LPI Linux Certification in a Nutshell (38 page)

• The
  /proc
  filesystem includes
  information on interrupts, I/O ports, and DMA in
  /proc/interrupts
  ,
  /proc/ioports
  , and
  /proc/dma
  .
  

• LILO has historically been the default Linux boot
  loader.
  

• LILO consists of the
  lilo
  command,
  which installs the boot loader, and the boot loader itself.
  

• LILO is configured using
  /etc/lilo.conf
  .
  

• Any modification to the
  /etc/lilo.conf
  file requires the
  lilo
  command to be
  rerun.
  

• GRUB can boot Linux as well as most other PC-based operating
  systems.
  

• GRUB relies on various files in the
  /boot/grub
  directory to support reading from
  various types of filesystems.
  

• GRUB is configured using
  /boot/grub/menu.lst
  (or
  /boot/grub/grub.conf
  on some
  distributions).
  

• GRUB can be configured to present a text or graphical menu
  interface and also has a command-line interface.
  

• Modifications to the GRUB configuration files do not require
  the
  grub
  command to be re-run (unlike
  LILO).
  

• System libraries provide many of the functions
  required by a program.
  

• A program that contains executable code from libraries is
  statically linked
  because
  it stands alone and contains
  all necessary code to execute.
  

• Since static linking leads to larger executable files and
  more resource consumption, system libraries can be shared among
  many executing programs at the same time.
  

• A program that contains references to external, shared
  libraries is
  dynamically linked
  at runtime by
  the dynamic linker,
  ld.so
  .
  

• New locations for shared libraries can be added to the
  LD_LIBRARY_PATH
variable. As an
  alternative, the locations can be added to
  /etc/ld.so.conf
  , which lists library file
  directories. After this, you must run
  /sbin/ldconfig
  to translate this file into
  the binary index
  /etc/ld.so.cache
  .
  

• dpkg
  automates the installation
  and maintenance of software packages and offers a number of
  options.
  

• dselect
  uses a text-based interactive
  menu to select (or deselect) packages for installation.
  

• alien
  can convert packages to and from
  the RPM and Debian package format.
  

• apt-get
  is a powerful tool that
  interfaces with online repositories of Debian packages to install
  and upgrade packages by name and resolves each package’s
  dependencies automatically.
  

• RPM automates the installation and maintenance of
  software packages.
  

• Package dependencies are defined but not resolved
  automatically.
  

• -i
  ,
  -e
  ,
  -U
  ,
  -v
  ,
  -h
  ,
  --nodeps
  , and
  --force
  are common options.
  

• The
  yum
  command is a frontend to RPM,
  interacting with online software repositories to download and
  install software automatically.
  

• The command
  yum update
  will search for
  updates to installed packages, download them, resolve
  dependencies, and install them automatically.
  

• A
  shell
  provides the command
  prompt and interprets commands.
  

• A
  shell variable
  holds a value that is
  accessible to shell programs.
  

• PATH
is a shell variable
  that contains a listing of directories that hold executable
  programs.
  

• Commands must be
  bash
  built-ins, found
  in the
  PATH
, or explicitly
  defined in order to succeed.
  

• When shell variables are
  exported
  , they
  become part of the
  environment
  .
  

• Commands are comprised of a valid command, with or without
  one or more options and arguments, followed by a carriage
  return.
  

• Interactive commands can include looping structures more
  often used in shell scripts.
  

• Shell sessions can be viewed as a conversation.
  History, expansion, and editing make that dialog more
  productive.
  

• Commands can be reissued, modified, and edited. Examples are
  shown in
  Table 10-3
  .
  

• Command substitution allows the
  result
  of a command to be placed into a shell variable.
  

Table 10-3. Shell expansion, editing, and substitution examples

!!

!n

^string1^string2

• Many commands contain either a
  -r
  or
  -R
  option for
  recursive execution through a directory hierarchy.
  

• The
  find
  command is inherently
  recursive, and is intended to descend through directories looking
  for files with certain attributes or executing commands.
  

cat
[file]


Print
file
to standard
output.

cut
[files]


Cut out selected columns or fields from one or more
files
.

expand
[files]


Convert Tabs to spaces in
files
.

fmt
[files]


Format text in
files
to a
specified width by filling lines and removing newline
characters.

head
[files]


Print the first few lines of
files
.

join
file1
file2


Print a line for each pair of input lines, one each from
file1
and
file2
, that have identical join
fields.

nl
[files]


Number the lines of
files
,
which are concatenated in the output.

od
[files]


Dump
files
in octal,
hexadecimal, ASCII, and other formats.

paste
files


Paste together corresponding lines of one or more files
into vertical columns.

pr
[file]


Convert a text file into a paginated, columnar version,
with headers and page fills.

sort
[file]


Sort lines in
file
alphabetically, numerically, or other ways.

split
[infile]
[outfile]


Split
infile
into a specified
number of line groups; the output will go into a succession of
files:
outfilea
a
,
outfilea
b
, and so on.

tac
[file]


Print
file
to standard output
in reverse line order.

tail
[files]


Print the last few lines of one or more files.

tr
[string1
[string2]]


Translate characters by mapping from
string1
to the corresponding
character in
string2
.

unexpand
[files]


Convert spaces to Tabs in
files
.

uniq
[files]


Display only unique lines in
files
that are already sorted.

wc
[files]


Print counts of characters, words, and lines for
files
.

