Ubuntu – Removal of colons in directory name

Well, you could just add this line to your ~/.profile¹:

HOME=/user/home/

However, that really isn't a good idea. Problems it would cause include (but are probably not limited to):

1. That will only work if /home/user is owned by your user. If it isn't, you won't even be able to log in.

2. This will work for your user only. For everyone else, your home directory will be whatever is stored in /etc/passwd. This means that, for example, cd ~user will fail. In other words, if I log in as bob and bob has the line HOME=/home/bob/foo in ~/.profile, then bob thinks that his home directory is /home/bob/foo but nobody else knows that:

    $ whoami
    bob
    $ echo $HOME
    /home/bob/foo
    $ cd ## this moves to your $HOME
    $ pwd
    /home/bob/foo
   

So far so good. But:

    $ whoami
    terdon
    $ cd ~bob
    $ pwd
    /home/bob

3. This will drive your sysadmin insane. You do not want to anger your sysadmin for you are crunchy and taste good with ketchup.

In any case, it is rarely a good idea to mess with variables like $HOME, as it can often have unintended consequences. Instead, a much cleaner solution would be to make sure every new shell session starts in the target directory. Just add this line to your ~/.bashrc:

cd /user/home/

Now, each time you log² in or open a terminal, you will find yourself in /user/home.

¹ _{Or ~/.bash_profile if it exists.}

² Log in to Debian-based systems like Ubuntu, anyway. For other distributions/OSs, you might need to add it to ~/.profile as well.See here.

Besides copying the binary from another machine and hoping it is the correct version, this can also be solved by using apt or apt-get to install the sed package and passing the --reinstall option so that the .deb file will be downloaded and installed even though Ubuntu's package manager thinks it is already present.

sudo apt install --reinstall sed

This works with apt-get instead of apt, too. It uninstalls and reinstalls the package in a single step. The situation in this problem is that the se package is already installed but the executable has been deleted, so reinstalling solves that. Without the --reinstall flag, the package manager assumes nothing has to be done, because the package is installed already.

You can run sudo apt update first if you want, though it will not typically be necessary in this case, unless you've modified your software sources without doing so. You can pass the --purge flag too if you want but it's unnecessary here, since that just causes conffiles for the package to be removed when it is uninstalled.

sed is a utility that programs are generally allowed to assume exists and to rely on themselves. As you noticed, the scripts that run when installing, removing, or upgrading software can use sed. That's the source of your specific error messages. In theory APT or dpkg could rely on it directly and be unable to reinstall it. In practice, this does not appear to happen, and I don't expect it to. I've tested this on Ubuntu 16.04 LTS.

I cannot think of any situation where replacing the binary fixes the problem but reinstalling neither works nor shows an error immediately (see comments on the post). Although I cannot be 100% sure that these instructions would work for you--because perhaps more was broken than documented in the question--they should generally work at least as reliably as manually replacing /bin/sed for others who experience this problem.