Beware of kubectl's -t (--tty) Option

I see this all over the place in documentation, blog posts, scripts, articles, etc:

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kubectl exec -it my-pod -- do_something_here

These type options have the following help text:

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  -i, --stdin=false: Pass stdin to the container
  -t, --tty=false: Stdin is a TTY

Using these options in a script can cause a tricky bug, though. Take a look at this example that just does a common kubectl exec to get some data from a pod:

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#!/bin/bash

EXPECTED_OUTPUT="hello world"
ACTUAL_OUTPUT=$(kubectl exec -it my-pod -- echo hello world)

echo "  Actual output: $ACTUAL_OUTPUT"
echo "Expected output: $EXPECTED_OUTPUT"

if [[ "$ACTUAL_OUTPUT" == "$EXPECTED_OUTPUT" ]]; then
    echo "Yes! $ACTUAL_OUTPUT matches $EXPECTED_OUTPUT"
else
    echo "No... $ACTUAL_OUTPUT does not match $EXPECTED_OUTPUT"
fi

Read the code and try to predict the output. The first two lines are expected (sort of):

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  Actual output: hello world
Expected output: hello world

These two strings are obviously the same. Right? Wrong! The third echo from the script starts to give us hints of that:

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 does not match hello world

That is shocking! Not only did [[ "$ACTUAL_OUTPUT" == "$EXPECTED_OUTPUT" ]] evaluate to false (we know that by the “does not match” part of the output), but the echo looks strange. It appears to be missing the “No… hello world” part of the output string.

So what is going on here? There is a subtle bug that you wouldn’t expect.

Let’s see what the byte output is when you use -t:

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 $ kubectl exec -it my-pod -- echo hello world | od -c -
0000000   h   e   l   l   o       w   o   r   l   d  \r  \n
0000015

There’s our problem! Because of the TTY, we have an additional carriage return (\r) added. Let’s remove -t (and naturally remove -i, because we don’t want an interactive session either):

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 $ kubectl exec my-pod -- echo hello world | od -c -
0000000   h   e   l   l   o       w   o   r   l   d  \n
0000014

This is what we would expect and desire: Just a \n and not \r\n in the output.

After you fix your script you should see the expected behavior:

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#!/bin/bash

EXPECTED_OUTPUT="hello world"
ACTUAL_OUTPUT=$(kubectl exec my-pod -- echo hello world)

echo "  Actual output: $ACTUAL_OUTPUT"
echo "Expected output: $EXPECTED_OUTPUT"

if [[ "$ACTUAL_OUTPUT" == "$EXPECTED_OUTPUT" ]]; then
    echo "Yes! $ACTUAL_OUTPUT matches $EXPECTED_OUTPUT"
else
    echo "No... $ACTUAL_OUTPUT does not match $EXPECTED_OUTPUT"
fi

Output:

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  Actual output: hello world
Expected output: hello world
Yes! hello world matches hello world

In summary, be mindful when you’re writing your kubectl exec commands, especially in a script! You should not be using -it in a script, or any other time that you don’t actually need an interactive session.