Simple fix for a debian linux upgrade to bullseye. Some packages will show an error message like `/usr/bin/perl: /lib64/libcrypt.so.1: version `XCRYPT_2.0' not found (required by /usr/bin/perl` and you have to link the new spot of the library. Update: I have to run it before installs and upgrades and everything is fine.

repair-debian-lib-XCRYPT_2.0.sh

#!/bin/bash
# Needed for debian upgrade
# atm from buster to bullseye the linked libs are broken 
# see: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=951880

LIB="libcrypt.so.1"


# Checks if the executer has super user permissions.
#
# @exit 2 if the user has no super user permissions
function validateRootPermissions() {
    if [[ $(/usr/bin/id -u) -ne 0 ]]; then
      echo "[ERR!] script must be run as root (sudo) and you're not root."
      exit 2
    fi
}

# Validates the existing of:
# - the source library exists (normally a link to the lib file)
# - both destinations are symlinks
function validateLibsExists() {
    if [[ ! -f "/lib/x86_64-linux-gnu/${LIB}" ]]; then
        echo "ERROR: Library file </lib/x86_64-linux-gnu/${LIB}> does not exists."
        exit 1
    fi

    if [[ ! -L "/lib64/${LIB}" ]]; then
        echo "ERROR: Library symlink </lib64/${LIB}> does not exists."
        exit 2
    fi

    if [[ ! -L "/lib/x86_64-linux-gnu/${LIB}" ]]; then
        echo "ERROR: Library symlink </lib/x86_64-linux-gnu/${LIB}> does not exists."
        exit 3
    fi
}

# Analyse the src and dest libs and if they link this src lib.
function analyseLibLinks() {
    validateLibsExists
    local correctPathSrcLib="$(readlink -f "/lib/x86_64-linux-gnu/${LIB}")"

    if [[ $(readlink -f "/lib64/${LIB}") = "${correctPathSrcLib}" ]]; then
        echo "[ ok ] $(ls -la "/lib64/${LIB}")"
    else
        echo "[ERR!] $(ls -la "/lib64/${LIB}")"
    fi

    if [[ $(readlink -f "/lib/x86_64-linux-gnu/${LIB}") = "${correctPathSrcLib}" ]]; then
        echo "[ ok ] $(ls -la "/lib/x86_64-linux-gnu/${LIB}")"
    else
        echo "[ERR!] $(ls -la "/lib/x86_64-linux-gnu/${LIB}")"
    fi
}

# Simple list of the symlinks to check.
function showLibLinks() {
    ls -la "/lib64/${LIB}"
    ls -la "/lib/x86_64-linux-gnu/${LIB}"
}

# Removes the old links and set the new symlinks.
function changeLibLinks() {
    sudo rm "/lib64/${LIB}"
    sudo rm "/lib/x86_64-linux-gnu/${LIB}"
    sudo ln -s "/usr/lib/x86_64-linux-gnu/${LIB}" "/lib64/${LIB}"
    sudo ln -s "/usr/lib/x86_64-linux-gnu/${LIB}" "/lib/x86_64-linux-gnu/${LIB}"
}

# Prints the help to stdout.
function printHelp() {
    cat <<EOF
    Project: rlxcrypt
    Desc:    Repair lib error XCRYPT2.x, XCRYPT4.4 not found.
    Version: 0.0.2
    Infos:   Martin Kock <code@deeagle.de>

    Possible options:
        -a | --analyse          Show the actual linked libraries
        -r | --repair           Change the library links to </usr/lib/x86_64-linux-gnu/${LIB}
        -h | --help             prints this help
EOF

}

# Catch arguments and go
if [[ $# -eq 0 ]]; then
    # starts the main function
    printHelp
else
    case "${1}" in
        -a | --analyse)
            analyseLibLinks
            ;;

        -r | --repair)
            validateRootPermissions
            changeLibLinks
            ;;

        -h | --help | *)
            printHelp
            ;;

  esac
fi

exit 0

Sorry but the solution you wrote did not solve for me.
I keep getting the error after the update and go to KP.
I have to restore with rsync to have a bootable system again.

@famarm I'm sorry about that, but good thing you have a backup of the pre-situation.

My system runs clean with this workaround but I run the following cli loop til apt says "nothing to do":

• apt upgrade -yq
• bash repair-libcrypt-v2.sh

Overall, though, this is a really terrible situation. Yes, it is testing, but I would have thought that the kind of bugs are found between unstable and testing.
Let's see how to support the community process in the future.

Good luck too you

After spending even more time on this issue, I found a simple way of fixing all the lib related issues that pop up during the upgrade, since there is a bunch.
Simply install usrmerge, modify the script to remove the files located in the /usr directories when it errors, run till it's finally happy and run the upgrade afterwards. Just works.