How to concatenate secret files mounted in a volume in kubernetes - kubernetes

Question

I have several secrets that are mounted and need to be read as a properties file. It seems kubernetes can't mount them as a single file so I'm trying to concatenate the files after the pod starts. I tried running a cat command in a postStart handler but it seems execute before the secrets are mounted as I get this error:
Error: failed to create containerd task: OCI runtime create failed: container_linux.go:380: starting container process caused: exec: "cat /properties/S3Secret /properties/S3Key >> /properties/dbPassword": stat cat /properties/S3Secret /properties/S3Key >> /properties/dbPassword: no such file or directory: unknown
Then here is the yaml.
apiVersion: apps/v1
kind: Deployment
metadata:
name: K8S_ID
spec:
selector:
matchLabels:
app: K8S_ID
replicas: 1
template:
metadata:
labels:
app: K8S_ID
spec:
containers:
- name: K8S_ID
image: IMAGE_NAME
ports:
- containerPort: 8080
env:
- name: PROPERTIES_FILE
value: "/properties/dbPassword"
volumeMounts:
- name: secret-properties
mountPath: "/properties"
lifecycle:
postStart:
exec:
command: ["cat /properties/S3Secret /properties/S3Key >> /properties/dbPassword"]
volumes:
- name: secret-properties
secret:
secretName: secret-properties
items:
- key: SECRET_ITEM
path: dbPassword
- key: S3Key
path: S3Key
- key: S3Secret
path: S3Secret

Answer 1

You need a shell session for your command like this:
...
lifecycle:
postStart:
exec:
command: ["/bin/sh","-c","cat /properties/S3Secret /properties/S3Key >> /properties/dbPassword"]
...