Original author(s) Written in C | Developer(s) Chet Ramey | |
Initial release 1989; 28 years ago (1989) Stable release 7.0 / September 15, 2016; 5 months ago (2016-09-15) Repository git.savannah.gnu.org/cgit/readline.git?h=devel |
GNU Readline is a software library that provides line-editing and history capabilities for interactive programs with a command-line interface, such as Bash. It is currently maintained by Chet Ramey as part of the GNU Project.
Contents
- Editing modes
- Emacs keyboard shortcuts
- Choice of the GPL as GNU Readlines license
- Implications of GNU Readlines GPL license
- Sample code
- References
It allows users to move the text cursor, search the command history, control a kill ring (a more flexible version of a copy/paste clipboard) and use tab completion on a text terminal. As a cross-platform library, readline allows applications on various systems to exhibit identical line-editing behavior.
Editing modes
Readline supports both Emacs and vi editing modes, which determine how keyboard input is interpreted as editor commands. See Editor war#Differences between vi and Emacs.
Emacs keyboard shortcuts
Emacs editing mode key bindings are taken from the text editor Emacs.
On some systems, Esc must be used instead of Alt, because the Alt shortcut conflicts with another shortcut. For example, pressing Alt+f in Xfce's terminal emulator window does not move the cursor forward one word, but activates "File" in the menu of the terminal window, unless that is disabled in the emulator's settings.
exit
). (Only if there is no text on the current line)clear
).bg
. To bring it back from background or suspension fg ['process name or job id']
(foreground) can be issued.Choice of the GPL as GNU Readline's license
GNU Readline is notable for being a free software library which is licensed under the GNU General Public License version 3 (GPLv3) instead of the GNU Lesser General Public License (LGPL). Free software libraries are often licensed under the LGPL, for example, the GNU C Library, GNU gettext and FLTK.
A developer of an application who chooses to link to an LGPL licensed library when building a new application is required to have the LGPL licensed library which it uses remain under the LGPL when distributing the combined resulting application. The part of the combined application excluding the LGPL licensed library can remain under the original license. This is in contrast to a developer choosing to use a GPL licensed library to create a new application, in which case the entire combined resulting application is required to be licensed under the GPL when distributed, to comply with section 5 of the GPL.
Implications of GNU Readline's GPL license
An important example of an application changing its licensing to comply with the copyleft conditions of GNU Readline is CLISP, an implementation of Common Lisp. Originally released in 1987, it changed to the GPL license in 1992, after an email exchange between one of CLISP's original authors, Bruno Haible, and Richard Stallman, in which Stallman argued that the linking of readline in CLISP meant that Haible was required to re-license CLISP under the GPL if he wished to distribute the implementation of CLISP which used readline.
Alternative command line editing libraries which are permissively licensed can be used by software projects which want to implement command line editing functionality, but wish to remain under a permissive license. For example, the Glasgow Haskell Compiler uses Haskeline (which is licensed under the 3 clause BSD license).
Sample code
The following code is in C and must be linked against the readline library by passing a -lreadline flag to the compiler: