Bash. It has enabled me to automatize a lot of sh*t I wouldn't do just because it would take me a lot of time/efford to do. There's a LOT you can do with a few simple scripts, a few examples:
- Remove files I don't need (images, rip/conversion logs, empty folders...).
- Compress and optimize folders of photos recursively.
- Apply watermarks to photos recursively.
- Convert filetypes (flac2mp3, pdf2cbr, webm2mp3,web2jpg...).
- Configure input devices (keyboards, mouse, graphic tablets).
- Autorename files (spaces to _, . to _, _ to spaces...).
- Remove audio from videos recursively.
- Remove audio/subtitle tracks from videos.
- Download images/videos/audio from websites.
- Update appimages automatically.
- Update/cleanup system/repos.
- Use different theming for different applications.
- Mount iso/bin... images.
- Extract zip|rar|tar.gz|.... files automatically.
- Modify pdfs.
- Get the weather for my location.
- Get stats from my PCs to be shown in panel applet.
- Alias program names to ones of my choosing.
- Open CUPS config in firefox.
- Refresh font caches and other management tools I don't care to remember.
- Fix permissions.
- Make a backup.
- Restore a backup.
- Copy files safely (rsync).
- Change volume level.
- Install all the packages and configs I need to make the OS/apps behave/look the way I like.
...you name it...
...in most cases just by typing one word in the terminal. It was kind of a mindblow coming from the crappy window$ crappysystem eons ago.