Seriously, stop typing `cd $_`

mkdir_and_cd.sh

#!/bin/sh
# The alias to call this script needs to source it, not just execute it in a
# new shell, or you'll make the directory but not cd to it in the parent shell.
# Like this: alias mcd='source ~/.scripts/mkdir_and_cd.sh'

if [ "$#" -ne 1 ]; then
    echo "Please pass exactly one directory name."
else
    mkdir "$1"
    cd "$1"
fi

mkdir_and_cd_adventurous.sh

#!/bin/sh
# Just like `mkdir_and_cd.sh`, but it will create you a randomly-named directory
# if you don't give it any arguments.
# 
# The alias to call this script needs to source it, not just execute it in a
# new shell, or you'll make the directory but not cd to it in the parent shell.
# Like this: alias mcda='source ~/.scripts/mkdir_and_cd_adventurous.sh'

case "$#" in
    0)
        DIR="$(shuf /usr/share/dict/words | head -n 1 | tr -dc 'a-zA-Z0-9')"
        mkdir "$DIR" && cd "$DIR"
        ;;
    1)
        mkdir "$1" && cd "$1"
        ;;
    *)
        echo "Don't you know I only take one argument?"
esac