Intercepting “command not found” in bash

On Debian, bash is patched with an interesting new function: command_not_found_handle. This intercepts exit code 127 (“command not found”) and allows you to do neat things. Debian uses it to pass it through the apt database, letting you know if a command you tried to invoke is not available, but can be found in the apt repos, and how to install it. Pretty spiffy.

This, of course, can be modified. Where I work, we use numbers to identify servers. I have a script that grabs login credentials from our internal systems and auto-logs me into servers based on their number. For example, I’d run `connect 12345′ to connect to server 12345.

By adding the following to my .bashrc:

[code lang=”bash”]function command_not_found_handle {
/home/kale/bin/command-not-found $1

And creating the following script, with regex in place to only care about numbers, placed in /home/kale/bin/command-not-found:

[code lang=”bash”]#!/bin/bash

MYBOOL=`echo $1 | awk ‘$1 ~ /^[0-9]+-*[0-9]*$/ {print $0}’ | wc -l | awk ‘{print $1}’`

if [ “$MYBOOL” == “1” ]; then
/home/kale/bin/connect $1
exit 127

(where `connect’ is the path to my connect script, previously written)

This now allows me to do this awesome deal:

[code lang=”bash”]kale@bastion:~$ 12345
Connecting to server 12345…

If you didn’t catch it, I don’t need to specify a command — just the argument. As there’s no application in my $PATH named `12345′, it falls through to the command_not_found_handle function, which then launches my connect script.

Who needs commands? I just saved hundreds of wasted seconds per night on typing “connect”!


  • Golgar

    March 6, 2009

    Kale is my hero. You win 5 internets.

  • Paul R

    February 7, 2010

    I like how you used the output of wc as a boolean. Great technique. Will borrow.

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