LESSOPENbased

LESSOPENbased refers to a design pattern in Unix-like systems where the pager less uses the LESSOPEN environment variable to preprocess files before display. In this model, a dedicated preprocessor chain, often implemented by the lesspipe script, converts or formats a file into plain text that can be rendered by less. The approach centers on automatic handling of many file types without manual preprocessing.

When less is invoked, it consults the LESSOPEN variable, which typically contains a command that reads a filename and outputs text. The common value is a pipeline through a lesspipe script, for example export LESSOPEN='| /usr/bin/lesspipe %s'. The script detects the file type, decompresses archives, expands markup languages, or extracts readable content from binary formats, and writes the result to standard output for less to display. The LESSCLOSE variable can also specify a command to run when a file is closed, but its usage is less common.

Users install or write a lesspipe-like script and configure their shell startup files to export LESSOPEN accordingly. Custom handlers can be added to support new formats (for example Markdown, reStructuredText, or custom logs). The approach ensures a consistent viewing experience by automatically presenting various content types in plain text form.

The LESSOPENbased approach provides immediate support for diverse formats and reduces manual preprocessing. It is extensible by adding new handlers and can improve readability of compressed or embedded content. However, it relies on external scripts whose quality and security affect system safety; misconfigurations can reveal sensitive data or degrade performance. Compatibility may vary across distributions and versions of less and shell environments.

See also: Less, lesspipe, LESSCLOSE, file command, shell environment variables.