cross-platform-care
"Works on my machine" is an environment assumption you didn't notice making (environment-first is the cure; this is the prevention). Weak portability is a script tested once on the author's laptop; strong portability names its targets and checks against the one it isn't on.
Before writing: name the targets
State which platforms and shells the artifact must run on (e.g. "Linux CI with /bin/sh = dash, macOS zsh, Windows PowerShell 7"). Check what the repo already commits to: CI matrix, existing shebangs, .editorconfig, .gitattributes, engines / requires-python. If it targets one platform only, say so in a comment at the top and stop portability work there.
1. Paths
Build paths with the path library, never string concatenation with / or \: pathlib / os.path.join, path.join, filepath.Join. Same for home (~ doesn't expand everywhere), temp dirs (platform API or the session scratchpad), and current-dir assumptions (derive absolute paths from a known root such as the script's own location). On Windows also avoid reserved names (CON, NUL, aux) and trailing dots or spaces in filenames.
2. Shell dialect
Declare the shell you actually target. Bashisms ([[, arrays, set -o pipefail, local, $'...') fail in POSIX sh; CI images often ship dash as /bin/sh; zsh differs on word splitting and globs. Use #!/usr/bin/env bash if you need bash, #!/bin/sh only if it is genuinely POSIX. PowerShell is a different language, not a dialect — never mix its syntax with sh.
3. Quoting and arguments
Quote every expansion ("$var", "$@"); unquoted expansion breaks on the first path with a space, which is every macOS user directory. Put -- before positional args to guard filenames with leading dashes. Never build a command by pasting user-supplied strings into a shell line — pass argv arrays.
4. Case-sensitivity
macOS and Windows filesystems are usually case-insensitive; Linux is not. import utils finds Utils.py locally and dies in CI. Imports, includes, asset URLs, and case-only renames must match the file's real casing — verify with git ls-files, not the editor's view.
5. Line endings and encodings
CRLF from Windows checkouts breaks shebangs, heredocs, and string comparisons; declare *.sh text eol=lf in .gitattributes. Declare encoding explicitly when reading or writing text (encoding="utf-8"); never inherit the platform default. Choose newline handling explicitly on write.
6. Tools and versions
Don't assume a tool exists or matches: GNU vs BSD flags differ (sed -i '', date -d vs -v, grep -P, readlink -f, xargs -r); python may be v2, absent, or a Store alias on Windows. Probe with command -v <tool> and fail with a message naming what's missing; version-check anything whose behavior you depend on; prefer the portable subset when it's cheap.
7. Time
Servers run UTC; laptops don't. Code that formats, parses, or compares timestamps names its zone explicitly; tests that depend on "today" pin the clock (see edge-case-sweep).
Exit check
Run a check under the target you're not on before shipping: shellcheck or sh -n / bash -n for a Linux target from macOS or Windows, WSL or a container for a full run, pwsh -NoProfile -File for PowerShell. Then answer: "would this run on a fresh Linux CI container and on a laptop with a space in the username?" Both, or the header says which one it targets.
