe this helps someone   I had this problem and tried everything  Volume bindings looked well and even if I mounted directory  not files   I had the file names in the mounted directory correctly but mounted as dirs   I tried to re-enable shared drives and Docker complained the firewall is active   After disabling the firewall all was working fine

User · Answer

I have same issue on my Windows 8 1  It turned out that it was due to case-sensitivity of path  I called docker-compose up from directory cd  c users alex  and inside container a file was turned into directory   But when I did cd  c Users alex   not Users capitalized  and called docker-compose up from there  it worked   In my system both Users dir and Alex dir are capitalized  though it seems like only Users dir matter

User · Answer

The way that worked for me is to use a bind mount   version   quot 3 7 quot        services    app      image  app latest     volumes        - type  bind         source    sourceFile yaml         target   location targetFile yaml  Thanks mike breed for the answer over at  Mount single file from volume using docker-compose You need to use the  quot long syntax quot  to express a bind mount using the volumes key  https   docs docker com compose compose-file  long-syntax-3

User · Answer

All above answers are Correct  but one thing that I found really helpful is that mounted file should exist inside docker host in advance otherwise docker will create a directory instead  for example   a file inside host hostFile txt  a file inside container containerFile txt  hostFile txt should exist in advance  otherwise you will receive this error  containerFile txt is a directory

User · Answer

Use mount  --mount  instead volume  -v   More info  https   docs docker com storage bind-mounts   Example   Ensure  tmp a txt exists on docker host  docker run -it --mount type bind source  tmp a txt target  root a txt alpine sh

User · Answer

You can also use a relative path in your docker-compose yml file like this  tested on Windows host  Linux container   volumes      -   test conf  fluentd etc test conf

User · Answer

You can mount files or directories folders it all depends on Source file or directory  And also you need to provide full path or if you are not sure you can use PWD  Here is a simple working example   In this example  I am mounting env-commands file which already exists in my working directory     docker run  --rm -it -v   PWD  env-commands  env-commands aravindgv eosdt 1 0 5  bin bash -c  cat  env-commands

User · Answer

I had been suffering from a similar issue  I was trying to import my config file to my container so that I can fix it every time I need without re-building the image   I mean I thought the below command would map   pwd  config py from Docker host to  root app config py into the container as a file   docker run -v   pwd  config py  root app config py my docker image   However  it always created a directory named config py  not a file   while looking for clue  I found the reason from here      If you use -v or --volume to bind-mount a file or directory that does   not yet exist on the Docker host  -v will create the endpoint for you    It is always created as a directory    Therefore  it is always created as a directory because my docker host does not have   pwd  config py   Even if I create config py in docker host     pwd  config py just overwirte  root app config py not exporting  root app config py

[docker] How to mount a single file in a volume

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