/etc/default/useradd is mandatory for the correct operation of useradd.

tests/split_groups/01_useradd_split_group/config/etc/default/useradd

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+# Default values for useradd(8)
+#
+# The SHELL variable specifies the default login shell on your
+# system.
+# Similar to DHSELL in adduser. However, we use "sh" here because
+# useradd is a low level utility and should be as general
+# as possible
+SHELL=/bin/sh
+#
+# The default group for users
+# 100=users on Debian systems
+# Same as USERS_GID in adduser
+# This argument is used when the -n flag is specified.
+# The default behavior (when -n and -g are not specified) is to create a
+# primary user group with the same name as the user being added to the
+# system.
+GROUP=100
+#
+# The default home directory. Same as DHOME for adduser
+HOME=/home
+#
+# The number of days after a password expires until the account 
+# is permanently disabled
+INACTIVE=-1
+#
+# The default expire date
+EXPIRE=
+#
+# The SKEL variable specifies the directory containing "skeletal" user
+# files; in other words, files such as a sample .profile that will be
+# copied to the new user's home directory when it is created.
+SKEL=/etc/skel
+#
+# Defines whether the mail spool should be created while
+# creating the account
+CREATE_MAIL_SPOOL=no

tests/split_groups/02_useradd_no_split_group/config/etc/default/useradd

@@ -0,0 +1,36 @@
+# Default values for useradd(8)
+#
+# The SHELL variable specifies the default login shell on your
+# system.
+# Similar to DHSELL in adduser. However, we use "sh" here because
+# useradd is a low level utility and should be as general
+# as possible
+SHELL=/bin/sh
+#
+# The default group for users
+# 100=users on Debian systems
+# Same as USERS_GID in adduser
+# This argument is used when the -n flag is specified.
+# The default behavior (when -n and -g are not specified) is to create a
+# primary user group with the same name as the user being added to the
+# system.
+GROUP=100
+#
+# The default home directory. Same as DHOME for adduser
+HOME=/home
+#
+# The number of days after a password expires until the account 
+# is permanently disabled
+INACTIVE=-1
+#
+# The default expire date
+EXPIRE=
+#
+# The SKEL variable specifies the directory containing "skeletal" user
+# files; in other words, files such as a sample .profile that will be
+# copied to the new user's home directory when it is created.
+SKEL=/etc/skel
+#
+# Defines whether the mail spool should be created while
+# creating the account
+CREATE_MAIL_SPOOL=no

tests/split_groups/03_useradd_split_group_already_split/config/etc/default/useradd

@@ -0,0 +1,36 @@
+# Default values for useradd(8)
+#
+# The SHELL variable specifies the default login shell on your
+# system.
+# Similar to DHSELL in adduser. However, we use "sh" here because
+# useradd is a low level utility and should be as general
+# as possible
+SHELL=/bin/sh
+#
+# The default group for users
+# 100=users on Debian systems
+# Same as USERS_GID in adduser
+# This argument is used when the -n flag is specified.
+# The default behavior (when -n and -g are not specified) is to create a
+# primary user group with the same name as the user being added to the
+# system.
+GROUP=100
+#
+# The default home directory. Same as DHOME for adduser
+HOME=/home
+#
+# The number of days after a password expires until the account 
+# is permanently disabled
+INACTIVE=-1
+#
+# The default expire date
+EXPIRE=
+#
+# The SKEL variable specifies the directory containing "skeletal" user
+# files; in other words, files such as a sample .profile that will be
+# copied to the new user's home directory when it is created.
+SKEL=/etc/skel
+#
+# Defines whether the mail spool should be created while
+# creating the account
+CREATE_MAIL_SPOOL=no

tests/split_groups/04_useradd_split_group_already_full/config/etc/default/useradd

@@ -0,0 +1,36 @@
+# Default values for useradd(8)
+#
+# The SHELL variable specifies the default login shell on your
+# system.
+# Similar to DHSELL in adduser. However, we use "sh" here because
+# useradd is a low level utility and should be as general
+# as possible
+SHELL=/bin/sh
+#
+# The default group for users
+# 100=users on Debian systems
+# Same as USERS_GID in adduser
+# This argument is used when the -n flag is specified.
+# The default behavior (when -n and -g are not specified) is to create a
+# primary user group with the same name as the user being added to the
+# system.
+GROUP=100
+#
+# The default home directory. Same as DHOME for adduser
+HOME=/home
+#
+# The number of days after a password expires until the account 
+# is permanently disabled
+INACTIVE=-1
+#
+# The default expire date
+EXPIRE=
+#
+# The SKEL variable specifies the directory containing "skeletal" user
+# files; in other words, files such as a sample .profile that will be
+# copied to the new user's home directory when it is created.
+SKEL=/etc/skel
+#
+# Defines whether the mail spool should be created while
+# creating the account
+CREATE_MAIL_SPOOL=no

tests/split_groups/07_useradd_split_group_already_split_user_in_both_lines/config/etc/default/useradd

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+# Default values for useradd(8)
+#
+# The SHELL variable specifies the default login shell on your
+# system.
+# Similar to DHSELL in adduser. However, we use "sh" here because
+# useradd is a low level utility and should be as general
+# as possible
+SHELL=/bin/sh
+#
+# The default group for users
+# 100=users on Debian systems
+# Same as USERS_GID in adduser
+# This argument is used when the -n flag is specified.
+# The default behavior (when -n and -g are not specified) is to create a
+# primary user group with the same name as the user being added to the
+# system.
+GROUP=100
+#
+# The default home directory. Same as DHOME for adduser
+HOME=/home
+#
+# The number of days after a password expires until the account 
+# is permanently disabled
+INACTIVE=-1
+#
+# The default expire date
+EXPIRE=
+#
+# The SKEL variable specifies the directory containing "skeletal" user
+# files; in other words, files such as a sample .profile that will be
+# copied to the new user's home directory when it is created.
+SKEL=/etc/skel
+#
+# Defines whether the mail spool should be created while
+# creating the account
+CREATE_MAIL_SPOOL=no