How to get the unicode characters for CTRL+B, CTRL+L, ALT+K, etc...?

Decoding Control Characters: Finding Unicode for CTRL+B, ALT+K, and More

Learn how to identify and represent special control characters like CTRL+B, CTRL+L, and ALT+K in Unicode, crucial for terminal applications and scripting in Linux/Unix environments.

In Linux and Unix-like terminal environments, certain key combinations like CTRL+B, CTRL+L, or ALT+K don't always produce a visible character. Instead, they often send special control sequences or characters that the terminal interprets for specific actions, such as moving the cursor, clearing the screen, or triggering application-specific functions. Understanding how to identify and represent these characters in Unicode is essential for debugging terminal issues, scripting, and configuring applications like tmux or Vim.

Understanding Control Characters and Their Unicode Representation

Control characters are non-printing characters that are part of the ASCII character set (and thus Unicode). They were originally designed to control hardware devices like printers and teletypes. In modern computing, they are primarily used for terminal control and data transmission. Each control character has a specific numerical value and a conventional representation. For example, CTRL+B typically corresponds to ASCII STX (Start of Text) with a decimal value of 2, while CTRL+L corresponds to FF (Form Feed) with a decimal value of 12.

flowchart TD
    A[User Presses Key Combination] --> B{Is it a Control Character?}
    B -- Yes --> C[Terminal Interprets Input]
    C --> D{Sends ASCII/Unicode Control Code}
    D --> E[Application/Shell Reacts]
    B -- No --> F[Sends Visible Character]
    F --> E

Flowchart illustrating how terminal input is processed for control characters.

Identifying Control Characters in Your Terminal

Several tools can help you identify the exact character or sequence sent by a key combination. The cat -v command is particularly useful for displaying non-printable characters in a human-readable format. For instance, CTRL+B might appear as ^B. For more detailed information, including the exact hexadecimal or decimal values, od (octal dump) and showkey are invaluable.

# Using cat -v to see control characters
# Type CTRL+B, then CTRL+D to exit
cat -v

# Using od -c to see character representation
# Type CTRL+B, then CTRL+D to exit
od -c

# Using showkey to see keycodes (requires root or specific permissions)
# Press CTRL+B, then ALT+K, then any other key to exit
showkey -a

Commands to identify control characters and keycodes in the terminal.

Representing Control Characters in Unicode and Scripts

Once you've identified the control character, you can represent it in various ways in scripts or configuration files. The most common way is using the caret notation (e.g., ^B for CTRL+B), but you can also use their ASCII decimal or hexadecimal values. For ALT key combinations, the behavior is often more complex, as ALT might send an escape sequence (e.g., ESC followed by the character) rather than a single control character.

# Python example: Representing control characters

# CTRL+B (ASCII 2, STX)
ctrl_b_char = '\x02'
print(f"CTRL+B (hex): {ctrl_b_char.encode('utf-8').hex()}")
print(f"CTRL+B (repr): {repr(ctrl_b_char)}")

# CTRL+L (ASCII 12, FF)
ctrl_l_char = '\x0c'
print(f"CTRL+L (hex): {ctrl_l_char.encode('utf-8').hex()}")
print(f"CTRL+L (repr): {repr(ctrl_l_char)}")

# ALT+K (often ESC followed by 'k')
alt_k_sequence = '\x1bk'
print(f"ALT+K (hex): {alt_k_sequence.encode('utf-8').hex()}")
print(f"ALT+K (repr): {repr(alt_k_sequence)}")

Python script demonstrating how to represent common control characters and ALT sequences.

1. Identify the Character

Use cat -v or od -c in your terminal. Press the key combination (e.g., CTRL+B), then CTRL+D to exit. Note the output (e.g., ^B).

2. Determine ASCII/Unicode Value

For ^B, this corresponds to ASCII STX (Start of Text), which is decimal 2 or hexadecimal 0x02. For ALT combinations, you might see ^[k, indicating ESC (0x1b) followed by k (0x6b).

3. Represent in Code/Config

Use the hexadecimal escape sequence (e.g., \x02 for CTRL+B, \x1bk for ALT+K) in programming languages or configuration files that support such escapes.