An incorrect path is displayed in my custom command line prompt in bash

Question

I would like to display the current and parent directories in my Linux command prompt. For example, if the current directory is /home/user/proj/src, I'd like the prompt to display proj/src.

I found the following from another post which more or less works:

PS1="${PWD#"${PWD%/*/*}/"} $ "

However, when I add it to my .bashrc script and create a new shell, it displays /home/user $ regardless of which directory I'm in. If I change directories, the prompt updates correctly.

Is there a way to set my command-line prompt to display only the current and parent directories and have it update without having to change directories?

Answer 1

To achieve the dynamic update of your shell prompt to display the current and parent directory names in every new shell session without the need to change directories, you can use a small script in your .bashrc that sets the PS1 variable. The issue with your current approach is that it sets PS1 to the result of the expansion at the time of sourcing .bashrc, but it doesn't dynamically update PS1 after that unless the directory changes.

To ensure PS1 updates dynamically with every command, you can use a shell function that recalculates PS1 before displaying the prompt. Bash provides a mechanism for this through the PROMPT_COMMAND environment variable. This variable can be set to a command that executes before the display of each prompt.

Here's how you can do it:

prompt_command() {
    # Capture the current working directory
    local cwd="${PWD}"
    # Extract the last two components of the path
    local last_two_dirs="${cwd#"${cwd%/*/*}/"}"
    # Set PS1 with these components
    PS1="${last_two_dirs} $ "
}

# Set PROMPT_COMMAND to call the prompt_command function
PROMPT_COMMAND=prompt_command

Here's a breakdown of what this script does:

• Defines a function prompt_command that dynamically calculates the last two components of the current working directory and sets PS1 accordingly.
• Sets PROMPT_COMMAND to call this function before each prompt is displayed. This way, PS1 is recalculated before showing the prompt, reflecting the current and parent directories accurately.

To use this script:

1. Open your .bashrc file in your favorite text editor. For example, you can use nano ~/.bashrc or vim ~/.bashrc.
2. Copy and paste the above script into your .bashrc file.
3. Save the changes and exit the editor.
4. To apply the changes to your current terminal session, source your .bashrc file by running source ~/.bashrc.

Now, your command prompt should dynamically display the current and parent directory names in every new command line prompt, updating correctly even in new shell sessions without the need to change directories first.

Answer 2

Unlike other situations, you have to use single quotes for the outer expansion and put it in the variable literally, as Bash will later do a second expansion every time the prompt is shown.

With double quotes, the value is expanded once at time of assignment and that's it.