Detecting the operating system from a script: uname, os-release, sw_vers, oslevel and a fallback order

Goal

Determine, from inside a script, which operating system and (where relevant) which distribution and version it is running on, reliably enough to select the correct command from the comparisons in this series.

Prerequisites

A POSIX shell (sh/bash) for Unix-like targets, or PowerShell for Windows; no assumptions about which utilities beyond the shell itself are pre-installed.

Steps

1. Start with uname -s in any POSIX shell. It distinguishes the kernel family cheaply and is present on every Unix-like system: Linux, Darwin (macOS), FreeBSD, AIX, and so on. A POSIX shell on Windows reports e.g. MINGW64_NT-... (Git Bash) or CYGWIN_NT-..., while WSL reports Linux.
2. If uname -s reports Linux, read /etc/os-release (or /usr/lib/os-release as a fallback if the former is absent) and use the ID= and VERSION_ID= fields to get the distribution and version without parsing free-text banners. This file is the standard, machine-readable identification source on modern Linux distributions.
3. If uname -s reports Darwin, run sw_vers -productVersion for the macOS version number; sw_vers -productName confirms it is macOS rather than another Darwin-based system.
4. If uname -s reports FreeBSD, run freebsd-version for the userland version, and separately uname -r for the running kernel version — the two can differ right after an upgrade until a reboot.
5. If uname -s reports AIX, run oslevel -s for the current maintenance/technology level string.
6. In a context where the shell itself might be PowerShell rather than POSIX (a cross-platform automation tool, PowerShell 7+ on Linux/macOS), check $IsWindows, $IsLinux, $IsMacOS first — these automatic variables exist in PowerShell 6 and later — and use $PSVersionTable.OS for detail. Windows PowerShell 5.1 has none of them: an undefined variable there evaluates to $null (or throws under Set-StrictMode), so test $PSVersionTable.PSEdition -eq 'Desktop' instead, which identifies Windows PowerShell 5.1 — a Windows-only host.
7. If every structured source above is unavailable (a minimal or unusual environment), fall back to parsing uname -a's free-text output as a last resort, and treat the result as low-confidence.

Expected result

A script obtains an OS family and, where applicable, a distribution/version string using the most specific structured source available for that family, with a documented fallback path rather than a single brittle check.

Limits and test basis

/etc/os-release is a convention documented and widely adopted across Linux distributions, not a kernel-enforced guarantee; a nonstandard or minimal image can still omit it, which is why step 7 exists. Detection tells you what the OS is, not whether it is still supported — pair this with the lifecycle check in this series before automating a fleet-wide action. No performance or timing claim is made about any of these commands; they are treated as correctness-only checks.