Bash edits the command line through the GNU Readline library. What does pressing Ctrl-R at a bash prompt do, and where do you make a change to bash's keybindings or editing mode permanent?

## What readline is Readline is a separate GNU library that bash links against to provide the editable command line: cursor movement, killing and yanking, history recall, and completion. It is not bash-specific — `gdb`, `psql` and many other tools use it too, which is why one `~/.inputrc` changes all of them at once. ## The search keys `Ctrl-R` invokes `reverse-search-history`. The prompt is replaced by `(reverse-i-search)` and the search is *incremental*: each keystroke re-searches the in-memory history list for the most recent line containing what you have typed so far. - `Ctrl-R` again — jump to the next older match. - Enter — execute the shown line. - Right arrow, `Ctrl-A`, or any cursor-movement key — accept it into the editing buffer without running it, so you can modify it first. This is the safe habit: never blind-Enter on a half-recognised destructive command. - `Ctrl-G` — abort and restore whatever you were typing before the search. `Ctrl-S` is the forward counterpart (`forward-search-history`), but in most terminals `Ctrl-S` is swallowed by the terminal driver's XON/XOFF flow control and appears to freeze the terminal (`Ctrl-Q` unfreezes it). `stty -ixon` in `~/.bashrc` frees the key. ## The default editing keys Readline starts in emacs mode. The set worth internalising is small: ``` Ctrl-A / Ctrl-E beginning / end of line Alt-B / Alt-F back / forward one word Ctrl-W kill the word before the cursor Ctrl-U kill from cursor to start of line Ctrl-K kill from cursor to end of line Ctrl-Y yank back the last killed text Ctrl-L clear the screen, keeping the current line Alt-. insert the last argument of the previous command ``` `Alt-.` in particular saves an enormous amount of retyping. ## Making changes permanent **`~/.inputrc`** is readline's configuration file (overridable with the `INPUTRC` environment variable; the system-wide file is usually `/etc/inputrc`). It holds two kinds of lines — `set VARIABLE value` for readline variables, and `"key sequence": function-or-macro` for bindings: ``` $include /etc/inputrc set editing-mode vi set completion-ignore-case on set show-all-if-ambiguous on "\e[A": history-search-backward "\e[B": history-search-forward ``` That last pair is the popular one: type a prefix, then press Up, and you cycle only through history entries starting with that prefix. **`bind` in `~/.bashrc`** does the same thing from bash. `bind '"\e[A": history-search-backward'` is equivalent to the inputrc line, `bind -f ~/.inputrc` re-reads the file, and `bind -x '"\C-o": some_shell_function'` binds a key directly to a shell command — something plain inputrc syntax cannot do. To discover names and current bindings, `bind -p` lists every binding with its function name and `bind -P` prints them in a readable form. **Editing mode.** `set -o vi` (bash) or `set editing-mode vi` (inputrc) switches to vi-style editing: you start in insert mode, `Esc` moves to command mode, normal vi motions apply, and `/` searches history. `set -o emacs` returns to the default. Adding `set show-mode-in-prompt on` to `~/.inputrc` makes the current vi mode visible, which removes most of the beginner pain. ## Two things worth knowing - Changes to `~/.inputrc` do not affect already-running shells until they re-read it — `Ctrl-X Ctrl-R` is bound to `re-read-init-file`, and `bind -f ~/.inputrc` does the same from the prompt. - If your keys emit odd characters instead of moving the cursor, the shell may not be using readline at all: `sh`, `dash` and `bash --noediting` give you a dumb line with no editing, which is why arrow keys print `^[[A` in a minimal container shell. ## Why this is worth configuring Every second spent retyping a long command is friction repeated hundreds of times a day. Prefix history search, case-insensitive completion and `Alt-.` are three small settings that remove most of it — and because they live in readline rather than in bash, they follow you into every other readline-based tool on the machine.