The echo command is commonly used to print user-specified strings to standard output. Sometimes, we may want to customize the color of the output to highlight important messages. For example, the following command prints "test passed" in green:

$ echo -e "\e[32mtest passed\e[0m"

Similarly, this command prints "test failed" in red:

$ echo -e "\e[31mtest failed\e[0m"

How Does This Work?

Let's break it down. The -e option in echo enables interpretation of backslash escapes. This means that \e in the string is recognized as the escape character (ESC) from the ASCII table.

Beyond the main message text ("test passed" and "test failed"), there are two additional sequences:

  • \e[32m (or \e[31m)
  • \e[0m

These sequences do not appear in the echo man page because they are terminal control sequences, defined in man console_codes. Since this reference is extensive, we will focus on explaining only the sequences used in our example.

Understand Terminal Control Sequences

According to man console_codes, the sequence ESC [ (represented as \e[ in our command) begins a Control Sequence Introducer (CSI). The final character in the sequence determines its function. In our case, it is "m", which tells the terminal to modify text attributes, such as color.

The numbers between ESC [ and "m" specify the attribute:

  • 31 → Sets the foreground color (i.e. font color) to red
  • 32 → Sets the foreground color to green
  • 0 → Resets all text attributes to the default

The final \e[0m is crucial because it ensures that only the intended line is colored. Without it, the color setting would persist across subsequent outputs.

Try experimenting with different color codes and attributes to enhance your terminal output. You can find more options in man console_codes.