What's GNU: Bash - The GNU Shell

While originally written by Brian Fox of the Free Software Foundation, bash is now maintained by Chet Ramey. In this article, Chet explains the nhistory of shells and then goes on to explore features specific to bash.
Editing and Completion

One area in which Bash shines is command line editing. Bash uses the readline library to read and edit lines when interactive. Readline is a powerful and flexible input facility that a user can configure to their own tastes. It allows lines to be edited using either emacs or vi commands, where those commands are appropriate. The full capability of emacs is not present-there is no way to execute a named command with M-x, for instance-but the existing commands are more than adequate. The vi mode is compliant with the command line editing standardized by POSIX.2.

Readline is fully customizable. In addition to the basic commands and key bindings, the library allows users to define additional key bindings using a startup file. The inputrc file, which defaults to the file ~/.inputrc , is read each time readline initializes, permitting users to maintain a consistent interface across a set of programs. Readline includes an extensible interface, so each program using the library can add its own bindable commands and program-specific key bindings. Bash uses this facility to add bindings that perform history expansion or shell word expansions on the current input line.

Readline interprets a number of variables which further tune its behavior. Variables exist to control whether or not eight-bit characters are directly read as input or converted to meta- prefixed key sequences (a meta-prefixed key sequence consists of the character with the eighth bit zeroed, preceded by the meta-prefix character, usually escape, which selects an alternate keymap), to decide whether to output characters with the eighth bit set directly or as a meta-prefixed key sequence, whether or not to wrap to a new screen line when a line being edited is longer than the screen width, the keymap to which subsequent key bindings should apply, or even what happens when readline wants to ring the terminal's bell. All of these variables can be set in the inputrc file.

The startup file understands a set of C preprocessor-like conditional constructs which allow variables or key bindings to be assigned based on the application using readline, the terminal currently being used, or the editing mode. Users can add program-specific bindings to make their lives easier: I have bindings that let me edit the value of $PATH and double-quote the current or previous word:

# Macros that are convenient for shell interaction $if Bash # edit the
path "\C-xp": "PATH=${PATH}\\C-e\C-a\f\C-f" # prepare to type a quoted
word-insert open and close double quotes # and move to just after the
open quote "\C-x\"": "\"\"\C-b" # Quote the current or previous word
"\C-xq": "\b\"\f\"" $endif

There is a readline command to re-read the file, so users can edit the file, change some bindings, and begin to use them almost immediately.

Bash implements the bind builtin for more dyamic control of readline than the startup file permits. Bind is used in several ways. In list mode, it can display the current key bindings, list all the readline editing directives available for binding, list which keys invoke a given directive, or output the current set of key bindings in a format that can be incorporated directly into an inputrc file. In batch mode, it reads a series of key bindings directly from a file and passes them to readline. In its most common usage, bind takes a single string and passes it directly to readline, which interprets the line as if it had just been read from the inputrc file. Both key bindings and variable assignments can appear in the string given to bind.

The readline library also provides an interface for word completion. When the completion character (usually TAB) is typed, readline looks at the word currently being entered and computes the set of filenames of which the current word is a valid prefix. If there is only one possible completion, the rest of the characters are inserted directly, otherwise the common prefix of the set of filenames is added to the current word. A second TAB character entered immediately after a non-unique completion causes readline to list the possible completions; there is an option to have the list displayed immediately. Readline provides hooks so that applications can provide specific types of completion before the default filename completion is attempted. This is quite flexible, though it is not completely user-programmable. Bash, for example, can complete filenames, command names (including aliases, builtins, shell reserved words, shell functions, and executables found in the file system), shell variables, usernames, and hostnames. It uses a set of heuristics that, while not perfect, is generally quite good at determining what type of completion to attempt.

This article will be continued next month.


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