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In this lesson, you’ll learn how to perform a “dry run” in your Bash or Unix shell scripts using the built-in no-op (:) command. A dry run lets you verify script logic and flow without modifying files or data—perfect for testing complex workflows before production.
The image shows a computer monitor with a network or blockchain icon on the left and a series of colored lines resembling code on the right, with the text "No op command" at the top.

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What Is a Dry Run?

The term “dry run” dates back to fire departments: they practice hose deployment without water, hence “dry.” In software, many tools offer a dry-run or no-op mode to preview changes safely.
The image shows two icons: a firefighter with a flame and a light bulb with a gear labeled "Wet run," under the title "No op command."

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Common Dry-Run Flags in DevOps Tools

ToolCommand ExampleDry-Run Flag
Ansibleansible-playbook -i inventory playbook.yml --check--check
Kuberneteskubectl apply -f deployment.yaml --dry-run=client -o yaml--dry-run=client
Puppetpuppet apply --noop my_manifest.pp--noop
Some tools distinguish client-side vs server-side dry runs. Always check the official docs for supported modes and output formats.

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Placeholder for Empty Branches: the : Command

Leaving an if or loop branch empty causes a Bash syntax error: 
#!/usr/bin/env bash

if [[ "$1" = "start" ]]; then
else
  echo "Invalid command."
fi

$ ./script.sh
script.sh: line 3: syntax error near unexpected token `else'
To satisfy Bash’s syntax without side effects, insert the no-op : command: 
#!/usr/bin/env bash

if [[ "$1" = "start" ]]; then
  :
else
  echo "Invalid command."
fi
The image describes a "No-op command" as a placeholder shell-built command with no programmed behavior, and it includes references to looping and if-else statements.
Because : is a shell builtin, it runs faster and cleaner than alternatives like echo "" or true.

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Interpreter Errors vs. Runtime Errors

Interpreter errors occur at parse time—even if that code path never runs: 
#!/usr/bin/env bash

if [[ "$1" = "start" ]]; then
  # empty block → interpreter error
else
  echo "Invalid command."
fi
Runtime errors only appear when execution reaches problematic code. For example, calling a non-existent command x: 
#!/usr/bin/env bash

if [[ "$1" = "start" ]]; then
  x
else
  echo "Invalid command."
fi
  • No arguments: 
    $ ./script2.sh
Invalid command.
  • With start: 
    $ ./script2.sh start
./script2.sh: line 4: x: command not found
Replacing x with : eliminates any error or output and exits cleanly: 
#!/usr/bin/env bash

if [[ "$1" = "start" ]]; then
  :
else
  echo "Invalid command."
fi

$ ./script2.sh start
# no output, exit code 0
Don’t confuse : with the external true command—: is built into the shell and more efficient.

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References and Further Reading

For more on advanced Bash scripting patterns and best practices, visit the Advanced Bash-Scripting Guide.

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