diff --git a/xCAT-test/unit/package_modular_pkgdir.t b/xCAT-test/unit/package_modular_pkgdir.t
new file mode 100644
index 000000000..35e4bd58e
--- /dev/null
+++ b/xCAT-test/unit/package_modular_pkgdir.t
@@ -0,0 +1,176 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+use File::Copy qw(copy);
+use File::Path qw(make_path);
+use File::Spec;
+use File::Temp qw(tempdir);
+use FindBin;
+use lib "$FindBin::Bin/../lib";
+use Test::More;
+
+use XCAT::Test::File qw(repo_path);
+
+my $library = repo_path('xCAT/postscripts/xcatpkgutils.sh');
+my $loader  = repo_path('xCAT/postscripts/xcatpkgutils-loader.sh');
+
+my @modular_osvers = qw(
+  rhel8 rhel9.6 rhel10.0 rhels8.10 rhels19
+  centos8-stream centos10 rocky9.4 rocky19
+  alma8 alma10 almalinux9 almalinux19 ol8 ol10
+);
+my @other_osvers = ( '', qw(rhel rhel1 rhel7.9 rhel20 ubuntu24.04 RHEL9 el9) );
+
+SKIP: {
+    skip 'the shared predicate is introduced by the production commit',
+      scalar( @modular_osvers + @other_osvers )
+      unless shared_predicate_available();
+
+    for my $osver (@modular_osvers) {
+        is( predicate_status($osver), 0,
+            "$osver uses modular package directories" );
+    }
+    for my $osver (@other_osvers) {
+        is( predicate_status($osver), 1,
+            ( $osver || 'an empty OS version' )
+              . ' does not use modular package directories' );
+    }
+}
+
+for my $caller (qw(ospkgs otherpkgs)) {
+    is_deeply(
+        [ caller_repository_paths( $caller, 'rocky9.4' ) ],
+        [
+            'package-test-server:INSTALLDIR/rocky9.4/x86_64/BaseOS',
+            'package-test-server:INSTALLDIR/rocky9.4/x86_64/AppStream',
+        ],
+        "$caller expands a modular OS into BaseOS and AppStream"
+    );
+    is_deeply(
+        [ caller_repository_paths( $caller, 'rocky7.9' ) ],
+        ['package-test-server:INSTALLDIR/rocky7.9/x86_64'],
+        "$caller retains the base directory for a non-modular OS"
+    );
+}
+
+done_testing();
+
+sub predicate_status {
+    my ($osver) = @_;
+    my $status = system(
+        '/bin/sh', '-c', '. "$1"; xcat_is_el_modular_pkgdir "$2"',
+        'package-modular-pkgdir-test', $library, $osver
+    );
+    die "Unable to execute /bin/sh: $!" if $status == -1;
+    die "/bin/sh terminated by signal " . ( $status & 127 )
+      if $status & 127;
+    return $status >> 8;
+}
+
+sub shared_predicate_available {
+    return system(
+        '/bin/sh', '-c',
+        '. "$1"; command -v xcat_is_el_modular_pkgdir >/dev/null 2>&1',
+        'package-modular-pkgdir-test', $library
+    ) == 0;
+}
+
+sub caller_repository_paths {
+    my ( $caller, $osver ) = @_;
+    my $tmpdir = tempdir( CLEANUP => 1 );
+    my $bindir = File::Spec->catdir( $tmpdir, 'bin' );
+    my $trace = File::Spec->catfile( $tmpdir, 'repository-paths.trace' );
+    make_path($bindir);
+
+    my $mount = File::Spec->catfile( $bindir, 'mount' );
+    write_fixture( $mount, "#!/bin/sh\nexit 1\n" );
+    chmod 0755, $mount or die "Unable to make $mount executable: $!";
+    my $uname = File::Spec->catfile( $bindir, 'uname' );
+    write_fixture( $uname, "#!/bin/sh\nprintf '%s\\n' Linux\n" );
+    chmod 0755, $uname or die "Unable to make $uname executable: $!";
+
+    for my $source (
+        $library, $loader, repo_path("xCAT/postscripts/$caller")
+      )
+    {
+        my $filename = ( File::Spec->splitpath($source) )[2];
+        my $destination = File::Spec->catfile( $tmpdir, $filename );
+        copy( $source, $destination )
+          or die "Unable to stage $source as $destination: $!";
+        chmod 0755, $destination
+          or die "Unable to make $destination executable: $!";
+    }
+
+    my $bash_env = File::Spec->catfile( $tmpdir, 'bash-env.sh' );
+    write_fixture(
+        $bash_env,
+        <<'SH'
+logger()
+{
+    case " $* " in
+        *" NFSSERVER="*)
+            size=$(array_get_size os_path)
+            index=0
+            while [ "$index" -lt "$size" ]; do
+                array_get_element os_path "$index"
+                index=$((index + 1))
+            done > "$XCAT_REPOSITORY_PATH_TRACE"
+            exit 73
+            ;;
+    esac
+    return 0
+}
+SH
+    );
+
+    local %ENV = %ENV;
+    delete @ENV{
+        qw(HTTPPORT KERNELDIR MASTER NODESETSTATE OTHERPKGDIR OSPKGDIR VERBOSE)
+    };
+    $ENV{ARCH} = 'x86_64';
+    $ENV{BASH_ENV} = $bash_env;
+    $ENV{INSTALLDIR} = 'INSTALLDIR';
+    $ENV{NFSSERVER} = 'package-test-server';
+    $ENV{OSVER} = $osver;
+    $ENV{PATH} = "$bindir:$ENV{PATH}";
+    $ENV{UPDATENODE} = 1;
+    $ENV{XCAT_REPOSITORY_PATH_TRACE} = $trace;
+    if ( $caller eq 'ospkgs' ) {
+        $ENV{OSPKGS} = 'package-test';
+        delete @ENV{qw(OTHERPKGS OTHERPKGS_INDEX)};
+    } else {
+        $ENV{OTHERPKGS_INDEX} = 1;
+        delete @ENV{qw(OSPKGS)};
+    }
+
+    my $script = File::Spec->catfile( $tmpdir, $caller );
+    my $status = system($script);
+    die "Unable to execute $script: $!" if $status == -1;
+    die "$script terminated by signal " . ( $status & 127 )
+      if $status & 127;
+    die "$script exited unexpectedly with " . ( $status >> 8 )
+      unless ( $status >> 8 ) == 73;
+
+    my $contents = read_fixture($trace);
+    chomp($contents);
+    return split /\n/, $contents;
+}
+
+sub write_fixture {
+    my ( $path, $contents ) = @_;
+    open( my $fh, '>:raw', $path )
+      or die "Unable to open $path for writing: $!";
+    print {$fh} $contents or die "Unable to write $path: $!";
+    close($fh) or die "Unable to close $path: $!";
+}
+
+sub read_fixture {
+    my ($path) = @_;
+    open( my $fh, '<:raw', $path )
+      or die "Unable to open $path for reading: $!";
+    my $contents = do { local $/; <$fh> };
+    die "Unable to read $path: $!" unless defined $contents;
+    close($fh) or die "Unable to close $path: $!";
+    return $contents;
+}