• Filesystem creation prepares a disk device (or
  partition) for use. Linux usually uses the native
  ext3
  (third extended) journaling filesystem,
  but it supports many other filesystem types. You can see a list of
  all the filesystems Linux supports by using the “l” option under
  the
  fdisk
  command.
  

• The Linux filesystem is arranged into a hierarchical
  structure anchored at the
  root directory
  , or
  /
  . Beneath this is a tree of directories and
  files.
  

• Identification information for a filesystem object is stored
  in its
  inode
  (index node), which holds
  location, modification, and security information. Filesystems are
  created with a finite number of inodes.
  

bzip2
[options]
[pattern]


Create or uncompress an archive with the bzip2
algorithm.

cp
file1
file2


cp
files
directory


Copy
file1
to
file2
, or copy
files
to
directory
.

cpio
[options]
[files]


Create or extract a binary archive, containing
either files or a recursive set of files and directories.

dd
[options]
[files]


Copy and convert files. The
dd
command can also copy data from raw devices, bypassing the
filesystem layer.

file
[file]


Determine the type of
file
by performing a number of
tests.

find
[directory] [options]
[pattern]


Search through
directory
looking for objects that
match
pattern
.

gunzip
[options]
[file]


Uncompress an archive created with
gzip
.

gzip
[options]
[pattern]


Create a compressed archive containing files and
directories that match
pattern
.

ls
[options]
[pattern]


List the contents of a directory, or list only
files that match
[pattern]
.

mkdir
directories


Create one or more
directories
.

mv
source
target


Move or rename files and directories.

rm
files


Delete one or more
files
from the filesystem. When used
recursively (with the
-r
option),
rm
also removes directories.

rmdir
directories


Delete
directories
,
which must be empty.

tar
[options]
[files]


Create or extract a Tape Archive, containing
either files or a recursive set of files and directories.

touch
files


Change the access and/or modification times of
files
by default to the present
time.

Table 10-4. File-naming wildcards

*

?

[
characters
]

[!
characters
]

[
a-z
]

[!
a-z
]

{
frag1
,
frag2
,
frag3
,...}

• A central concept for Linux and Unix systems is that
  everything is a file
  .
  

• Many system devices are represented in the filesystem using
  a
  device file
  , such as
  /dev/ttyS0
  for a serial port.
  

• The shell provides the
  standard I/O
  capability, offering three default file descriptors to running
  programs:
  

  • Standard input
    (
    STDIN
    ) is a text input stream, by
    default attached to the keyboard.
    

  • Standard output
    (
    STDOUT
    ) is an output stream for normal
    program output. By default, this is the screen.
    

  • Standard error
    (
    STDERR
    ) is an output stream meant for
    error messages. By default, this is the screen.
    

• It is possible to tie the output of one program to the input
  of another. This is known as a
  pipe
  and is
  created by joining commands using the pipe symbol (
  |
).
  

• Pipes are a special form of
  redirection
  , which allows you
  to manage the origin of input streams and the destination of
  output streams. Redirection syntax for various shells differs
  slightly. See
  Table 10-5
  for
  examples of common redirection operators.
  

  Table 10-5. Common redirection operators
  

  Redirection function	Syntax for bash
  Send STDOUT to file .	$ cmd > file
  	$ cmd 1> file
  Send STDERR to file .	$ cmd 2> file
  Send both STDOUT and STDERR to file .	$ cmd > file 2>&1
  	$ cmd > file 2> file
  Receive STDIN from file .	$ cmd < file
  Append STDOUT to file .	$ cmd >> file
  	$ cmd 1>> file
  Append STDERR to file .	$ cmd 2>> file
  Append both STDOUT and STDERR to file .	$ cmd >> file 2>&1
  Pipe STDOUT from cmd1 to cmd2 .	$ cmd1 | cmd2
  Pipe STDOUT and STDERR from cmd1 to cmd2 .	$ cmd1 2>&1 | cmd2
  Pipe STDOUT from cmd1 to cmd2 while simultaneously writing it to file1 using tee .	$ cmd1 | tee file1 | cmd2