Passing Commands and Arguments to Kubernetes Resources
Passing Commands and Arguments to Kubernetes Resources
Commands and Arguments
Let us make use of help, to understand the usage of commands and arguments.
As shown here, with the help of --command option we can pass a different command and custom arguments to our container.
pradeep@learnk8s$ kubectl run -h
Create and run a particular image in a pod.
Examples:
<SNIP>
# Start the nginx pod using the default command, but use custom arguments (arg1 .. argN) for that command
kubectl run nginx --image=nginx -- <arg1> <arg2> ... <argN>
# Start the nginx pod using a different command and custom arguments
kubectl run nginx --image=nginx --command -- <cmd> <arg1> ... <argN>
Options:
<SNIP>
As an example, let us try to create a Ubuntu container that just prints date.
pradeep@learnk8s$ kubectl run ubuntu-date --image=ubuntu --command date
pod/ubuntu-date created
Look at the Command header under Containers section.
pradeep@learnk8s$ kubectl describe pods ubuntu-date
Name: ubuntu-date
Namespace: default
Priority: 0
Node: k8s-m02/192.168.177.30
Start Time: Tue, 15 Feb 2022 13:55:40 +0530
Labels: run=ubuntu-date
Annotations: <none>
Status: Running
IP: 10.244.1.21
IPs:
IP: 10.244.1.21
Containers:
ubuntu-date:
Container ID: docker://3c6c897fe4ad67e0a90540ec086901eb63a5f8ab1d4afae85b7d0a734e363a9c
Image: ubuntu
Image ID: docker-pullable://ubuntu@sha256:669e010b58baf5beb2836b253c1fd5768333f0d1dbcb834f7c07a4dc93f474be
Port: <none>
Host Port: <none>
Command:
date
State: Terminated
Reason: Completed
Exit Code: 0
Started: Tue, 15 Feb 2022 13:55:45 +0530
Finished: Tue, 15 Feb 2022 13:55:45 +0530
Ready: False
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-87bh2 (ro)
Conditions:
Type Status
Initialized True
Ready False
ContainersReady False
PodScheduled True
Volumes:
kube-api-access-87bh2:
Type: Projected (a volume that contains injected data from multiple sources)
TokenExpirationSeconds: 3607
ConfigMapName: kube-root-ca.crt
ConfigMapOptional: <nil>
DownwardAPI: true
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 9s default-scheduler Successfully assigned default/ubuntu-date to k8s-m02
Normal Pulled 4s kubelet Successfully pulled image "ubuntu" in 4.705982332s
Normal Created 4s kubelet Created container ubuntu-date
Normal Started 4s kubelet Started container ubuntu-date
Normal Pulling 3s (x2 over 8s) kubelet Pulling image "ubuntu"
The pod is Terminated with status Completed.
pradeep@learnk8s$ kubectl get pods | grep date
ubuntu-date 0/1 CrashLoopBackOff 3 (55s ago) 111s
Let us create another container that uses an argument as well, in addition to the command.
pradeep@learnk8s$ kubectl run ubuntu-sleep --image=ubuntu --command sleep 5000
pod/ubuntu-sleep created
pradeep@learnk8s$ kubectl describe pods ubuntu-sleep
Name: ubuntu-sleep
Namespace: default
Priority: 0
Node: k8s-m02/192.168.177.30
Start Time: Tue, 15 Feb 2022 13:59:46 +0530
Labels: run=ubuntu-sleep
Annotations: <none>
Status: Running
IP: 10.244.1.22
IPs:
IP: 10.244.1.22
Containers:
ubuntu-sleep:
Container ID: docker://4a12725e6951fd785eadb5bb692b25755ecfac7c664d2eb97305f46369f9d898
Image: ubuntu
Image ID: docker-pullable://ubuntu@sha256:669e010b58baf5beb2836b253c1fd5768333f0d1dbcb834f7c07a4dc93f474be
Port: <none>
Host Port: <none>
Command:
sleep
5000
State: Running
Started: Tue, 15 Feb 2022 13:59:56 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-gpjg5 (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
kube-api-access-gpjg5:
Type: Projected (a volume that contains injected data from multiple sources)
TokenExpirationSeconds: 3607
ConfigMapName: kube-root-ca.crt
ConfigMapOptional: <nil>
DownwardAPI: true
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 26s default-scheduler Successfully assigned default/ubuntu-sleep to k8s-m02
Normal Pulling 25s kubelet Pulling image "ubuntu"
Normal Pulled 17s kubelet Successfully pulled image "ubuntu" in 8.387291792s
Normal Created 17s kubelet Created container ubuntu-sleep
Normal Started 16s kubelet Started container ubuntu-sleep
Let us take a look at the definition of this Pod with command and arguments in YAML.
