Understand
Linux Configuration Files
What are system
configuration files?
The
kernel itself may be considered a "program." Why does the
kernel need configuration files? The kernel needs to know the list of
users and groups in the system, and manage file permissions (that is,
determine if a file can be opened by a specific user, according to
the permissions, UNIX_USERS). Note that these files are not
specifically read by programs, but by a function provided by a system
library, and used by the kernel. For instance, a program needing the
(encrypted) password of a user should not open the /etc/passwd file.
Instead, it should call the system library function getpw(). This
kind of function is also known as a system call. It is up to the
kernel (through the system library) to open the /etc/passwd file and
after that, search for the password of the requested user.
Most
of the configuration files in the Red Hat Linux system are in the
/etc directory unless otherwise specified. The configuration files
can be broadly classified into the following categories:
Access files
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/etc/host.conf
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Tells
the network domain server how to look up hostnames. (Normally
/etc/hosts, then name server; it can be changed through netconf.)
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/etc/hosts
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Contains
a list of known hosts (in the local network). Can be used if the
IP of the system is not dynamically generated. For simple
hostname resolution (to dotted notation), /etc/hosts.conf
normally tells the resolver to look here before asking the
network nameserver, DNS or NIS.
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/etc/hosts.allow
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Man
page same as hosts_access. Read by tcpd at least.
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/etc/hosts.deny
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Man
page same as hosts_access. Read by tcpd at least.
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Booting and
login/logout
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/etc/issue
& /etc/issue.net
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These
files are read by mingetty (and similar programs) to display a
"welcome" string to the user connecting from a terminal
(issue) or through a telnet session (issue.net). They include a
few lines stating the Red Hat release number, name, and Kernel
ID. They are used by rc.local.
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/etc/redhat-release
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Includes
one line stating the Red Hat release number and name. Used by
rc.local.
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/etc/rc.d/rc
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Normally
run for all run levels with level passed as argument. For
example, to boot your machine in the Graphics mode (X-Server),
run the following command from your command line: init
5. The runlevel 5
is starts the system in graphics mode.
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/etc/rc.d/rc.local
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Not
official. May be called from rc, rc.sysinit, or /etc/inittab.
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/etc/rc.d/rc.sysinit
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Normally
the first script run for all run levels.
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/etc/rc.d/rc/rcX.d
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Scripts
run from rc (X
stands for any number from 1 to 5). These directories are
"run-level" specific directories. When a system starts
up, it identifies the run-level to be initiated, and then it
calls all the startup scripts present in the specific directory
for that run-level. For example, the system usually starts up and
the message "entering run-level 3" is shown after the
boot messages; this means that all the init scripts in the
directory /etc/rc.d/rc3.d/ will be called.
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File system
The
kernel provides an interface to display some of its data structures
that can be useful for determining the system parameters like
interrupts used, devices initialised, memory statistics, etc. This
interface is provided as a separate but dummy filesystem known as the
/proc filesystem. Many system utilities use the values present in
this filesystemf or displaying the system statistics. For example,
the file /proc/modules lists the currently loaded modules in the
system. This information is read by the command lsmod, which then
displays it in a human readable format. In the same way, the file
mtab specified in the following table reads the /proc/mount file,
which contains the currently mounted filesystems.
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/etc/mtab
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This
changes continuously as the file /proc/mount changes. In other
words, when filesystems are mounted and unmounted, the change is
immediately reflected in this file.
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/etc/fstab
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Lists
the filesystems currently "mountable" by the computer.
This is important because when the computer boots, it runs the
command mount
-a, which takes
care of mounting every file system marked with a "1" in
the next-to-last column of fstab.
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/etc/mtools.conf
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Configuration
for all the operations (mkdir, copy, format, etc.) on a DOS-type
filesystem.
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System
administration
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/etc/group
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Contains
the valid group names and the users included in the specified
groups. A single user can be present in more than one group if he
performs multiple tasks. For example, is a "user" is
the administrator as well as a member of the project group
"project 1", then his entry in the group file will look
like: user:
* : group-id : project1
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/etc/nologin
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If
the file /etc/nologin exists, login(1) will allow access only to
root. Other users will be shown the contents of this file and
their logins refused.
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etc/passwd
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See
"man passwd". Holds some user account info including
passwords (when not "shadowed").
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/etc/rpmrc
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rpm
command configuration. All the rpm command line options can be
set together in this file so that all of the options apply
globally when any rpm command is run on that system.
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/etc/securetty
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Contains
the device names of tty lines (one per line, without leading
/dev/) on which root is allowed to login.
