apt alternative for proxy environment with bash shell

As I am new to bash scripting, and want to use apt-get in my university. I know that many people have issues when trying to do so. My focus is simplicity and ease of use, but still need to be somewhat robust.

#!/bin/bash 
string="install"
errormsg="\n\tInvalid input\n You should run this script with the following structure:\n\n sudo apt-proxy-install.sh install USERNAME PASSWORD\n\n "
DIRECTORY=~/bin/
if [ "$1" = "$string" ] && { ! ([ -z "$3" ] || [ -z "$3" ]) }
   then 
         if [ ! -d "$DIRECTORY" ]; then
            mkdir ~/bin/
         fi
         printf "#!/bin/bash\nhttp_proxy=""http://$2:$3@10.20.10.50:3128"" sudo apt-get \${@:2}" > ~/bin/apt-proxy
         chmod 777 ~/bin/apt-proxy
         PATH=~/bin:$PATH
         addpath="export $PATH"
         sudo cat ~/.bashrc $addpath > ~/.bashrc
   else
         printf "$errormsg"
fi

My question is: is this code acceptable, or should I improve it? If the answer is not, then please give me some hints.

Storing passwords in text files is never a good idea, especially if you give that file world-readable permissions.

Try this:

#!/bin/bash 

mkdir -p ~/bin/

# add ~/bin to PATH if not already there
echo '[[ :"$PATH": == *:"$HOME/bin":* ]] || PATH="$HOME/bin:$PATH"' >> .bashrc

# create the apt-proxy script
cat  ~/bin/apt-proxy
#!/bin/bash

stty -echo
printf "Password for %s: " "$LOGNAME"
read password
stty echo
echo

export http_proxy="http://${LOGNAME}:${password}@10.20.10.50:3128"
sudo -S apt-get "$@" <<< "$password"
END_SCRIPT

chmod 755 ~/bin/apt-proxy

cat <<INSTRUCTIONS
The apt-proxy script has been installed. 
You may need to log out and log back in before you can use it.

usage: apt-proxy package ...
INSTRUCTIONS

• mkdir -p dir will silently do nothing if the directory already exists

• you only need to add the user's bin dir to the PATH if it is not already there

• DO NOT store the user's password. Have the user enter it each time.

• you should not even need to use sudo to edit files in your home directory.

• 777 permissions is overly generous: the world does not have to be able to edit the file

• use sudo -S and pass the user's password via stdin

StackExchange Code Review Q#36986, answer score: 3