Learn the scripting language of your particular environment. In Linux or Mac this is most likely bash. Windows has a few different scripting languages, of which PowerShell is king. Unfortunately, PowerShell depends heavily on .NET, so if you don’t already know .NET you won’t be able to pick up PowerShell in a day. Visual Basic for Applications is another powerful scripting language. It’s bundled with Microsoft Office, and as its name implies it’s intended to work with applications. I’ve used it for system-level scripting, and I learned the basics in a day. Back in the 1990s I used to write “batch files” for DOS/Windows. I’m not sure if those still work in modern versions of Windows, and I’m fairly sure that the API hooks are lacking. I’d recommend learning VBA for Windows; it’s garbage but it will get you there.
If you’ve ever needed to rename 10,000 files according to a pattern or do any sort of repetitive computing work, then you need to learn how to script! Python is great, and IMO better for scripting than any of the native scripting languages I’ve used, but it’s not available on every system. If you find yourself working on a computer where you are not allowed to install software, then it will behoove you to know something of the local scripting language.
If you already know how to script, then learn regular expressions. Regex is a way of searching for specific patterns. Any programmer that doesn’t know regex is effectively crippled. Regular expressions take only a few hours to learn, but a lifetime to master.