lfs-book/chapter04/settingenviron.xml

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<sect1 id="ch-preps-settingenviron">
  <?dbhtml filename="settingenvironment.html"?>

  <title>Setting Up the Environment</title>

  <para>Set up a good working environment by creating two new startup files
  for the <command>bash</command> shell. While logged in as user
  <systemitem class="username">lfs</systemitem>, issue the following command
  to create a new <filename>.bash_profile</filename>:</para>

<screen><userinput>cat &gt; ~/.bash_profile &lt;&lt; "EOF"
<literal>exec env -i HOME=$HOME TERM=$TERM PS1='\u:\w\$ ' /bin/bash</literal>
EOF</userinput></screen>

  <para>When logged on as user <systemitem class="username">lfs</systemitem>,
  the initial shell is usually a <emphasis>login</emphasis> shell which reads
  the <filename>/etc/profile</filename> of the host (probably containing some
  settings and environment variables) and then <filename>.bash_profile</filename>.
  The <command>exec env -i.../bin/bash</command> command in the
  <filename>.bash_profile</filename> file replaces the running shell with a new
  one with a completely empty environment, except for the <envar>HOME</envar>,
  <envar>TERM</envar>, and <envar>PS1</envar> variables. This ensures that no
  unwanted and potentially hazardous environment variables from the host system
  leak into the build environment. The technique used here achieves the goal of
  ensuring a clean environment.</para>

  <para>The new instance of the shell is a <emphasis>non-login</emphasis>
  shell, which does not read the <filename>/etc/profile</filename> or
  <filename>.bash_profile</filename> files, but rather reads the
  <filename>.bashrc</filename> file instead. Create the
  <filename>.bashrc</filename> file now:</para>

<screen><userinput>cat &gt; ~/.bashrc &lt;&lt; "EOF"
<literal>set +h
umask 022
LFS=/mnt/lfs
LC_ALL=POSIX
LFS_TGT=$(uname -m)-lfs-linux-gnu
PATH=/tools/bin:/bin:/usr/bin
export LFS LC_ALL LFS_TGT PATH</literal>
EOF</userinput></screen>

  <para>The <command>set +h</command> command turns off
  <command>bash</command>'s hash function. Hashing is ordinarily a useful
  feature&mdash;<command>bash</command> uses a hash table to remember the
  full path of executable files to avoid searching the <envar>PATH</envar>
  time and again to find the same executable. However, the new tools should
  be used as soon as they are installed. By switching off the hash function,
  the shell will always search the <envar>PATH</envar> when a program is to
  be run. As such, the shell will find the newly compiled tools in
  <filename class="directory">$LFS/tools</filename> as soon as they are
  available without remembering a previous version of the same program in a
  different location.</para>

  <para>Setting the user file-creation mask (umask) to 022 ensures that newly
  created files and directories are only writable by their owner, but are
  readable and executable by anyone (assuming default modes are used by the
  <function>open(2)</function> system call, new files will end up with permission
  mode 644 and directories with mode 755).</para>

  <para>The <envar>LFS</envar> variable should be set to the chosen mount
  point.</para>

  <para>The <envar>LC_ALL</envar> variable controls the localization of certain
  programs, making their messages follow the conventions of a specified country.
  Setting <envar>LC_ALL</envar> to <quote>POSIX</quote> or <quote>C</quote>
  (the two are equivalent) ensures that everything will work as expected in
  the chroot environment.</para>

  <para>The <envar>LFS_TGT</envar> variable sets a non-default, but compatible machine
  description for use when building our cross compiler and linker and when cross
  compiling our temporary toolchain. More information is contained in
  <xref linkend="ch-tools-toolchaintechnotes" role=""/>.</para>

  <para>By putting <filename class="directory">/tools/bin</filename> ahead of the
  standard <envar>PATH</envar>, all the programs installed in <xref
  linkend="chapter-temporary-tools"/> are picked up by the shell immediately after
  their installation. This, combined with turning off hashing, limits the risk
  that old programs are used from the host when the same programs are available in
  the chapter 5 environment.</para>

  <para>Finally, to have the environment fully prepared for building the
  temporary tools, source the just-created user profile:</para>

<screen><userinput>source ~/.bash_profile</userinput></screen>

</sect1>