Deterministic Command Choices

Prefer commands whose output does not depend on locale, globbing or shell state

Use when: an agent constructs commands that a shell will interpret before the tool sees them

Avoid when: the command is a fixed literal with no interpolation

Procedure

1. Identify every part of the command that the shell expands
2. Replace globs with explicit file lists where the set matters
3. Quote every path and value that could contain a space
4. Avoid locale-sensitive sorting by specifying the collation or sorting explicitly
5. Pin time formats to a fixed zone and format
6. Re-run the command and confirm identical output

Signals

• two runs produce byte-identical output
• no path in the command is unquoted
• the file set passed to the tool is explicit
• timestamps carry a zone and a fixed format

Patterns

1. Explicit list

cmd a.txt b.txt instead of cmd *.txt

2. Quoted path

"results/my file.json"

3. Fixed time

--date 2026-10-29T00:00:00Z

Pitfalls

• Relying on glob expansion order which differs by filesystem and locale
• Leaving a date unformatted so the output changes every run
• Passing an unquoted path that breaks silently on a space

Done when

• the command produces identical output on a rerun
• no shell expansion affects which files or values are used

References

• POSIX shell quoting
• Python - os.environ