10

I want to run a command before and after every single command that gets executed in a terminal (i.e. clear before and then reset). I want this to happen automatically and not with a short alias or similar.

I have looked into bashrc and /usr/share/bash-completion/bash_completion although the latter seems to be just for the auto completion feature with the Tab?

Where should I make my changes?

Katu
  • 3,593
  • 26
  • 42
  • You want to run a command before each command, what will be run before that command? It's a recursion here – Anwar Nov 24 '16 at 12:35
  • @Anwar I meant run a command before each command written in the console. – Katu Nov 24 '16 at 12:55
  • That's a good opportunity to switch to Zsh, which can do this natively. In Bash it requires some tinkering; see http://superuser.com/questions/175799/does-bash-have-a-hook-that-is-run-before-executing-a-command – fkraiem Nov 24 '16 at 13:27
  • Also see: http://unix.stackexchange.com/questions/44713/can-i-configure-bash-to-execute-clear-before-every-command-typed-in-the-consol – muru Nov 26 '16 at 07:39
  • In zsh you can simply define a function: precmd() { echo "Hi" ; } – Raúl Salinas-Monteagudo Jun 03 '23 at 06:38

1 Answers1

12

In your .bashrc:

function process_command() {
  ... do something with $BASH_COMMAND ...
}
trap process_command DEBUG

For a good example see "Make gnome-terminal show the command running as title" on AskUbuntu.

To solve the original poster's request to run clear before the command and reset after the command, add to .bashrc:

function before_command() { 
  case "$BASH_COMMAND" in
        $PROMPT_COMMAND)
          ;;
       *)
          clear;
  esac
}

function before_prompt() { reset ; }

trap before_command DEBUG
PROMPT_COMMAND=before_prompt

In man bash look for PROMPT_COMMAND and trap .-lp.

Community
  • 1
AlexP
  • 10,197
  • Would it be { clear; $BASH_COMMAND; reset; } ? – wjandrea Nov 25 '16 at 23:07
  • No. Please read the provided link and the manual. You cannot execute $BASH_COMMAND, it's provided for informative purposes only. What the original poster asked can be done by clearing the screen in a function called just before executing a command, and resetting the terminal in a function run before outputting the prompt. I have edited the answer to provide an example. – AlexP Nov 26 '16 at 06:22
  • @AlexP your answer is useful, however, I have tried it with the command date and found the following: Hitting enter correctly runs before_command and before_prompt. However, if I write the command ls and then hit enter, the before_command runs twice, once before and once after ls. The command before_prompt correctly runs once after ls. I'm still looking for a complete solution for this, any tips are much appreciated. – Katu Nov 28 '16 at 10:33
  • The function set as a DEBUG trap will run before any command is executed, which means that it will also run before $PROMPT_COMMAND. You can test $BASH_COMMAND to see whether the command to be run is $PROMPT_COMMAND or not. – AlexP Nov 28 '16 at 10:45