Autosuggestions

Fish-style autosuggestions show command completions as you type.

Source: src/io/readline.f90:update_autosuggestion

Overview

As you type, fortsh shows a grayed-out suggestion based on your command history. The suggestion appears inline after your cursor.

Accepting Suggestions

The suggestion is appended to your current input.

How Suggestions Work

1. fortsh searches your history backwards (newest first)
2. Finds the first entry that starts with your current input
3. Shows the remainder as a grayed-out suggestion
4. Stops at newlines (to avoid heredoc content)

From the source:

! Search history backwards for matching prefix
do i = command_history%count, 1, -1
    call get_history_line(i, hist_line, found)
    if (found .and. starts_with(hist_line, current_input)) then
        suggestion = hist_line(len(current_input)+1:)
        return
    end if
end do

Path Suggestions

fortsh also suggests file paths. When your last word looks like a partial path (contains /), path completion takes priority over history.

cd /ho        # Suggests: /home
cat ~/pro     # Suggests: ~/projects

Display Behavior

• Suggestions only appear when cursor is at end of line
• Grayed out using ANSI color code 90 (bright black)
• Limited to terminal width to prevent wrapping
• Not shown during incremental search or menu selection

Configuration

Autosuggestions are enabled by default. To disable:

export FORTSH_NO_SUGGESTIONS=1

Comparison with Fish

Implementation Notes

• Max suggestion length: MAX_LINE_LEN (1024 bytes on Linux, 128 on macOS ARM64)
• Suggestions respect the current command's context
• Multi-line commands (heredocs) are handled by stopping at newlines