一直好奇Ubuntu20 原生的bash 每次输入命令行，没有该命令后会提示你如何安装这个软件包，前一段时间一直在找，直到我使用oh-my-zsh添加command-not-found插件后，找到了根源。

cat command-not-found.plugin.zsh 
## Platforms with a built-in command-not-found handler init file

for file (
  # Arch Linux. Must have pkgfile installed: https://wiki.archlinux.org/index.php/Pkgfile#Command_not_found
  /usr/share/doc/pkgfile/command-not-found.zsh
  # macOS (M1 and classic Homebrew): https://github.com/Homebrew/homebrew-command-not-found
  /opt/homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh
  /usr/local/Homebrew/Library/Taps/homebrew/homebrew-command-not-found/handler.sh
); do
  if [[ -r "$file" ]]; then
    source "$file"
    unset file
    return 0
  fi
done
unset file


## Platforms with manual command_not_found_handler() setup

# Debian and derivatives: https://launchpad.net/ubuntu/+source/command-not-found
if [[ -x /usr/lib/command-not-found || -x /usr/share/command-not-found/command-not-found ]]; then
  command_not_found_handler() {
    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 "zsh: command not found: %s\n" "$1" >&2
      return 127
    fi
  }
fi

# Fedora: https://fedoraproject.org/wiki/Features/PackageKitCommandNotFound
if [[ -x /usr/libexec/pk-command-not-found ]]; then
  command_not_found_handler() {
    if [[ -S /var/run/dbus/system_bus_socket && -x /usr/libexec/packagekitd ]]; then
      /usr/libexec/pk-command-not-found "$@"
      return $?
    fi

    printf "zsh: command not found: %s\n" "$1" >&2
    return 127
  }
fi

# NixOS: https://github.com/NixOS/nixpkgs/tree/master/nixos/modules/programs/command-not-found
if [[ -x /run/current-system/sw/bin/command-not-found ]]; then
  command_not_found_handler() {
    /run/current-system/sw/bin/command-not-found "$@"
  }
fi

# Termux: https://github.com/termux/command-not-found
if [[ -x /data/data/com.termux/files/usr/libexec/termux/command-not-found ]]; then
  command_not_found_handler() {
    /data/data/com.termux/files/usr/libexec/termux/command-not-found "$1"
  }
fi

# SUSE and derivates: https://www.unix.com/man-page/suse/1/command-not-found/
if [[ -x /usr/bin/command-not-found ]]; then
  command_not_found_handler() {
    /usr/bin/command-not-found "$1"
  }
fi

发现debian衍生系列的发现版会用/usr/lib/command-not-found脚本，如果懂python，又要兴趣的朋友可以深入研究一下。提升python能力。

cat /usr/lib/command-not-found
#!/usr/bin/python3
# (c) Zygmunt Krynicki 2005, 2006, 2007, 2008
# Licensed under GPL, see COPYING for the whole text

from __future__ import absolute_import, print_function


__version__ = "0.3"
BUG_REPORT_URL = "https://bugs.launchpad.net/command-not-found/+filebug"

try:
    import sys
    if sys.path and sys.path[0] == '/usr/lib':
        # Avoid ImportError noise due to odd installation location.
        sys.path.pop(0)
    if sys.version < '3':
        # We might end up being executed with Python 2 due to an old
        # /etc/bash.bashrc.
        import os
        if "COMMAND_NOT_FOUND_FORCE_PYTHON2" not in os.environ:
            os.execvp("/usr/bin/python3", [sys.argv[0]] + sys.argv)

    import gettext
    import locale
    from optparse import OptionParser

    from CommandNotFound.util import crash_guard
    from CommandNotFound import CommandNotFound
except KeyboardInterrupt:
    import sys
    sys.exit(127)


def enable_i18n():
    cnf = gettext.translation("command-not-found", fallback=True)
    kwargs = {}
    if sys.version < '3':
        kwargs["unicode"] = True
    cnf.install(**kwargs)
    try:
        locale.setlocale(locale.LC_ALL, '')
    except locale.Error:
        locale.setlocale(locale.LC_ALL, 'C')


def fix_sys_argv(encoding=None):
    """
    Fix sys.argv to have only unicode strings, not binary strings.
    This is required by various places where such argument might be
    automatically coerced to unicode string for formatting
    """
    if encoding is None:
        encoding = locale.getpreferredencoding()
    sys.argv = [arg.decode(encoding) for arg in sys.argv]


class LocaleOptionParser(OptionParser):
    """
    OptionParser is broken as its implementation of _get_encoding() uses
    sys.getdefaultencoding() which is ascii, what it should be using is
    locale.getpreferredencoding() which returns value based on LC_CTYPE (most
    likely) and allows for UTF-8 encoding to be used.
    """
    def _get_encoding(self, file):
        encoding = getattr(file, "encoding", None)
        if not encoding:
            encoding = locale.getpreferredencoding()
        return encoding


def main():
    enable_i18n()
    if sys.version < '3':
        fix_sys_argv()
    parser = LocaleOptionParser(
        version=__version__,
        usage=_("%prog [options] <command-name>"))
    parser.add_option('-d', '--data-dir', action='store',
                      default="/usr/share/command-not-found",
                      help=_("use this path to locate data fields"))
    parser.add_option('--ignore-installed', '--ignore-installed',
                      action='store_true',  default=False,
                      help=_("ignore local binaries and display the available packages"))
    parser.add_option('--no-failure-msg',
                      action='store_true', default=False,
                      help=_("don't print '<command-name>: command not found'"))
    (options, args) = parser.parse_args()
    if len(args) == 1:
        try:
            cnf = CommandNotFound.CommandNotFound(options.data_dir)
        except FileNotFoundError:
            print(_("Could not find command-not-found database. Run 'sudo apt update' to populate it."), file=sys.stderr)
            print(_("%s: command not found") % args[0], file=sys.stderr)
            return
        if not cnf.advise(args[0], options.ignore_installed) and not options.no_failure_msg:
            print(_("%s: command not found") % args[0], file=sys.stderr)


if __name__ == "__main__":
    crash_guard(main, BUG_REPORT_URL, __version__)