GNU Readline is a software library that adds line editing to interactive command-line programs. The project’s own description states that the Readline library provides a set of functions for use by applications that allow users to edit command lines as they are typed in. Rather than each program reinventing how the user fixes a typo, recalls a previous command, or moves the cursor mid-line, a program links Readline and inherits a complete, consistent editing experience at its prompt.
The most visible part of Readline is its keybindings. The library supports both Emacs and vi editing modes, so the same finger habits a user knows from a text editor work at the shell prompt: moving by word, killing to end of line, transposing characters. Emacs-style bindings are the default, which is a direct reason the GNU command line and Emacs feel like one environment. A user can switch to vi mode and get modal editing instead, matching the other great editor tradition.
Readline’s second pillar is history. It ships with a companion History library that maintains a record of previously entered command lines and lets the user recall and re-edit them, plus csh-like history expansion for manipulating earlier commands. The History library is distributed with Readline but can be used on its own by applications that want recall without full line editing. Together, editing and history turn a bare prompt into something a person can work in comfortably.
The reason Readline matters out of proportion to its size is reach. It is the line-editing layer beneath the bash shell, and it is linked into a long list of other interactive programs, language interpreters, database clients, and REPLs, so a single library defines how editing feels across much of the command line. Because it is free software under the GNU General Public License, any program using it must also be free, which both spread Readline widely and shaped the licensing of the tools that adopted it.
Readline is a quiet piece of infrastructure: most users never name it, yet they rely on it every time they press the up arrow or jump to the start of a line in a terminal. By packaging editor-grade editing into a reusable library, it brought the discipline of text editors to the humble command prompt and made that experience uniform across the free-software toolchain.