backup user has not valid shell any more

The login shell of the backup user seems to have changed in Ubuntu 14.04 LTS from /bin/sh to /usr/sbin/nologin during the upgrade from 12.04 LTS.

This is a problem for backupninja that uses "su backup" at some places, for example for the mysql backup:

/usr/share/backupninja/mysql in line 248:

databases=$(set -o pipefail ; su $user -c "$MYSQL $defaultsfile -N -B -e 'show databases'" | sed 's/|//g;/\+----/d')

This command will fail with no further explanation. "su backup" does not work any more, the error message that occurs when executing su backup manually is:

This account is currently not available

When changing the shell back to /bin/sh everything seems to work again.