I am learning kubernetes and have the following question related to command and argument syntax for POD.

Are there any specific syntax that we need to follow to write a shell script kind of code in the arguments of a POD? For example

In the following code, how will I know that the while true need to end with a semicolon ; why there is no semi colon after do but after If etc

   while true;
  do
  echo $i;
  if [ $i -eq 5 ];
  then
    echo "Exiting out";
    break;
  fi;
      i=$((i 1));
      sleep "1";
  done

We don't write shell script in the similar way from semicolon prespective so why do we have to do this in POD.

I tried the command in /bin/bash format as well

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: bash
  name: bash
spec:
  containers:
  - image: nginx
    name: nginx
    args:
    - /bin/bash
    - -c
    - >
       for i in 1 2 3 4 5
       do
         echo "Welcome $i times"
       done
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}

Error with new code

/bin/bash: -c: line 2: syntax error near unexpected token `echo'
/bin/bash: -c: line 2: `  echo "Welcome $i times"'

CodePudding user response：

Are there any specific syntax that we need to follow to write a shell script kind of code in the arguments of a POD?

No, shell syntax is the same across.

...how will I know that the while true need to end with a semicolon

Used | for your text block to be treated like an ordinary shell script:

...
args:
- /bin/bash
- -c
- |
  for i in 1 2 3 4 5
  do
    echo "Welcome $i times"
  done

When you use > your text block is merge into a single line where newline is replaced with white space. Your command become invalid in such case. If you want your command to be a single line, then write them with ; like you would in ordinary terminal. This is shell scripting standard and is not K8s specific.

If you must use >, you need to either add empty line or indented the next line correctly:

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: bash
  name: bash
spec:
  containers:
  - image: nginx
    name: nginx
    args:
    - /bin/bash
    - -c
    - >
      for i in 1 2 3 4 5
        do
          echo "Welcome $i times"
        done
  restartPolicy: Never

kubectl logs bash to see the 5 echos and kubectl delete pod bash to clean-up.

CodePudding user response：

You are doing syntax errors in your bash script, run this and note the position of semicolon after for and before do. Also, consider validating your shellcode on shellcheck.net. As you mentioned you are using shells like ash/dash on command line command works. You are mixing the syntax of different shells. As in your pod manifest you have used /bin/bash then keep the syntax of bash , not dash(where your command worked on command line)

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: bash
  name: bash
spec:
  containers:
  - image: nginx
    name: nginx
    args:
    - /bin/bash
    - -c
    - >
       for i in 1 2 3 4 5;
       do
         echo "Welcome $i times"
       done
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}