Executing multi-line statements in the one-line command-line

Mastering Multi-Line Statements in Your Command Line

Learn how to execute complex, multi-line commands and scripts directly from your shell's one-line prompt, enhancing productivity and scripting capabilities.

Working in the command line often involves executing simple, single-line commands. However, many tasks, especially in scripting languages like Python or complex shell operations, require multiple lines of code. This article explores various techniques to input and execute multi-line statements directly within your shell's one-line prompt, saving you from creating temporary script files for every small task.

Understanding Shell Line Continuation

Most modern shells (Bash, Zsh, PowerShell) offer mechanisms to indicate that a command continues on the next line. The most common method is using the backslash (\) character at the end of a line. When the shell encounters a backslash, it interprets the newline character immediately following it as a space or continuation, allowing you to type the rest of your command on the next line without executing the current one.

echo "This is a very long command that I want to break up \ 
> into multiple lines for readability and organization." \ 
> "It will still execute as a single command."

Using backslash for shell line continuation

Executing Multi-Line Python Code

Python is a prime example where multi-line statements are common. While you can always save a .py file and run it, for quick tests or one-off executions, entering multi-line Python directly into the shell is incredibly useful. This can be achieved using the python -c command or by leveraging the shell's ability to pass a string to standard input.

flowchart TD
    A[Start] --> B{"Need Multi-Line Python?"}
    B -->|Yes| C{Choose Method}
    C --> D["python -c 'code; more code'"]
    C --> E["python <<< $'code\nmore code'"]
    D --> F[Execute]
    E --> F[Execute]
    F --> G[End]
    B -->|No| G

Decision flow for executing multi-line Python

Using python -c

python -c " import os print(os.getcwd()) for i in range(3): print(f'Iteration {i}') "

Using heredoc (Bash/Zsh)

python <<EOF import sys print(f'Python version: {sys.version.split()[0]}') EOF

Using echo and pipe

echo "import math\nprint(math.pi)" | python

Advanced Shell Scripting with Multi-Line Commands

Beyond simple continuations, shells offer more robust ways to handle multi-line input, especially for complex scripts or functions. Heredocs (<<EOF) are particularly powerful for embedding blocks of text or code directly into a script or command, which can then be piped to another program or executed by the shell itself.

cat << 'EOT'
This is a multi-line
text block that will be
printed exactly as is.
Variables like $USER will not be expanded here.
EOT

Using a 'quoted' heredoc to prevent variable expansion

1. Start a multi-line command

For shell commands, end the first line with a backslash (\). For Python, use python -c "..." or a heredoc.

2. Continue on subsequent lines

The shell will typically provide a secondary prompt (e.g., > or ...) indicating it's awaiting more input. Type your additional lines of code or command.

3. Terminate the multi-line input

For backslash continuations, simply press Enter on the last line without a backslash. For python -c, close the string with a double quote. For heredocs, type the delimiter (e.g., EOF) on a new line by itself.

4. Execute the command

Once terminated correctly, the shell will execute the entire multi-line input as a single command or script.