`cle` EMACS-like Command Line Editor

Overview

The cle library provides an EMACS-like command line editor for NuttX applications. It is similar to GNU readline but much smaller, making it suitable for resource-constrained embedded systems. cle is not a standalone command; it is a library that applications call to get interactive line editing with history and key bindings.

The editor assumes a VT100-compatible terminal and a fixed-width character set (such as Courier). It does not support word-oriented operations or keypad cursor control.

Configuration

Enable the editor with CONFIG_SYSTEM_CLE. The following options are available when CONFIG_SYSTEM_CLE is enabled:

CONFIG_SYSTEM_COLOR_CLE

Add simple color highlighting to the prompt, command text, and output. The colors are hardcoded: yellow for the prompt, cyan for command text, and green for output.

CONFIG_SYSTEM_CLE_CMD_HISTORY

Enable Unix-style command history using the up and down arrow keys. The history is stored in an in-memory circular buffer.

In FLAT and PROTECTED builds the history is shared by all threads; in KERNEL builds each process has its own history buffer.

CONFIG_SYSTEM_CLE_CMD_HISTORY_LINELEN

Maximum length of one command line in the history buffer. Default: 64 (when CONFIG_DEFAULT_SMALL is set) or 80.

CONFIG_SYSTEM_CLE_CMD_HISTORY_LEN

Number of history records to keep. Default: 4 (when CONFIG_DEFAULT_SMALL is set) or 16.

CONFIG_SYSTEM_CLE_DEBUGLEVEL

Debug output level. 0 = off, 1 = errors on console, 2 = full debug information. Debug output is sent via syslog().

Usage

cle is a library, not a standalone command. Applications call one of the following functions:

#include <system/cle.h>

int cle_fd(FAR char *line, FAR const char *prompt, uint16_t linelen,
           int infd, int outfd);

#ifdef CONFIG_FILE_STREAM
int cle(FAR char *line, FAR const char *prompt, uint16_t linelen,
        FAR FILE *instream, FAR FILE *outstream);
#endif

Parameters

line: Buffer to store the edited line.
prompt: Prompt string displayed before the cursor.
linelen: Maximum length of the line buffer.
infd / instream: Input file descriptor (cle_fd) or FILE stream (cle).
outfd / outstream: Output file descriptor (cle_fd) or FILE stream (cle).

Return Value

Both functions return the number of characters in the edited line, or a negative error code on failure.

Key Bindings

cle uses EMACS-style control key bindings:

Ctrl-A: Move cursor to the start of the current line.
Ctrl-B: Move cursor left one character.
Ctrl-D: Delete the character at the cursor position.
Ctrl-E: Move cursor to the end of the current line.
Ctrl-F: Move cursor right one character.
Ctrl-H (Backspace): Delete the character to the left of the cursor.
Ctrl-K: Delete from the cursor to the end of the line.
Ctrl-L: Clear the screen and redraw the current line.
Ctrl-N: Move down one line (when command history is enabled).
Ctrl-P: Move up one line (when command history is enabled).
Ctrl-U: Delete the entire line.
\\ (backslash): Quote the next character; the quoted character is inserted literally.

Examples

Use cle in an application to get interactive line editing:

#include <system/cle.h>
#include <stdio.h>

int main(void)
{
  char line[128];
  int len;

  printf("Enter a command: ");
  len = cle(line, "> ", sizeof(line), stdin, stdout);
  if (len > 0)
    {
      printf("You typed: %s\n", line);
    }

  return 0;
}

Notes

cle requires a VT100-compatible terminal. It uses VT100 escape sequences for cursor movement, line editing, and screen clearing.
The editor uses a fixed-width character set assumption. Variable-width fonts may cause display issues.
When CONFIG_SYSTEM_CLE_CMD_HISTORY is enabled, the history buffer is shared across all threads in FLAT and PROTECTED builds. In KERNEL builds, each process has its own history buffer.
Memory usage is approximately 1.5–2 KB.
cle does not support word-oriented operations (move by word, delete word, etc.) or keypad cursor control.

cle EMACS-like Command Line Editor