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/etc/usertty
/etc/shadow
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Contains the encrypted password
information for users' accounts and optionally the password aging
information. Included fields are:
Login
name
Encrypted
password
Days
since Jan 1, 1970 that password was last changed
Days
before password may be changed
Days
after which password must be changed
Days
before password is to expire that user is warned
Days
after password expires that account is disabled
Days
since Jan 1, 1970 that account is disabled
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/etc/shells
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Holds
the list of possible "shells" available to the system.
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/etc/motd
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Message
Of The Day; used if an administrator wants to convey some message
to all the users of a Linux server.
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Networking
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/etc/gated.conf
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Configuration
for gated. Used only by the gated daemon.
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/etc/gated.version
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Contains
the version number of the gated daemon.
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/etc/gateway
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Optionally
used by the routed daemon.
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/etc/networks
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Lists
names and addresses of networks accessible from the network to
which the machine is connected. Used by route command. Allows use
of name for network.
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/etc/protocols
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Lists
the currently available protocols. See the NAG (Network
Administrators Guide) and man page.
C interface is
getprotoent. Should never change.
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/etc/resolv.conf
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Tells
the kernel which name server should be queried when a program
asks to "resolve" an IP Address.
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/etc/rpc
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Contains
instructions/rules for RPC, which can be used in NFS calls,
remote file system mounting, etc.
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/etc/exports
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The
file system to be exported (NFS) and permissions for it.
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/etc/services
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Translates
network service names to port number/protocol. Read by inetd,
telnet, tcpdump, and some other programs. There are C access
routines.
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/etc/inetd.conf
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Config
file for inetd. See the inetd man page. Holds an entry for each
network service for which inetd must control daemons or other
servicers. Note that services will be running, but comment them
out in /etc/services so they will not be available even if
running. Format: <service_name> <sock_type> <proto>
<flags> <user> <server_path> <args>
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/etc/sendmail.cf
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The
Mail program sendmail's configuration file. Cryptic to
understand.
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/etc/sysconfig/network
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Indicates
NETWORKING=yes or no. Read by rc.sysinit at least.
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/etc/sysconfig/network-scripts/if*
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Red
Hat network configuration scripts.
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System commands
System
commands are meant exclusively to control the system, and make
everything work properly. All the programs like login (performing the
authentication phase of a user on the console) or bash (providing the
interaction between a user and the computer) are system commands. The
files associated with them are therefore particularly important. This
category has the following files of interest to users and
administrators.
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/etc/lilo.conf
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Contains
the system's default boot command line parameters and also the
different images to boot with. You can see this list by pressing
Tab at the LILO prompt.
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/etc/logrotate.conf
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Maintains
the log files present in the /var/log directory.
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/etc/identd.conf
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Identd
is a server that implements the TCP/IP proposed standard IDENT
user identification protocol as specified in the RFC 1413
document. identd operates by looking up specific TCP/IP
connections and returning the user name of the process owning the
connection. It can optionally return other information instead of
a user name. See the identd man page.
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/etc/ld.so.conf
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Configuration
for the Dynamic Linker.
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/etc/inittab
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This
is chronologically the first configuration file in UNIX. The
first program launched after a UNIX machine is switched on is
init, which knows what to launch, thanks to inittab. It is read
by init at run level changes, and controls the startup of the
main process.
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/etc/termcap
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A
database containing all of the possible terminal types and their
capabilities.
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Daemons
A
daemon is a program running in non-interactive mode. Typically,
daemon tasks are related to the networking area: they wait for
connections, so that they can provide services through them. Many
daemons are available for Linux, ranging from Web servers to ftp
servers.
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/etc/syslogd.conf
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The
configuration file for the syslogd daemon. syslogd is the daemon
that takes care of logging (writing to disk) messages coming from
other programs to the system. This service, in particular, is
used by daemons that would not otherwise have any means of
signaling the presence of possible problems or sending messages
to users.
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/etc/httpd.conf
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The
configuration file for Apache, the Web server. This file is
typically not in /etc. It may be in /usr/local/httpd/conf/ or
/etc/httpd/conf/, but to make sure, you need to check the
particular Apache installation.
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/etc/conf.modules
or /etc/modules.conf
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The
configuration file for kerneld. Ironically, it is not the kernel
"as a daemon". It is rather a daemon that takes care of
loading additional kernel modules "on the fly" when
needed.
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User programs
In
Linux (and UNIX in general), there are countless "user"
programs. A most common user program config file is /etc/lynx.cfg.
