Ubuntu – How to find the command history if a user used a space before the command

Do you want it to work without typing an extra command, e.g.

$ rm file

Or only when the user types something like

$ confirm rm file

Or only when the user tries to run certain commands, e.g.

$ rm file

but not for

$ echo "Hello"

If option 1, that can be done using the preexec hook in zsh, or the DEBUG trap in bash.

If option 2, put something like this in /etc/bash.bashrc or other shell startup file.

confirm() {
    echo -n "Do you want to run $*? [N/y] "
    read -N 1 REPLY
    echo
    if test "$REPLY" = "y" -o "$REPLY" = "Y"; then
        "$@"
    else
        echo "Cancelled by user"
    fi
}

If option 3, you could modify the confirm script above, or, some commands have an option to ask before doing something, e.g. rm -i. You could put alias rm='rm -i' in /etc/bash.bashrc.

You can reverse the default Ubuntu settings by setting two variables (HISTIGNORE an HISTCONTROL), use the commands like below:

HISTIGNORE='!( *)'
HISTCONTROL=ignoredups

HISTCONTROL=ignoredups changes history behaviour to only ignore duplicate lines, and not ignore lines starting with a space. (You can also set HISTCONTROL to the empty string (with HISTCONTROL= ) if you do want to keep duplicates, but that is usually not wanted.)

HISTIGNORE='!( *)' makes history ignore every line which doesn't start with a space: ( *) would match every line starting with a space, but the leading ! negates the match, so it matches everything which doesn't start with a space. And everything what is matched by the HISTIGNORE pattern will be ignored by history. (This latter option requires that bash is run with extglob turned on, with shopt -s extglob, but that is the default setting on Ubuntu.)

If you want to make this permanent, don't forget to put the above two commands into your ~/.bashrc.