Broken my Debian install: 'sh' not found in PATH

Question

I'm running a custom build of Debian buster/sid. I have broken a lot of things seemingly by mistakenly running apt-get dist-upgrade (I think that's what did it). So the desktop manager\desktop environment no longer loads at all and I just get a command-line shell.

Aptitude and apt-get are broken as is dpkg.

apt update, aptitude update and apt-get update fails and gives a lot of errors when attempted to run, however my /etc/apt/sources.list looks fine

deb http://ftp.ie.debian.org/debian unstable main contrib non-free
deb-src http://ftp.ie.debian.org/debian unstable main contrib non-free
deb http://ftp.ie.debian.org/debian experimental main contrib non-free
deb-src http:// ftp.ie.debian.org/debian experimental main contrib non free
deb https://dl.winehq.org/wine-builds/debian sid main
deb https://dl.winehq.org/wine-builds/ubuntu sid main

errors on update like this:

Err:2 http://ftp.ie.debian.org/debian unstable InRelease
Couldn't execute /usr/bin/apt-key to check /var/lib/apt/lists/partial/ftp.ie.debian.org_debian_dists_experimental_InRelease

and

E: Problem executing scripts APT::Update::Post-Invoke-Success '/usr/bin/test -e /usr/share/dbus-1/system-services/org.freedesktop.PackageKit.service && /usr/bin/test -S /var/run/dbus/system_bus_socket && /usr/bin/gdbus call --system --dest org.freedesktop.PackageKit --object-path /org/freedesktop/PackageKit --timeout 4 --method org.freedesktop.PackageKit.StateHasChanged cache-update > /dev/null; /bin/echo > /dev/null'

E: Sub-process returned an error code

aptitude upgrade, apt upgrade or apt-get upgrade appear to be broken due to dpkg being broken

Dpkg gives this error:

dpkg: warning: 'sh' not found in PATH or not executable
dpkg: error: 1 expected

however echo $PATH returns, which looks fine to me

/home/ferg/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/games:/usr/games

if I try sh I get

The program 'sh' is currently not installed. To run 'sh' please ask your administrator to install the package 'dash'

If I try sudo apt-get install dash

it tells me dash is already the newest version (0.5.8-2.6)

and if I continue

E: Sub-process /usr/sbin/dpkg-preconfigure --apt || true returned an error code (100)
E: Failure running script /usr/sbin/dpkg/preconfigure --apt || true

If I run sudo nano /etc/apt/apt.conf.d/70debconf and comment out the line DPkg::Pre-Install-Pkgs {"/usr/sbian/dpkg-preconfigure --apt || true";};

then aptitude fails with

dpkg: warning: 'sh' not found in PATH or not executable
dpkg: error: 1 expected

So I can't reinstall necessary packages to restore the system because dpkg (and therefore aptitude, apt-get, apt) are broken, seemingly because sh is not in my PATH. However all the correct directories are in my PATH. I'm assuming that fixing this issue with sh would allow dpkg and aptitude to work again but I'm wondering what precisely is the problem with sh?

Thank you in advance for any help.

---

Update:

ls -l /bin/bash

returns

-rwxr-xr-x 1 root root 1099016 May 15  2017 /bin/bash

ls -l /bin/sh returns

lrwxrwxrwx 1 root root 4 Jun 28  2017 /bin/sh -> dash

ls -l /bin/dash returns

ls: cannot access 'bin/dash': No such file or directory

So /bin/sh seems to redirect to /bin/dash which has been deleted somehow?

sudo apt-get install --reinstall dash fails with:

1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.    
E: Internal Error, No file name for dash:amd64

Running sudo ln -sf bash /bin/sh and then sudo apt-get install --reinstall dash fails with the same error as above.

Searching for this issue and variants of it on Google I've found numerous solutions related to the user's PATH and permissions, I've checked the files which relate to that including /etc/sudoers /root/.bashrc and /root/.profile and /etc/profile. In most of those cases the users were able to solve their issue by simply defining their path correctly (usually the /sbin or /usr/sbin directories were missing). As stated above my PATH seems to contain all the relevant directories whether I run echo $PATH normally or as root or sudo echo $PATH. Some users were also having issues with permissions or with sudo itself. I've also tried executing various aptitude apt and apt-get and dpkg commands as root using sudo -i which suggests that my issue is different.

Answer 1

You were bitten by a nasty bug in dash which caused /bin/sh to be removed. This has since been fixed, which is why another apt update and apt full-upgrade fixed it (after restoring /bin/sh to point to bash presumably, your comments aren’t clear about that).

Answer 2

As stated by @Stephen Kitt I had encountered this bug in dash which had broken sh (the Bourne shell) as /bin/sh is a symlink to /bin/dash/ in my particular distribution.

Running sudo ln -sf bash /bin/sh in order to point sh to bash and then running sudo apt-get update followed by sudo apt-get install --reinstall dash followed by sudo apt-get full-upgrade solved the issue.