This is the configuration file for lynx, the well-known textual
browser. Through this file you can define the proxy server, the
character set to use, and so on. The following code sample shows a
part of the lynx.cfg file that can be modified to change the proxy
settings of the Linux system. These settings apply (by default) to
all the users running lynx in their respective shells, unless a user
overrides the default config file by specifying --cfg
= "mylynx.cfg.
Proxy
settings in /etc/lynx.cfg
.h1 proxy
.h2 HTTP_PROXY
.h2 HTTPS_PROXY
.h2 FTP_PROXY
.h2 GOPHER_PROXY
.h2 NEWS_PROXY
.h2 NNTP_PROXY
# Lynx version 2.2 and beyond supports the use of proxy servers that can act as
# firewall gateways and caching servers. They are preferable to the older
# gateway servers. Each protocol used by Lynx can be mapped separately using
# PROTOCOL_proxy environment variables (see Lynx Users Guide). If you have
# not set them externally, you can set them at run time via this configuration file.
# They will not override external settings. The no_proxy variable can be used
# to inhibit proxying to selected regions of the Web (see below). Note that on
# VMS these proxy variables are set as process logicals rather than symbols, to
# preserve lowercasing, and will outlive the Lynx image.
#
.ex 15
http_proxy:http://proxy3.in.ibm.com:80/
ftp_proxy:http://proxy3.in.ibm.com:80/
#http_proxy:http://penguin.in.ibm.com:8080
#ftp_proxy:http://penguin.in.ibm.com:8080/
.h2 NO_PROXY
# The no_proxy variable can be a comma-separated list of strings defining
# no-proxy zones in the DNS domain name space. If a tail substring of the
# domain-path for a host matches one of these strings, transactions with that
# node will not be proxied.
.ex
no_proxy:demiurge.in.ibm.com, demiurge
Changing
configuration files
When
changing a configuration file, make sure that the program using that
configuration is restarted if it's not controlled by the system
administrator or the kernel. A normal user doesn't usually have
privileges to start or stop system programs and/or daemons.
The
kernel
Changing
configuration files in the kernel immediately affects the system. For
example, changing the passwd file to add a user immediately enables
that user. Also there are some kernel tunable parameters in the
/proc/sys directory on any Linux system. The write-access to all
these files is given only to the super-user; other users have only
readonly access. The files in this directory are classified in the
same manner as the Linux kernel source. Every file in this directory
represents a kernel data structure that can be dynamically modified
to change the system performance.
Note:
Before changing any value in any of these files, make sure you know
everything about the file to avoid irreparable damage to the system.
Files
in the /proc/sys/kernel/ directory
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File
name
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Description
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threads-max
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The
maximum number of tasks the kernel can run.
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ctrl-alt-del
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If
1, then pressing this key sequence cleanly reboots the system.
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sysrq
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If
1, then Alt-SysRq is active.
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osrelease
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Displays
the release of the operating system.
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ostype
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Displays
the type of the operating system.
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hostname
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The
host name of the system.
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domainname
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Network
domain of which the system is a part.
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modprobe
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Specifies
whether modprobe should be automatically run at startup, and load
the necessary modules.
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Daemons
and system programs
A
daemon is a program that is always running in background, quietly
carrying out its task. Common ones are in.ftpd (ftp server daemon),
in.telnetd (telnet server daemon), and syslogd (system logging
daemon). Some daemons, while running, keep a close watch on the
configuration file and reload it automatically when it changes. But
most of the daemons do not reload automatically. We need to "tell"
them somehow that the configuration file has changed and that it
should be reloaded. This can be achieved (on Red Hat Linux systems)
by restarting the services using the service command.
For
example, if we have changed the network configuration, we need to
issue: service
network restart.
Note:
The services are most commonly the scripts present in the
/etc/rc.d/init.d/* directory and are started by the init when the
system is booted. So, to restart the service you can also do the
following: /etc/rc.d/init.d/<script-for-the-service>
start | stop | status
start,
stop, and status are the values that these scripts take as input to
perform the action.
User
programs
A
user or system program reads its configuration file every time it is
launched. Remember, though, that some system programs are spawned
when the computer is turned on, and their behaviour depends on what
they read in the configuration files in /etc/. So, the first time a
user program is started, the default configuration is read from the
files present in the /etc/ directory. Later, the user can customise
the programs by using rc and . (dot) files as explained in the next
section.
User configuration
files: . (dot) files and rc files
We
have seen how programs can be easily configured. But what if someone
does not like the way a program has been configured in /etc/? A
"normal" user cannot simply go into /etc and change the
configuration files; they are owned -- from the filesystem's point of
view -- by root! This is why most user programs define two
configuration files: the first one at a "system" level,
located in /etc/; and the other one, "private" to the user,
that can be found in his or her home directory.
