shellsafe

Run shell commands safely using Python 3.14 template strings. Values can never turn into commands.

from shellsafe import run

message = get_user_input()          # "fix; rm -rf ~"
run(t"git commit -m {message}")
# argv: ["git", "commit", "-m", "fix; rm -rf ~"]
# one command; the scary text is just an argument

Why this package exists

Python 3.14 added template strings (PEP 750). Now Python keeps your fixed text and your values separate at the language level.

Shell commands are the first place people want to use this. That is because f-strings inside shell commands have caused real security bugs for ten years:

subprocess.run(f"git commit -m {message}", shell=True)
# if message = "fix; rm -rf ~"  ->  two commands run. The second one is bad.

Python planned to solve this officially (PEP 787), but that plan was postponed. So today there is no standard way to run shell commands safely with templates. This package fills that gap.

Install

pip install shellsafe

Needs Python 3.14 or newer.

How to use

Run a command. Your values always stay one argument each:

from shellsafe import run

run(t"mkdir {path}")
run(t"docker build -t {tag} .", check=True, timeout=300)

Get the output as text:

from shellsafe import capture

res = capture(t"grep {pattern} {file}")
print(res.stdout, res.returncode)

Need pipes? Works on Linux and macOS. Your values are quoted safely first:

from shellsafe import shx

shx(t"cat {file} | wc -l")

Want to see exactly what will run?

from shellsafe import plan

print(plan(t"git commit -m {message}"))
# argv: ["git","commit","-m","fix; rm -rf ~"]

Safety rules

Case	What happens
Any value you pass	becomes one argument, exactly as given
Value used as the program name	error: program names must be written as fixed text
Shell features on Windows	error: we cannot make Windows safe this way, so we say no
RAW() misuse	error: one value only, used as-is, no nesting

RAW(...) marks content you have already made safe by hand. It is loud and easy to find in code review, so trust is never hidden.

Find old dangerous patterns

Already have code using f-strings in shell commands? The scanner finds them:

shellsafe audit src/

It detects f-strings passed to os.system, subprocess.run(shell=True), and other shell executors. Get machine-readable output:

shellsafe audit src/ --json

Filter by severity:

shellsafe audit src/ --severity warning

Limits

Shell features (pipes, redirections) work on Linux and macOS only. Windows supports plain commands only.
run() refuses templates that contain shell metacharacters. Use shx() for those.
Byte values are rejected. Decode them first.
We keep your command safe to build and run. Testing what your command does is still your job.

Needs

Python 3.14 or newer
Linux, macOS, Windows (pipes work on Linux and macOS only)

Source and issues: github.com/rahulXs/shellsafe
Want to help? See CONTRIBUTING.md in the repository.

License: MIT

Development Status
- 3 - Alpha
Intended Audience
- Developers
License
- OSI Approved :: MIT License
Programming Language
- Python :: 3
- Python :: 3.14
Topic
- Security
- Software Development :: Libraries
Typing
- Typed

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