postinst is in exim4-base 4.76-3ubuntu3.
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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 | #!/bin/sh
set -e
. /usr/share/debconf/confmodule
if [ -n "$EX4DEBUG" ]; then
  echo "now debugging $0 $@"
  set -x
fi
db_version 2.0
BDBVERSION=5.1
case "$1" in
  configure)
	if ! getent passwd Debian-exim > /dev/null ; then
	  echo 'Adding system-user for exim (v4)' 1>&2
	  adduser --system --group --quiet --home /var/spool/exim4 \
	    --no-create-home --disabled-login --force-badname Debian-exim
	fi
	# Create directories for log etc
	# install also fixes permissions.
	install -d -oDebian-exim -gadm -m2750 /var/log/exim4
	install -d -oDebian-exim -gDebian-exim -m750 /var/run/exim4
	install -d -oDebian-exim -gDebian-exim -m750 /var/spool/exim4
	install -d -oDebian-exim -gDebian-exim -m750 /var/spool/exim4/db \
		/var/spool/exim4/input /var/spool/exim4/msglog
	# fix permissions on upgrades
	if dpkg --compare-versions "$2" le "4.30-1" ; then
		find /var/log/exim4 /var/spool/exim4 -group mail \
			\( -type f -or -type d \) -print0 | \
			xargs -0r chgrp Debian-exim
		find /var/log/exim4 /var/spool/exim4 -user mail \
			\( -type f -or -type d \) -print0 | \
			xargs -0r chown Debian-exim
	fi
	
	# Paranoia check: On any db upgrade throw away hints
	# databases.
	if test -r /var/lib/exim4/berkeleydbvers.txt ; then
		OLDBDB=`head -n1 /var/lib/exim4/berkeleydbvers.txt`
	else
		OLDBDB="unknown"
	fi
	if [ "$BDBVERSION" != "$OLDBDB" ] ; then
		echo exim: DB upgrade, deleting hints-db 1>&2
		rm -f /var/spool/exim4/db/misc-* /var/spool/exim4/db/wait-* \
			/var/spool/exim4/db/callout* \
			/var/spool/exim4/db/retry* \
			/var/spool/exim4/db/ratelimit* \
			/var/spool/exim4/db/__db.retry \
			/var/spool/exim4/db/__db.misc* \
			/var/spool/exim4/db/__db.callout \
			/var/spool/exim4/db/__db.ratelimit \
			/var/spool/exim4/db/__db.wait* \
			/var/spool/exim4/db/log.*
		echo "$BDBVERSION" > /var/lib/exim4/berkeleydbvers.txt
	fi
	# Check that db files are readable by this Exim's db library
	dbfiles=""
	for f in /var/spool/exim4/db/misc-* /var/spool/exim4/db/wait-* \
	         /var/spool/exim4/db/callout* /var/spool/exim4/db/retry* \
		 /var/spool/exim4/db/ratelimit* ; do
	    if [ -f "$f" ]; then
		if echo $f | grep \.lockfile\$ >/dev/null 2>&1; then
		    : # ignore lock files
		else
		    dbfiles="$dbfiles $(basename $f)"
		fi
	    fi
	done
	for dbfile in $dbfiles; do
	    if exim_dumpdb /var/spool/exim4 $dbfile >/dev/null 2>&1; then
		: # File OK
	    else
		echo "Resetting invalid $dbfile hints db" 1>&2
		rm -f /var/spool/exim4/db/$dbfile \
			/var/spool/exim4/db/$dbfile.* \
			/var/spool/exim4/db/__db.${dbfile}.* \
			/var/spool/exim4/db/log.*
	    fi
	done
       if [ -x "/etc/init.d/exim4" ]; then
           update-rc.d exim4 defaults >/dev/null
       fi
	# honor dpkg-statoverride settings for files not managed with dpkg
	for pat in /var/\*/exim4 /var/\*/exim4/\*; do
		[ $EX4DEBUG ] && eval echo "evaluate statoverride $pat"
		eval dpkg-statoverride --list $pat | while read USER GROUP MODE FILE; do
			[ $EX4DEBUG ] && echo "statoverride $USER $GROUP $MODE $FILE"
			chown ${USER}:${GROUP} $FILE
			chmod $MODE $FILE
		done
	done
	;;
esac
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