postinst is in libnss-sss 1.16.1-1ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 | #!/bin/sh
set -e
# This code was taken from libnss-myhostname, which got it from nss-mdns:
log() {
    echo "$*"
}
# try to insert sss entries to the passwd, group, shadow and netgroup
# lines in /etc/nsswitch.conf to automatically enable libnss-sss
# support; do not change the configuration if the lines already
# references some sss lookups
insert_nss_entry() {
    log "Checking NSS setup..."
    # abort if /etc/nsswitch.conf does not exist
    if ! [ -e /etc/nsswitch.conf ]; then
        log "Could not find /etc/nsswitch.conf."
        return
    fi
    # append 'sss' to the end of the line if it's not found already
    sed -i --regexp-extended '
      /^(passwd|group|shadow|netgroup|services):/ {
        /\bsss\b/! s/$/ sss/
      }
    ' /etc/nsswitch.conf
}
action="$1"
if [ configure = "$action" ]; then
    if [ -z "$2" ]; then
        log "First installation detected..."
        # first install: setup the recommended configuration (unless
        # nsswitch.conf already contains sss entries)
        insert_nss_entry
    else
        # upgrade
        version="$2"
        # Nothing to do here yet
    fi
fi
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