pradeep@learnk8s$ kubectl get pods ubuntu-sleep -o yaml > pod-command-arg.yaml
pradeep@learnk8s$ cat pod-command-arg.yaml
apiVersion: v1
kind: Pod
metadata:
creationTimestamp: "2022-02-15T08:29:46Z"
labels:
run: ubuntu-sleep
name: ubuntu-sleep
namespace: default
resourceVersion: "5745"
uid: 971c6c08-da5e-4360-901b-7b8ca65a693a
spec:
containers:
- command:
- sleep
- "5000"
image: ubuntu
imagePullPolicy: Always
name: ubuntu-sleep
resources: {}
terminationMessagePath: /dev/termination-log
terminationMessagePolicy: File
volumeMounts:
- mountPath: /var/run/secrets/kubernetes.io/serviceaccount
name: kube-api-access-gpjg5
readOnly: true
dnsPolicy: ClusterFirst
<SNIP>
Now let us use the same kodecloud/web-app image and try to pass the color argument while initializing the pod.
Looking at the source code of this container kodekloudhub/webapp-color, we can find out that, A color can be specified in two ways: either as a command line argument or as an environment variable.
from flask import Flask
from flask import render_template
import socket
import random
import os
import argparse
app = Flask(__name__)
color_codes = {
"red": "#e74c3c",
"green": "#16a085",
"blue": "#2980b9",
"blue2": "#30336b",
"pink": "#be2edd",
"darkblue": "#130f40"
}
SUPPORTED_COLORS = ",".join(color_codes.keys())
# Get color from Environment variable
COLOR_FROM_ENV = os.environ.get('APP_COLOR')
# Generate a random color
COLOR = random.choice(["red", "green", "blue", "blue2", "darkblue", "pink"])
@app.route("/")
def main():
# return 'Hello'
return render_template('hello.html', name=socket.gethostname(), color=color_codes[COLOR])
if __name__ == "__main__":
print(" This is a sample web application that displays a colored background. \n"
" A color can be specified in two ways. \n"
"\n"
" 1. As a command line argument with --color as the argument. Accepts one of " + SUPPORTED_COLORS + " \n"
" 2. As an Environment variable APP_COLOR. Accepts one of " + SUPPORTED_COLORS + " \n"
" 3. If none of the above then a random color is picked from the above list. \n"
" Note: Command line argument precedes over environment variable.\n"
"\n"
"")
# Check for Command Line Parameters for color
parser = argparse.ArgumentParser()
parser.add_argument('--color', required=False)
args = parser.parse_args()
if args.color:
print("Color from command line argument =" + args.color)
COLOR = args.color
if COLOR_FROM_ENV:
print("A color was set through environment variable -" + COLOR_FROM_ENV + ". However, color from command line argument takes precendence.")
elif COLOR_FROM_ENV:
print("No Command line argument. Color from environment variable =" + COLOR_FROM_ENV)
COLOR = COLOR_FROM_ENV
else:
print("No command line argument or environment variable. Picking a Random Color =" + COLOR)
# Check if input color is a supported one
if COLOR not in color_codes:
print("Color not supported. Received '" + COLOR + "' expected one of " + SUPPORTED_COLORS)
exit(1)
# Run Flask Application
app.run(host="0.0.0.0", port=8080)
Let us try the first method that we just discussed: as a command argument.
