MTD 248-645A Manual de usuario descargar pdf (Pagina 49)

Linux From Scratch - Version 7.0

When logged on as user lfs, the initial shell is usually a login shell which reads the /etc/profile of the host

(probably containing some settings and environment variables) and then .bash_profile. The exec env -i.../bin/

bash command in the .bash_profile file replaces the running shell with a new one with a completely empty

environment, except for the HOME, TERM, and PS1 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.

The new instance of the shell is a non-login shell, which does not read the /etc/profile or .bash_profile

files, but rather reads the .bashrc file instead. Create the .bashrc file now:

cat > ~/.bashrc << "EOF"

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

EOF

The set +h command turns off bash's hash function. Hashing is ordinarily a useful feature—bash uses a hash table

to remember the full path of executable files to avoid searching the PATH 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 PATH when a program is to be run. As such, the shell will find the newly compiled tools

in $LFS/tools as soon as they are available without remembering a previous version of the same program in a

different location.

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 open(2) system

call, new files will end up with permission mode 644 and directories with mode 755).

The LFS variable should be set to the chosen mount point.

The LC_ALL variable controls the localization of certain programs, making their messages follow the conventions of

a specified country. If the host system uses a version of Glibc older than 2.2.4, having LC_ALL set to something other

than “POSIX” or “C” (during this chapter) may cause issues if you exit the chroot environment and wish to return

later. Setting LC_ALL to “POSIX” or “C” (the two are equivalent) ensures that everything will work as expected

in the chroot environment.

The LFS_TGT 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 Section 5.2,

“Toolchain Technical Notes”.

By putting /tools/bin ahead of the standard PATH, all the programs installed in Chapter 5 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.

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

source ~/.bash_profile

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MTD 248-645A Manual de usuario Pagina 49