Comparing Version strings in zsh

Another excerpt from the book “Moving to zsh.” I found this one so useful, I thought I’d like to share it.

You can get the version of zsh with the ZSH_VERSION variable:

% echo $ZSH_VERSION
5.7.1

And you can get the version of macOS with the sw_vers command:

% sw_vers -productVersion
10.15.1

Comparing version strings is usually fraught with potential errors. Strings are compared by the character code for each character.‘2’ is alphabetically greater than ‘10’ when compared as strings, because the character code for 2 is greater than the character code for 1. So, a string comparison of macOS version numbers will return that 10.9.5 is greater than 10.15.1.

Zsh, however, provides a function is-at-least which helps with version string comparisons.

With a single argument, is-at-least will return if the current zsh version matches or is higher than a given number:

if ! is-at-least 2.6-17; then
  echo "is-at-least is not available"
fi

When you provide two arguments to is-at-least, then the second argument is compared (using version string rules) with the first and needs to match or be higher:

autoload is-at-least
if is-at-least 10.9 $(sw_vers -productVersion); then
  echo "can run Catalina installer"
else
  echo "cannot run Catalina installer"
fi

Note: when used in a script, you will probably have to autoload is-at-least before using it. In an interactive shell, it is often already loaded, because many other autoloader functions will have already loaded it.