LPI Linux Certification in a Nutshell (74 page)

User account information is stored in
/etc/passwd
.

Each line in
/etc/passwd
contains a
username, password, UID, GID, user’s name, home directory, and
default shell.

Group information is stored in
/etc/group
.

Each line in
/etc/group
contains a
group name, group password, GID, and group member list.

passwd
and
group
are world-readable.

To prevent users from obtaining encrypted passwords from
passwd
and
group
, shadow
files are implemented.

Encrypted passwords are moved to a new file, which is
readable only by
root
.

The shadow file for
/etc/passwd
is
/etc/shadow
.

The shadow file for
/etc/group
is
/etc/gshadow
.

The
cron
facility consists of
crond
, the
cron
daemon, and
crontab
files containing
job-scheduling information.

cron
is intended for the execution of
commands on a periodic basis.

crond
examines all
crontab
files every minute.

Each system user has access to
cron
through a personal
crontab
file.

The
crontab
command allows the
crontab
file to be viewed and, with the
–e
option, edited.

Entries in the
crontab
file are in the
form of:

minute hour day month dayofweek command

Asterisks in any of the time fields match all possible
values.

In addition to personal
crontab
files,
the system has its own
crontab
files in
/etc/crontab
, as well as files in
/etc/cron.d
.

The
at
facility, shown here, is
for setting up one-time future command
execution
:

The
atd
daemon must be running in order
for
at
commands to be processed.

Access to
cron
can be controlled using
lists of users in
cron.allow
and
cron.deny
.

Access to
at
can be controlled using
lists of users in
at.allow
and
at.deny
.

The syslog system displays and records messages
describing system events.

The syslog program is made up of two processes:
syslogd
, which logs user-level events, and
klogd
, which logs kernel events.

Messages can be placed on the console, in logfiles, and on
the text screens of users.

Syslog is configured by
/etc/syslog.conf
in the form
facility
.
level
action
:

Together,
facility.levels
comprise the
message selector
.

Most syslog messages go to
/var/log/messages
.

Most system logfiles are rotated to expire old information
and prevent disks from filling up.

logrotate
accomplishes log rotation and
is configured using
/etc/logrotate.conf
.

Files in
/var/log
(such as
messages
) and elsewhere can be examined using
utilities such as
tail
,
less
, and
grep
.

Information in
syslogd
logfiles
includes date, time, origin hostname, message sender, and
descriptive text.

To debug problems using logfile information, first look at
the hostname and sender, and then at the message text.

The Common Unix Printing System (CUPS) is the
current standard for printing on Linux.

The
cupsd
daemon handles print
spooling. Configuration files are stored in
/etc/cups/
, and configuration usually
happens through a web interface accessed through
http://localhost:631/
.

CUPS maintains backward compatibility with
lpd
.

CUPS supports printer configuration in the file
/etc/printcap
for backward
compatibility
.

Printers are assigned to queues, which are managed
by
lpd
, the print daemon.
lpd
listens for inbound print requests,
forking a copy of itself for each active print queue.

lpr
submits jobs to print
queues.

lpq
queries and displays queue
status.

lprm
allows jobs to be removed from
print queues.

lpc
allows root to administer queues;
it has both interactive and command-line forms.

Filters translate data formats into a printer definition
language.

Spool directories hold spooled job data.

New printer definitions are added to
/etc/printcap
:

lp|ljet:\
:sd=/var/spool/lpd/lp:\
:mx#0:\
:sh:\
:lp=/dev/lp0:\
:if=/var/spool/lpd/lp/filter:
:lf=/var/spool/lpd/lp/log:

The lines in this example are defined as follows: