Customize "command not found" message in Bash

Question

Is there someway to alter the Bash system error message template so that you can print something in addition to the original message? For example:

Macbook Air:~/Public]$ lfe
-bash: lfe: WTF command not found

or

Macbook Air:~/Public]$ lfe
-bash: lfe: #!&**! command not found

Depends on your setup. For me (Ubuntu 16.04), there is a Python script at `/usr/lib/command-not-found` that prints the message that you could edit. It is called from a function declared in `/etc/bash.bashrc`, and that is sourced in `/etc/profile`. — Benjamin W., Mar 17 '17 at 19:22
You can check where the function is for you if you turn on `set -x` and issue a non-existing command. — Benjamin W., Mar 17 '17 at 19:24
Is there a way to 'undo' the `set -x` command? It'd be nice to add a message for other types of error messages too. — pmagunia, Mar 17 '17 at 20:30
The set command prints the last command to the console I think. — pmagunia, Mar 17 '17 at 20:37
I just realize that `command_not_found_handle` was introduced in Bash 4.0, so you won't have it on a Mac. — Benjamin W., Mar 17 '17 at 21:08

score 4 · Answer 1 · answered Mar 17 '17 at 21:32

Since Bash 4.0, if the search for a command is unsuccessful, the shell searches for a function called command_not_found_handle. If it doesn't exist, Bash prints a message like this and exits with status 127:

$ foo
-bash: foo: command not found
$ echo $?
127

If it does exist, it is called with the command and its arguments as arguments, so if you have something like

command_not_found_handle () {
    echo "It's my handle!"
    echo "Arguments: $@"
}

in your .bashrc, Bash will react like this:

$ foo bar
It's my handle!
Arguments: foo bar

Most systems have something much more sophisticated in place, though. My Ubuntu, for example, has this in /etc/bash.bashrc:

# if the command-not-found package is installed, use it
if [ -x /usr/lib/command-not-found -o -x /usr/share/command-not-found/command-not-found ]; then
    function command_not_found_handle {
            # check because c-n-f could've been removed in the meantime
                if [ -x /usr/lib/command-not-found ]; then
           /usr/lib/command-not-found -- "$1"
                   return $?
                elif [ -x /usr/share/command-not-found/command-not-found ]; then
           /usr/share/command-not-found/command-not-found -- "$1"
                   return $?
        else
           printf "%s: command not found\n" "$1" >&2
           return 127
        fi
    }
fi

and this is sourced from /etc/profile. /usr/lib/command-not-found is a Python script that uses some more Python (CommandNotFound) to basically look up packages that are named like the unknown command, or sound similar:

$ sl
The program 'sl' is currently not installed. You can install it by typing:
sudo apt install sl
$ sedd
No command 'sedd' found, did you mean:
 Command 'sed' from package 'sed' (main)
 Command 'seedd' from package 'bit-babbler' (universe)
 Command 'send' from package 'nmh' (universe)
 Command 'send' from package 'mailutils-mh' (universe)
sedd: command not found

So if you want simple customization, you can provide your own command_not_found_handle, and if you want to customize the existing system, you can modify the Python scripts.

But, as mentioned, this requires Bash 4.0 or higher.

score 2 · Accepted Answer · answered Mar 17 '17 at 19:43

Maybe something like:

curl https://raw.githubusercontent.com/rcaloras/bash-preexec/master/bash-preexec.sh -o ~/.bash-preexec.sh
echo '[[ -f ~/.bash-preexec.sh ]] && source ~/.bash-preexec.sh' >> ~/.bashrc

then add the following to .bashrc too

preexec() { type "$1" >/dev/null 2>&1 || echo -n 'WTF??? '; }

reload your shell, then try enter some nonexistent command, like bububu

$ bububu

will print

WTF??? -bash: bububu: command not found

Important: read https://github.com/rcaloras/bash-preexec

Customize "command not found" message in Bash

2 Answers2

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