How to mount a specific file configured in the ConfigMap into the Pod

You can mount a specific file configured in ConfigMap using “subPath” as follows. This configuration steps are available on both OpenShift and Kubernetes.

Configuration steps

Create a test project, a test pod and a test text file for ConfigMap.

// Create test pod and deploymentconfig.
$ oc new-project test-configmap
$ oc run test — image registry.redhat.io/rhel7 — tail -f /dev/null
deploymentconfig.apps.openshift.io/test created
// Create test.txt
$ cat <<EOF > test.txt
Test file
EOF

Create ConfigMap using created text file and configure the ConfigMap mount options to the deploymentConfig using “subPath”.

// Create testmap ConfigMap using above test.txt file.
$ oc create configmap testmap --from-file=test.txt

// Modify volumes and volumeMounts for mounting only test.txt file to "/etc/test.txt".
$ oc edit dc/test
:
containers:
- name: test
  :
  volumeMounts:
  - mountPath: /etc/test.txt
    name: testtxt
    subPath: test.txt
:
terminationGracePeriodSeconds: 30
volumes:
- configMap:
    defaultMode: 420
    items:
    - key: test.txt
      path: test.txt
    name: testmap
  name: testtxt
:

Test the configuration result.

// You can verify test.txt after redeploying test pod after modification.
$ oc rsh dc/test cat /etc/test.txt
Test file

// you can also verify test.txt file is only mounted to /etc directory as one specified file by subPath, not all directory.
$ oc rsh dc/test ls -l /etc/
total 888
:
drwxr-xr-x.  4 root root          151 Aug  3 09:13 systemd
drwxr-xr-x.  2 root root            6 Aug 15  2017 terminfo
-rw-r--r--.  1 root 1000110000     10 Aug 14 16:02 test.txt
:
drwxr-xr-x.  1 root root            6 Aug  3 09:37 yum.repos.d

Thank you for reading.