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wjandrea
Asked: 2017-12-27 15:24:23 +0800 CST

How can I edit a variable in a running shell?

  • 772

I'm editing my PS1 prompt and I don't want to keep sourcing my bashrc to update it (since my bashrc resets other things too). I also don't want to copy and paste it over and over. Is there a way to edit the variable interactively, like using nano (my default editor)?

The PS1 is mostly an example, because I want to do the same thing with other vars. The reason I want to do it interactively is that the variable is already quite long, and I just need to edit a few characters in it. And I only need to change it in the current shell.

command-line

3 Answers

  1. This can be done. Type var=$var and then expand and edit it. To expand, use Esc+Ctrle (the default shortcut, check output of bind -p | grep shell-expand-line to confirm).

    So: 

    muru@muru-1604:~$ PS1=$PS1

    will become:

    muru@muru-1604:~$ PS1=${debian_chroot:+($debian_chroot)}\u@\h:\w\$

    Which you can then edit in an editor with Ctrlx Ctrle (edit-and-execute-command in readline terms). When you save and quit, the saved content will be executed by the shell.

    If you already have PS1=... in your history, you can just go back to that and Ctrlx Ctrle.

    From the bash manual:

    shell-expand-line (M-C-e)

    Expand the line as the shell does. This performs alias and history expansion as well as all of the shell word expansions.

    edit-and-execute-command (C-xC-e)

    Invoke an editor on the current command line, and execute the result as shell commands. Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the editor, in that order.

    • 8

  2. Instead of sourcing the bashrc, source another file, which just contains the variable.

    1. Create it:

      echo "PS1='$PS1'" > /tmp/PS1

      • Note: If the variable contains single quotes, you will need to use a command that can escape them, e.g.:

        declare -p PS1 | cut -d' ' -f3- > /tmp/PS1

    2. Open it in an editor, e.g. nano /tmp/PS1.

      • To avoid running nano over and over, you could run it in another terminal/TTY, or use a graphical editor.

    3. Make your changes and save.

    4. Source it:

      source /tmp/PS1

    5. Repeat steps 3 and 4 as needed.

    • 1
  3. Best Answer

    Zsh has a builtin called vared that lets you edit a variable inline. I wrote my own version, originally posted on Super User:

    vared(){
    # Based on the zsh builtin of the same name.
    IFS= read -rei "${!1}" "$1"
}

    Note this has some subtle differences from the Zsh builtin, e.g:

    • Won't error if the variable is unset
    • Checks the validity of the variable name before printing its value (e.g. vared $)
    • Truncates multi-line variables.

    Then to edit the PS1, just run vared PS1.

    • 1