Your Permanent Toolkit

It's next month. You're sitting in ~/finances/ with a fresh bank statement. You know you built a script that handles CSV parsing perfectly — it dealt with quoted fields, commas inside merchant names, the works. But where did you put it? Was it in ~/projects/chapter8/? Or ~/Desktop/scripts/? You try python3 sum-expenses.py and get No such file or directory. The script exists somewhere on your machine, and you can't find it.

If you have to remember where a tool lives, it's not a tool yet.

This isn't hypothetical. People build useful scripts, forget to make them permanent, and six months later rebuild the same thing from scratch because they can't find the original. The script worked. The workflow worked. But it was never installed, so it rotted in a forgotten folder. Twenty minutes of setup now saves hours of "I know I built this already" frustration later.

Real tools are available everywhere, instantly, by name. You type ls from any folder, not /usr/bin/ls. Your scripts deserve the same treatment. This lesson makes that happen.

The Problem

Here's what that frustration looks like in practice:

cd ~/finances
cat january-statement.csv | python3 sum-expenses.py

Output:

python3: can't open file 'sum-expenses.py': No such file or directory

You'd need the full path:

cat january-statement.csv | python3 ~/projects/chapter8/sum-expenses.py

That works, but it's fragile. You have to remember the exact path. If you move the script, every command breaks. This is the gap between "a script that works" and "a tool you actually use."

The Conversation

Ask Claude Code to solve this permanently:

You: I want to use sum-expenses from anywhere on my system.
Make it a permanent command I can run like: cat file.csv | sum-expenses

Claude Code: I'll set that up. First, let me create a dedicated
tools directory and move your script there.

[Runs: mkdir -p ~/tools]
[Runs: cp sum-expenses.py ~/tools/sum-expenses.py]
[Runs: chmod +x ~/tools/sum-expenses.py]

Now I need to check which shell you're using.

[Runs: echo $SHELL]
/bin/zsh

You're using zsh, so I'll add an alias to your .zshrc.

[Appends to ~/.zshrc:]
alias sum-expenses='python3 ~/tools/sum-expenses.py'

[Runs: source ~/.zshrc]

Done! Try it from any directory:
cat any_file.csv | sum-expenses

Notice what the agent did: it didn't just create the alias. It organized your script into ~/tools, made it executable, checked your shell, and reloaded the config. You described the outcome you wanted. The agent handled every step to get there.

And notice where you refined the interaction: you specified the exact command format you wanted (cat file.csv | sum-expenses). That specificity told the agent you wanted a pipe-compatible alias, not just "make it accessible." Your clear intent shaped the solution.

The Agent's Toolkit

Each command the agent ran serves a specific purpose:

Command	What It Does	Memory Trick
mkdir -p ~/tools	Creates your personal tools folder	p = create parents too
chmod +x script.py	Makes file executable	change mode + execute
alias name='command'	Creates a shortcut	Like a nickname for a command
source ~/.zshrc	Reloads shell config	Load the source of settings

The Shebang Line

Your script starts with this line:

#!/usr/bin/env python3

This is called a shebang (the #! characters). It tells your operating system: "When someone runs this file directly, use python3 to execute it."

Without the shebang, your OS wouldn't know this is a Python script — it would try to run it as raw shell commands and fail. The env part finds python3 wherever it's installed on your system, making the script portable across different setups.

Shell Config Files

Every time you open a terminal, your shell reads a config file and runs whatever's inside it. Which file depends on your shell:

echo $SHELL

Output (one of these):

/bin/zsh     → Edit ~/.zshrc
/bin/bash    → Edit ~/.bashrc

When the agent added alias sum-expenses='python3 ~/tools/sum-expenses.py' to your .zshrc, it put the alias where your shell will find it every single time you open a terminal. That's what makes it permanent — not magic, just a config file that runs on startup.

source ~/.zshrc reloads that file in your current terminal so you don't have to close and reopen it.

The Pattern

Whenever you build a script worth keeping:

"I want to use [script] from anywhere. Make it a permanent command."

The agent will follow the same steps: organize, make executable, alias, reload. This pattern works for any script in any language.

Checkpoint: Prove It's Permanent

This is the moment where a script becomes a tool. Don't skip it.

1. Close your terminal completely
2. Open a brand new terminal
3. Navigate to your home directory: cd ~
4. Type: sum-expenses
5. If you see usage info or an error about missing input — your command is installed
6. If you see "command not found" — go back and check your alias

The new terminal has no memory of what you did before. It only knows what's in your shell config file. If sum-expenses works here, it works everywhere, forever.

Your tools are installed, permanent, and available from any directory on your system. You can sum any CSV with a single pipe command. But summing is all they do — they can't tell the difference between a pharmacy visit and a grocery run. Your accountant doesn't want "Total: $4,215.52." They want medical, charitable, and business broken out separately. And buried in your bank data, Dr. Pepper is waiting to be misclassified as a doctor visit.

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Try With AI

Prompt 1: Batch Install Multiple Scripts

I have 3 scripts I use regularly: sum.py, sum-expenses.py, and a
count-lines.py I wrote earlier. Help me set up ~/tools with all of
them and create aliases for each one. Show me the final state of
my .zshrc aliases section.

What you're learning: Batch tool installation. Instead of repeating the process three times, you describe the full scope and the agent handles the repetitive setup for multiple scripts at once. This is the "300 files vs 3 files" principle from Chapter 7 — when the task is repetitive, hand it to the agent.

Prompt 2: PATH vs Aliases

Instead of aliases, can I add ~/tools to my PATH so I can run
scripts directly by name without an alias? What are the pros and
cons of PATH vs aliases?

What you're learning: Advanced tool installation. PATH modification is how professional developers make tools available system-wide. The agent explains when aliases are sufficient and when modifying PATH is the better approach — and helps you understand the tradeoff between simplicity and flexibility.