Not every task requires a graphical user interface. Often it is quicker and more efficient to simply type a command. Command line tools allow you to perform powerful operations with just a few keystrokes and make it possible to automate repetitive tasks. Below is a selection of tools that I find particularly useful. All of them are free and open-source.

Bash

Here are some basic commands for the linux shell:

cd ~/         (change directory)
pwd           (print working directory)
ls            (list files and directories)
tree          (display content in tree like format)
mkdir name    (create new directory)
rm -r name    (delete file or directory)
mv old new    (move file or directory)
cp old new    (copy file or directory)
top           (display system information)
  -d seconds  (refresh rate)
  u name      (only for user name)
du -sh        (display disk usage)
uptime -p     (how long is the system running)
clear         (clear terminal)
ssh           (remote login)
scp           (copy file to other computer)
command &     (runs command in background)
ctrl+z        (pause running job)
bg            (continue running in background)
fg            (continue running in terminal)
jobs          (lists running jobs)

ImageMagick

ImageMagick can convert and resize images.

magick input_image.jpg -resize 300x output_image.webp

we can convert a SVG to a png

magick -background none -size 64x64 input.svg output.png

or combine multiple png files to create a .ico file

magick icon_small.png icon_medium.png icon_large.png icon.ico

ExifTool

The metadata is ExifTool can be used to modify the metadata of an image.

exiftool image.jpg

But we are more interested to remove it

exiftool -all:all= -r -overwrite_original image.jpg

FFmpeg

FFmpeg can be used to convert audio and video files.

ffmpeg -i input.mp3 -vn -acodec copy -ss 00:00:10 -to 00:00:30 output.mp3

wget

This will download the entire website (including css files).

wget -mkE www.example.com
wget --user= --ask-password www.example.com