Linux+ Chapter9

Front 1

Env variable holding computer's name

set an environment variable manually via dm

Back 1

$HOSTNAME

$ export HOSTNAME=carson.example.com

Front 2

find out how environment variables are confi gured by typing

Back 2

env cmd

Front 3

Display env variable HOSTNAME with help from grep

Back 3

$ env | grep HOSTNAME

Front 4

Env. variable that is your current username

Env. variable that is your current shell

Back 4

$USER / $USERNAME

$SHELL

Front 5

Env. variable that prints the working dir

Env. variable that point to you home dir

Back 5

$PWD

$HOME

Front 6

Env. variable $PATH

Back 6

Sets the path for a session, which is a colon-delimited list of dir in which Linux searches for executable programs

Front 7

Env. variable that holds the location of the users mailbox

Env. variable that hold your current language

Back 7

$MAIL

$LANG

Front 8

Env. variable $LD_LIBRARY_PATH desc

Back 8

$LD_LIBRARY_PATH - Program uses this env. var to indicate dir in which library files may be found

Front 9

Env. variable $PS1

Back 9

This is the default prompt in bash. It generally includes variables of its own, such as \u (for the username), \h (for the hostname), and \W (for the current working directory). This value is frequently set in /etc/profile, but it’s often overridden by users.

Front 10

Env. var $NNTPSERVER

Back 10

Some Usenet news reader programs use this environment variable to specify the name of the news server system. This value may be set in /etc/profile or in the user’s configuration files.

Front 11

Env. var $TERM

Env. var $DISPLAY

Env. var $EDITOR

Back 11

Current terminal type

Identifies the display used by X: Usually is :0.0

Some programs launch the program point by this env. var when the need to call a text editor to use

Front 12

The PATH variable sometimes includes the current directory indicator (.) why is this a security risk?

Back 12

The PATH variable sometimes includes the current directory indicator (.) so that programs in the current directory can be run This practice poses a security risk, though, because a miscreant can create a program with the same name as some other program (such as ls) and trick another user into running it by simply leaving it in a directory the victim frequents Even the root user may be victimized this way For this reason, it’s best to omit the
current directory from the PATH variable, especially for the superuser If it’s really needed for ordinary users, put it at the end of the path

Front 13

run a program in the background by typing

Back 13

its name and appending an ampersand (&)

Front 14

To implement an alias, you use the following syntax:

If you want to use an option as the default for your alias, you can use alias

Back 14

alias alias_name=’commands‘

$ alias ls=’ls --color’

Front 15

Common bash config files global - login and non login

Back 15

Login File Location
/etc/profile
/etc/profile.d

Non-Login File Location
/etc/bashrc
/etc/bash.bashrc

Front 16

Common bash config files user - login and non-login

Back 16

Login file location
~/.bash_login
~/.profiles
~/.bash_profile

Non-Login file location
~/.bashrc

Front 17

bash logout scripts location

Back 17

~/.bash_logout

most disto dont put them in the user home dir

Front 18

bash configuration file which helps customize your keyboard config

Back 18

~/.inputrc
Consist of line like this
M-Control-u: universal-argument
above line means Meta-Ctrl-U is mapped to the universal-argument action

meta = ESC

Front 19

A shell script begins with a line that identifies the shell that’s used to run it, such as the following:

Back 19

#!/bin/sh

#! - special code that tells the Linux kernel that this is a script and to use the rest of the line as a pathname to the program that’s to interpret the script.

Front 20

comment character in bash scripts

Back 20

#

for the #!/bin/sh
the script utility ignores this line
the kernel dosent

Front 21

Once you are done writing a shell script you should modify it so its executable by using the _______ cmd

Back 21

chmod +x
+x adds executable permissions

to make my-script exec you should issue
$ chmod a+x my-scirpt

Front 22

to make my-script exec you should issue

Back 22

$ chmod a+x my-scirpt

Front 23

to tell linux to run the script in the current dir you should precede by ____

Back 23

./

Front 24

if you want to run an non exec script run called my-script

Back 24

bash my-sciprt

Front 25

Simple Script That Launches Three Programs

Back 25

#!/bin/bash
/usr/bin/xterm &
/usr/bin/xterm &
/usr/bin/kmail &

Front 26

What if you have an & at the end of a cmd in a script

Back 26

tells the shell to go on to the next line without waiting for the first line to finish

