less -R versus less -r for preserving ANSI color in piped output

The Recommendation

For the vast majority of users and environments, -R should be the default setting for the LESS environment variable. It provides the necessary balance between preserving color and protecting the terminal emulator from corruption.

Functional Differences

The core difference lies in how less handles non-printable control characters before passing them to your terminal:

• -R (Raw Control Characters): Only passes through valid ANSI SGR (Select Graphic Rendition) escape sequences. It strips other control characters, such as those used for changing window titles or triggering system bells.
• -r (Raw): Passes every single byte from the input directly to the terminal. This includes SGR codes, OSC (Operating System Command) sequences, private modes, and arbitrary binary data.

Risk Assessment

The risk of terminal state change outweighs the benefit of raw pass-through whenever the input source is untrusted or complex. Using -r can lead to several issues if the piped output contains non-SGR sequences:

• Persistent State: An unclosed color sequence or a private mode command can leave your terminal text permanently colored or invisible even after less exits.
• OSC Interference: Sequences that change the terminal window title or modify the scroll region can be triggered unexpectedly.
• Binary Corruption: If a binary file is accidentally piped to less -r, the terminal may interpret random bytes as control codes, potentially changing the keyboard mapping or character set.

Comparison Summary

Feature	less -R	less -r
ANSI Colors (SGR)	Preserved	Preserved
Window Titles (OSC)	Stripped	Preserved
Binary/Control Bytes	Stripped	Preserved
Terminal Safety	High	Low

Implementation and Verification

To set -R as your default, add the following to your shell profile (e.g., .bashrc or .zshrc):

export LESS='-R'

To verify the behavior, you can test how each flag handles a mix of SGR (color) and OSC (title) sequences:

# Create a test string with Green text and a Title change request
printf "\e[32mGreen Text\e[0m \e]0;New Title\a"

# Test -R: Should show green text, but NOT change terminal title
printf "\e[32mGreen Text\e[0m \e]0;New Title\a" | less -R

# Test -r: Should show green text AND change terminal title
printf "\e[32mGreen Text\e[0m \e]0;New Title\a" | less -r

Diagnostic Note: Does your specific workflow require the preservation of terminal window titles or custom Xterm private modes? If so, -r is necessary, but should be used as a one-off flag rather than a global environment setting.