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When a Compose service cannot read or write a mounted host directory, check the identity of the process inside the container against the directory’s numeric owner and group on the host. Compose’s user setting can run the service process as a chosen UID and GID, but the right values depend on your system and image; there is no universal number that fixes every setup.
What the Compose user setting changes
A service’s user field sets the user used to run its container process. Docker’s Compose reference says, “The default value is the user that starts the container.” If the setting is absent, the image’s default applies; if the image has no default, the process runs as root. See the Compose service reference.
You can specify a numeric user and group, for example:
services:
app:
image: your-image
user: "UID:GID"
volumes:
- ./data:/dataReplace UID:GID with the numeric identity appropriate for your host directory and image. It is a placeholder, not a literal value or a recommendation to use a particular number. Setting user changes the process identity; it does not change the host directory’s ownership or permissions.
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Why a read-write bind mount can still deny access
A bind mount makes a host path available at a path inside the container. Compose’s short volume syntax is read-write by default, but that describes the mount’s access mode, not whether the process is allowed by the host filesystem to read or write its files. The host’s ownership and permission bits still matter. Docker describes bind mounts in its bind mounts documentation.
For example, a mount can be read-write while the process running your application lacks write permission to the mounted directory. The relevant comparison is between the process’s effective UID and groups and the host path’s numeric owner, group, and permission bits.
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Diagnose the mismatch before changing permissions
- Find the exact mount. In the service’s
volumesentry, identify both the host path and the container target. Confirm the application is accessing the target you expect. - Inspect the host path. Check the numeric owner UID, group GID, and permission bits for the directory and any files the application needs to access. Inspect the relevant files as well as the top-level directory; their permissions may differ.
- Check the process identity. Determine the effective UID and groups of the process that performs the failing read or write inside the container. Do not assume that the image’s startup scripts or configuration variables describe the identity of every process it launches.
- Read the image’s documentation. Some images support variables such as
PUIDandPGID, but these are image-specific conventions. Verify that your image supports them and how it applies them before adding them to Compose. They are not interchangeable with Compose’s container-levelusersetting. - Check ID mapping if the numbers seem to match. Docker user-namespace remapping changes how container IDs map to host IDs, which can complicate access to bind mounts. See Docker’s user namespace remapping documentation.
- Make the smallest justified change, then reproduce the real operation. Adjust the process identity, ownership, or permissions only after identifying the mismatch. Test the application’s actual read or write action against the mounted path; a successful container start alone does not prove the access problem is fixed.
Choose a fix that matches the cause
| What you find | What to investigate |
|---|---|
| The process UID or groups do not have access under the host path’s owner, group, and mode. | Check whether setting Compose user to an appropriate numeric identity is compatible with the image, or whether a justified host ownership or permission change is safer. |
The image documents support for PUID or PGID. |
Follow that image’s instructions. Do not assume those variables work in other images or behave like Compose’s user field. |
| Container and host IDs appear aligned, but access is still denied. | Check whether user-namespace remapping changes the host-side identity, and investigate the host filesystem or network-share behavior. |
| The mount is configured as read-write, but writes fail. | Check the process identity and host permission bits. Read-write mount mode does not grant the process filesystem write permission. |
Why there is no universal UID:GID to copy
The correct numeric identity depends on the host directory’s ownership, the process that accesses it, the image’s behavior, and any user-namespace mapping. Without knowing those details, a value such as 1000:1000 is only a guess—not a reliable fix. Establish the identities and permissions on both sides first, then change only what the evidence points to.
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