shadow/man/login.1.xml

<?xml version="1.0" encoding="UTF-8"?>
<refentry id='login.1'>
  <!--  $Id$ -->
  <refmeta>
    <refentrytitle>login</refentrytitle>
    <manvolnum>1</manvolnum>
    <refmiscinfo class="sectdesc">User Commands</refmiscinfo>
  </refmeta>
  <refnamediv id='name'>
    <refname>login</refname>
    <refpurpose>begin session on the system</refpurpose>
  </refnamediv>
  <!-- body begins here -->
  <refsynopsisdiv id='synopsis'>
    <cmdsynopsis>
      <command>login</command>
      <arg choice='opt'>-p </arg>
      <arg choice='opt'>
      <replaceable>username</replaceable></arg>
      <arg choice='opt' rep='repeat'> <replaceable>ENV=VAR</replaceable></arg>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>login</command>
      <arg choice='opt'>-p </arg>
      <arg choice='opt'>-h <replaceable>host</replaceable></arg>
      <arg choice='opt'>-f <replaceable>username</replaceable></arg>
    </cmdsynopsis>
    <cmdsynopsis>
      <command>login</command>
      <arg choice='opt'>-p </arg>
      <arg choice='plain'>-r <replaceable>host</replaceable></arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1 id='description'>
    <title>DESCRIPTION</title>
    <para>
      The <command>login</command> program is used to establish a new session
      with the system. It is normally invoked automatically by responding to
      the <emphasis remap='I'>login:</emphasis> prompt on the user's
      terminal. <command>login</command> may be special to the shell and may
      not be invoked as a sub-process. Typically, <command>login</command>
      is treated by the shell as <emphasis remap='B'>exec login</emphasis>
      which causes the user to exit from the current shell. Attempting to
      execute <command>login</command> from any shell but the login shell
      will produce an error message.
    </para>

    <para>
      The user is then prompted for a password, where appropriate. Echoing
      is disabled to prevent revealing the password. Only a small number of
      password failures are permitted before <command>login</command> exits
      and the communications link is severed.
    </para>

    <para>
      If password aging has been enabled for your account, you may be
      prompted for a new password before proceeding. You will be forced to
      provide your old password and the new password before continuing. 
      Please refer to <citerefentry>
      <refentrytitle>passwd</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry> for more information.
    </para>

    <para>
      After a successful login, you will be informed of any system messages
      and the presence of mail. You may turn off the printing of the system
      message file, <filename>/etc/motd</filename>, by creating a
      zero-length file <filename>.hushlogin</filename> in your login directory. 
      The mail message will be one of "<emphasis>You have new
      mail.</emphasis>", "<emphasis>You have mail.</emphasis>", or
      "<emphasis>No Mail.</emphasis>" according to the condition of your
      mailbox.
    </para>

    <para>
      Your user and group ID will be set according to their values in the
      <filename>/etc/passwd</filename> file. The value for
      <envar>$HOME</envar>, <envar>$SHELL</envar>, <envar>$PATH</envar>,
      <envar>$LOGNAME</envar>, and <envar>$MAIL</envar> are set according
      to the appropriate fields in the password entry. Ulimit, umask and nice
      values may also be set according to entries in the GECOS field.
    </para>

    <para>
      On some installations, the environmental variable
      <envar>$TERM</envar> will be initialized to the terminal type on
      your tty line, as specified in <filename>/etc/ttytype</filename>.
    </para>

    <para>
      An initialization script for your command interpreter may also be
      executed. Please see the appropriate manual section for more
      information on this function.
    </para>

    <para>
      A subsystem login is indicated by the presence of a "*" as the first
      character of the login shell. The given home directory will be used as
      the root of a new file system which the user is actually logged into.
    </para>

    <para>
      The <command>login</command> program is NOT responsible for removing
      users from the utmp file. It is the responsibility of
      <citerefentry><refentrytitle>getty</refentrytitle>
      <manvolnum>8</manvolnum></citerefentry> and
      <citerefentry><refentrytitle>init</refentrytitle>
      <manvolnum>8</manvolnum></citerefentry> to clean up apparent ownership
      of a terminal session. If you use <command>login</command> from the
      shell prompt without <command>exec</command>, the user you use will
      continue to appear to be logged in even after you log out of the
      "subsession".
    </para>

  </refsect1>

  <refsect1 id='options'>
    <title>OPTIONS</title>
    <variablelist remap='IP'>
      <varlistentry>
	<term>
	  <option>-f</option>
	</term>
	<listitem>
	  <para>Do not perform authentication, user is preauthenticated.
	  </para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-h</option>
	</term>
	<listitem>
	  <para>Name of the remote host for this login.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-p</option>
	</term>
	<listitem>
	  <para>Preserve environment.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term>
	  <option>-r</option>
	</term>
	<listitem>
	  <para>Perform autologin protocol for rlogin.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <para>
      The <option>-r</option>, <option>-h</option> and <option>-f</option>
      options are only used when <command>login</command> is invoked by
      root.
    </para>
  </refsect1>

  <refsect1 id='caveats'>
    <title>CAVEATS</title>
    <para>
      This version of <command>login</command> has many compilation options,
      only some of which may be in use at any particular site.
    </para>

    <para>The location of files is subject to differences in system
      configuration.
    </para>

    <para>
      The <command>login</command> program is NOT responsible for removing
      users from the utmp file. It is the responsibility of <citerefentry>
      <refentrytitle>getty</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry> and <citerefentry>
      <refentrytitle>init</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry> to clean up apparent ownership of a terminal session. 
      If you use <command>login</command> from the shell prompt without
      <command>exec</command>, the user you use will continue to appear to
      be logged in even after you log out of the "subsession".
    </para>

    <para>
      As with any program, <command>login</command>'s appearance can be faked. 
      If non-trusted users have physical access to a machine, an
      attacker could use this to obtain the password of the next person
      coming to sit in front of the machine. Under Linux, the SAK mechanism can be
      used by users to initiate a trusted path and prevent this kind of
      attack.
    </para>

  </refsect1>

  <refsect1 id='files'>
    <title>FILES</title>
    <variablelist>
      <varlistentry>
	<term><filename>/var/run/utmp</filename></term>
	<listitem>
	  <para>List of current login sessions.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/var/log/wtmp</filename></term>
	<listitem>
	  <para>List of previous login sessions.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/passwd</filename></term>
	<listitem>
	  <para>User account information.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/shadow</filename></term>
	<listitem>
	  <para>Secure user account information.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/motd</filename></term>
	<listitem>
	  <para>System message of the day file.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/nologin</filename></term>
	<listitem>
	  <para>Prevent non-root users from logging in.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>/etc/ttytype</filename></term>
	<listitem>
	  <para>List of terminal types.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><filename>$HOME/.hushlogin</filename></term>
	<listitem>
	  <para>Suppress printing of system messages.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id='see_also'>
    <title>SEE ALSO</title>
    <para>
      <citerefentry>
	<refentrytitle>mail</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>passwd</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>sh</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>su</refentrytitle><manvolnum>1</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>login.defs</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>nologin</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>passwd</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>securetty</refentrytitle><manvolnum>5</manvolnum>
      </citerefentry>,
      <citerefentry>
	<refentrytitle>getty</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>.
    </para>
  </refsect1>
</refentry>