POSTIN

/bin/sh

# Add fish to the list of allowed shells in /etc/shells
if [ "$1" = 1 ]; then
  if ! [ -f "/etc/shells" ] || ! grep -q '^/usr/bin/fish$' "/etc/shells"; then
    echo '/usr/bin/fish' >>"/etc/shells"
  fi
fi

POSTUN

/bin/sh

# Remove fish from the list of allowed shells in /etc/shells
if [ "$1" = 0 ]; then
  sed -i -e '\#^/usr/bin/fish$#d' "/etc/shells"
fi