For
example, in my system I have installed the very useful wget utility.
In /etc/ there is an /etc/wgetrc file. In my home directory, there is
a file named .wgetrc, which describes my customised configuration
(which will be loaded only when I, the user run the wget command).
Other users may also have the .wgetrc file in their home directory
(/home/other); this file will be read, of course, only when the user
runs the wget command. In other words, the /etc/wgetrc file provides
"default" values for wget, while the /home/xxx/.wgetrc file
lists the "customisations" for a certain user. It is
important to understand that this is the "general rule,"
and is not necessarily true for all cases. A program like pine, for
instance, does not have any files in /etc/, but only the custom
configuration in the users' home directory, in a file named .pinerc.
Other programs may only have a default configuration file in /etc/,
and may not let users "customize" them (it's the case with
only a few of the config. files in the /etc dir.).
Commonly
used rc and . (dot) files
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Filename
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Description
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~/.bash_login
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Look
at "man bash". Treated by bash like ~/.bash_profile if
that doesn't exist.
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~/.bash_logout
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Look
at "man bash".Sourced by bash login shells at exit.
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~/.bash_profile
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Sourced
by bash login shells after /etc/profile.
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~/.bash_history
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The
list of commands executed previously.
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~/.bashrcÂ
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Look
at "man bash". Sourced by bash non-login interactive
shells (no other files are). Non-interactive shells source
nothing unless BASH_ENV or ENV are set.
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~/.emacs
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Read
by emacs at startup.
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~/.forward
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If
this contains an e-mail address, then all mail to owner of ~ will
be forwarded to that e-mail address.
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~/.fvwmrc
~/.fvwm2rc
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Config
files for fvwm and fvwm2 (the basic X Window manager).
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~/.hushlogin
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Look
at "man login". Causes a "quiet" login (no
mail notice, last login info, or MOD).
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~/.mail.rc
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User
init file for mail program.
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~/.ncftp/
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Directory
for ncftp program; contains bookmarks, log, macros, preferences,
trace. See man ncftp. The purpose of ncftp is to provide a
powerful and flexible interface to the Internet standard File
Transfer Protocol. It is intended to replace the stock ftp
program that comes with the system.
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~/.profile
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Look
at "man bash". Treated by bash like ~/.bash_profile if
that and ~/.bash_login don't exist, and used by other
Bourn-heritage shells too.
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~/.pinerc
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Pine
configuration
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~/.muttrc
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Mutt
configuration
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~/.exrc
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Configuration
of vi can be controlled by this file.
Example: set ai sm
ruler
Writing the above line in this file makes vi set the
auto-indentation, matching brackets and displaying line number
and rows-columns options.
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~/.vimrc
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Default
"Vim" configuration file. Same as .exrc.
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~/.gtkrc
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GNOME
Toolkit.
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~/.kderc
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KDE
configuration.
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~/.netrc
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Default
login names and passwords for ftp.
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~/.rhosts
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Used
by the r-tools: rsh, rlogin, etc. Very weak security since host
impersonation is easy.
Must
be owned by user (owner of ~/) or superuser.
Lists
hosts from which users may access this account.
Ignored
if it is a symbolic link.
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~/.rpmrc
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See
"man rpm". Read by rpm if /etc/rpmrc is not present.
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~/.signature
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Message
text that will be appended automatically to the mail sent from
this account.
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~/.twmrc
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Config
file for twm (The
Window
Manager).
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~/.xinitrc
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Read
by X at startup (not by xinit script). Mostly starts some progs.
Example: exec /usr/sbin/startkde
If the above line is
present in this file, then the KDE Window Manager is started in
when the startx command is issued from this account.
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~/.xmodmaprc
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This
file is passed to the xmodmap program, and could be named
anything (~/.Xmodmap and ~/.keymap.km, for example).
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~/.xserverrc
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Run
by xinit as the X server if it can find X to execute.
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~/News/Sent-Message-IDs
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Default
mail history file for gnus.
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~/.Xauthority
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Read
and written by xdm program to handle authorization. See the X,
xdm, and xauth man pages.
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~/.Xdefaults,
~/.Xdefaults-hostname
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Read
by X applications during startup on hostname. If the -hostname
file can't be found, .Xdefaults is looked for.
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~/.Xmodmap
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Points
to .xmodmaprc;
Red Hat had (has) .xinitrc using this name.
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~/.Xresources
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Usually
the name for the file passed to xrdb to load the X resources
database, to avoid the need for applications to read a long
.Xdefaults file. (~/.Xres has been used by some.)
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~/mbox
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User's
old mail.
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