Create a pod using kodekloud/webapp-color:v3 but pass the –color argument to change the default color from red to blue. Earlier, when we ran this image (with defaults), the color was red.
This can also be confirmed by looking at the Dockerfile definition.
pradeep@learnk8s$ kubectl run kodekloud-change-color --image=kodekloud/webapp-color:v3 --dry-run=client -o yaml -- "--color" "blue" > pod-change-color.yaml
Now we can see we are passing the --color and blue as arguements to this container.
pradeep@learnk8s$ cat pod-change-color.yaml
apiVersion: v1
kind: Pod
metadata:
creationTimestamp: null
labels:
run: kodekloud-change-color
name: kodekloud-change-color
spec:
containers:
- args:
- --color
- blue
image: kodekloud/webapp-color:v3
name: kodekloud-change-color
resources: {}
dnsPolicy: ClusterFirst
restartPolicy: Always
status: {}
Let us create a pod from this definition file.
pradeep@learnk8s$ kubectl create -f pod-change-color.yaml
pod/kodekloud-change-color created
pradeep@learnk8s$ kubectl describe pods kodekloud-change-color
Name: kodekloud-change-color
Namespace: default
Priority: 0
Node: k8s-m02/192.168.177.30
Start Time: Tue, 15 Feb 2022 16:31:54 +0530
Labels: run=kodekloud-change-color
Annotations: <none>
Status: Running
IP: 10.244.1.29
IPs:
IP: 10.244.1.29
Containers:
kodekloud-change-color:
Container ID: docker://3c6bf9642cb90fcc2f969a3043f3879d9a42917d7e15ed26905b2a340461609e
Image: kodekloud/webapp-color:v3
Image ID: docker-pullable://kodekloud/webapp-color@sha256:3ecd19b1b85db381a0b6f78272458c3c274ac2a38e878d65700393899adb3177
Port: <none>
Host Port: <none>
Args:
--color
blue
State: Running
Started: Tue, 15 Feb 2022 16:31:57 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-6l7n4 (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
kube-api-access-6l7n4:
Type: Projected (a volume that contains injected data from multiple sources)
TokenExpirationSeconds: 3607
ConfigMapName: kube-root-ca.crt
ConfigMapOptional: <nil>
DownwardAPI: true
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 12s default-scheduler Successfully assigned default/kodekloud-change-color to k8s-m02
Normal Pulled 10s kubelet Container image "kodekloud/webapp-color:v3" already present on machine
Normal Created 10s kubelet Created container kodekloud-change-color
Normal Started 9s kubelet Started container kodekloud-change-color
pradeep@learnk8s$ kubectl get pods -o wide | grep color
kodekloud-change-color 1/1 Running 0 40s 10.244.1.29 k8s-m02 <none> <none>
Verify the color of the webapp now, it should say blue instead of the default red.
pradeep@learnk8s$ minikube ssh -p k8s
_ _
_ _ ( ) ( )
___ ___ (_) ___ (_)| |/') _ _ | |_ __
/' _ ` _ `\| |/' _ `\| || , < ( ) ( )| '_`\ /'__`\
| ( ) ( ) || || ( ) || || |\`\ | (_) || |_) )( ___/
(_) (_) (_)(_)(_) (_)(_)(_) (_)`\___/'(_,__/'`\____)
$ curl 10.244.1.29:8080
<!doctype html>
<title>Hello from Flask</title>
<body style="background: #2980b9;"></body>
<div style="color: #e4e4e4;
text-align: center;
height: 90px;
vertical-align: middle;">
<h1>Hello from kodekloud-change-color!</h1>
<h2>
Application Version: v3
</h2>
</div>$
$ curl 10.244.1.29:8080/color
blue$ exit
logout
It is confirmed the arguments that we passed worked! (background: #2980b9 indicates blue, as defined in the color_code, "blue": "#2980b9").
