Linux – how to properly use $1 in an alias with two arguments

If you look at the function _cd() in /etc/bash_completion, you'll see that it appends the trailing slash itself and that complete gets called with the option -o nospace for cd.

You can do the same for xyz, but to have to verify separately if the found match is a directory (if so, append slash) or a file (if so, append space). This should be done in a for loop to process all found matches.

Also, to properly handle paths that contain spaces, you have to set the internal file separator to only newline and escape the spaces. Using IFS=$'\n' in combination with printf %q makes completion work with almost all characters.¹ Special care has to be taken to not escape the trailing space.

The following should work:

_xyz ()
{
    local IFS=$'\n'
    local LASTCHAR=' '

    COMPREPLY=($(compgen -o plusdirs -f -X '!*.txt' \
        -- "${COMP_WORDS[COMP_CWORD]}"))

    if [ ${#COMPREPLY[@]} = 1 ]; then
        [ -d "$COMPREPLY" ] && LASTCHAR=/
        COMPREPLY=$(printf %q%s "$COMPREPLY" "$LASTCHAR")
    else
        for ((i=0; i < ${#COMPREPLY[@]}; i++)); do
            [ -d "${COMPREPLY[$i]}" ] && COMPREPLY[$i]=${COMPREPLY[$i]}/
        done
    fi

    return 0
}

complete -o nospace -F _xyz xyz

_{¹ The newline character is the obvious exception here, since it is an internal file separator.}

By default neither sudo nor su reads a password from stdin. They try to use a terminal device directly (some implementations may even complain when used in a pipe). There is sudo -S option to change this behavior, but as far as I know there is no similar option for su.

Solution: use expect.

expect is a program that "talks" to other interactive programs according to a script. Following the script, expect knows what can be expected from a program and what the correct response should be.

[…]

In general, expect is useful for running any program which requires interaction between the program and the user. All that is necessary is that the interaction can be characterized programmatically.

In your case the script may be:

#!/usr/bin/expect 

log_user 0
spawn /bin/su - otherusr
expect "Password: "
send "mypassword\n"
interact

I advise to make this script accessible only to you (chmod go-rwx). Other users shouldn't be allowed to read it because it contains mypassword in plaintext; they shouldn't be allowed to run it, because it gives access to otherusr's shell.