Here's a neat way to do an in-place update from a script.

The idea is;

1. export the configmap to YAML (kubectl get cm -o yaml)
2. use sed to do a command-line replace of an old value with a new value (sed "s|from|to")
3. push it back to the cluster using kubectl apply

In this worked example, I'm updating a log level variable from 'info' level logging to 'warn' level logging.

So, step 1, read the current config;

$ kubectl get cm common-config -o yaml

apiVersion: v1
data:
  CR_COMMON_LOG_LEVEL: info
kind: ConfigMap

Step 2, you modify it locally with a regular expression search-and-replace, using sed:

$ kubectl get cm common-config -o yaml | \
  sed -e 's|CR_COMMON_LOG_LEVEL: info|CR_COMMON_LOG_LEVEL: warn|'

apiVersion: v1
data:
  CR_COMMON_LOG_LEVEL: warn
kind: ConfigMap

You can see the value has changed. Let's push it back up to the cluster;

Step 3; use kubectl apply -f -, which tells kubectl to read from stdin and apply it to the cluster;

$ kubectl get cm common-config -o yaml | \
  sed -e 's|CR_COMMON_LOG_LEVEL: info|CR_COMMON_LOG_LEVEL: warn|' | \
  kubectl apply -f -

configmap/common-config configured

Another option is actually you can use this command:

kubectl create configmap some-config --from-file=some-key=some-config.yaml -n some-namespace -o yaml --dry-run | kubectl apply -f -

Refer to Github issue: Support updating config map and secret with --from-file

Now you can. Just throw: kubectl edit configmap <name of the configmap> on your command line. Then you can edit your configuration.

Here is how you can add/modify/remove files in a configmap with some help from jq:

export configmap to a JSON file:

CM_FILE=$(mktemp -d)/config-map.json
oc get cm <configmap name> -o json > $CM_FILE

DATA_FILES_DIR=$(mktemp -d)
files=$(cat $CM_FILE | jq '.data' | jq -r 'keys[]')
for k in $files; do
    name=".data[\"$k\"]"
    cat $CM_FILE | jq -r $name > $DATA_FILES_DIR/$k;
done

add/modify a file:

echo '<paste file contents here>' > $DATA_FILES_DIR/<file name>.conf

remove a file:

rm <file name>.conf

when done, update the configmap:

kubectl create configmap <configmap name> --from-file $DATA_FILES_DIR -o yaml --dry-run | kubectl apply -f -

delete temporary files and folders:

rm -rf CM_FILE
rm -rf DATA_FILES_DIR

kubectl edit configmap -n <namespace> <configMapName> -o yaml

This opens up a vim editor with the configmap in yaml format. Now simply edit it and save it.

Here is a complete shell script to add new file to configmap (or replace existing one) based on @Bruce S. answer https://stackoverflow.com/a/54876249/2862663

#!/bin/bash
# Requires jq to be installed
if [ -z "$1" ]
  then
    echo "usage: update-config-map.sh <config map name> <config file to add>"
    return
fi 
if [ -z "$2" ]
  then
    echo "usage: update-config-map.sh <config map name> <config file to add>"
    return
fi 

CM_FILE=$(mktemp -d)/config-map.json
kubectl get cm $1 -o json > $CM_FILE

DATA_FILES_DIR=$(mktemp -d)
files=$(cat $CM_FILE | jq '.data' | jq -r 'keys[]')
for k in $files; do
    name=".data[\"$k\"]"
    cat $CM_FILE | jq -r $name > $DATA_FILES_DIR/$k;
done

echo cunfigmap: $CM_FILE tempdir: $DATA_FILES_DIR

echo will add file $2 to config

cp $2 $DATA_FILES_DIR

kubectl create configmap $1 --from-file $DATA_FILES_DIR -o yaml --dry-run |  kubectl apply -f -
echo Done
echo removing temp dirs
rm -rf $CM_FILE
rm -rf $DATA_FILES_DIR