Front 27

For instance, users’ home directories appear in the sixth colon-delimited field of the /etc/ passwd file. You can therefore type _____________ to extract this inforamtion

Back 27

cut -f 6 -d “:” /etc/passwd

Front 28

variables that are passed to the script are frequently called _____________ and are represented by a _______________ sign followed by number from 0 to 9

Back 28

parameters
dollar

$0 stands for the name of the script
$1 for the first parameter

Front 29

shift command expl

Back 29

shifts the parameter variables, so that what would ordinarily be $2 becomes $1, what would be $3 becomes $2, and so on Adding a number, as in shift 3, shifts the assignments by that number of units The shift command does not alter the $0 variable

Front 30

using variable create a script that
creates an account
changes the account’s password (you’ll be prompted to enter the password when you run the script)
creates a directory in the /shared directory tree corresponding to the account,
sets a symbolic link to that directory from the new user’s home directory
adjusts ownership and permissions in a way that may be useful, depending on your system’s ownership and permissions policies.

Back 30

#!/bin/sh
useradd -m $1
passwd $1
mkdir -p /shared/$1
chown $1.users /shared/$1
chmod 775 /shared/$1
ln -s /shared/$1 /home/$1/shared
chown $1.users /home/$1/shared

2, you need type only three things: the script name with the desired username and the password (twice).

mkuser ajones

Front 31

declare variable in a script

Back 31

ping="/bin/ping"
ip=`route -n | grep UG | tr -s “ “ | cut -f 2 -d “ “`

when you’re assigning a value to a variable from the output of a command, that command should be enclosed in back-quote characters (`),

Front 32

script with user interaction for entering variable

Back 32

#!/bin/sh
echo -n “Enter a username: “
read name
useradd -m $name

Front 33

bash script conditional statement if expl

Back 33

The if keyword’s conditional expression appears in brackets after the if keyword and can take many forms.

Front 34

bash if statment primary expressions
if [-f file]
if [-s file]
if [ STRING1 == STRING2 ]

Back 34

-f - True if FILE exists and is a regular file.
-s - True if FILE exists and has a size greater than zero.
[ STRING1 == STRING2 ] True if the strings are equal. "=" may be used instead o

Front 35

bash if statment primary expressions
if [-a file]
if [-d file]
if [ -x file]

Back 35

[ -a FILE ] True if FILE exists.
[ -d FILE ] True if FILE exists and is a directory.
[ -x FILE ] True if FILE exists and is executable.

Front 36

Conditionals may be combined together with the logical and (&&) or logical or (||) operators. When conditionals are combined with &&, both sides of the operator must be ____________ for the condition as a whole to be true
When || is used, if either side of the operator is true, the condition as a whole is __________

Back 36

true

true

Front 37

script to exit if the file /tmp/tempstuff is present and is larger than 0 bytes

Back 37

if [ -s /tmp/tempstuff ]
then
echo “/tmp/tempstuff found; aborting!”
exit
fi

fi (if backward) marks the end of the if block

Front 38

An alternative form for a conditional expression uses the test keyword rather than square brackets around the conditional

Back 38

if test -s /tmp/tempstuff

Front 39

You can also test a command’s return value by using the command as the condition

Back 39

if [ command ]
then
additional-commands
fi

in this example additional-commands will be run only if command completes successfully

Front 40

Conditional expressions may be expanded by use of the ___________ clause

Back 40

else

if [ conditional-expression ]
then
commands
else
other-commands
fi

Front 41

What do you do if more than two outcomes are possible—for instance, if a user may
provide any one of four possible inputs?

Back 41

case word in
pattern1) command(s) ;;
pattern2) command(s) ;;
...
esac

word is a variable

each patter is a value

Front 42

Script That Executes a Command on Every Matching File in a Directory

Back 42

#!/bin/bash
for d in `ls *.wav` ; do
aplay $d
done

aplay command is a basic audio file player

Front 43

seq command

Back 43

can be useful in creating for loops
This command generates a list of numbers starting from its first argument and continuing to its last one. For instance, typing seq 1 10 generates 10 lines, each with a number between
You can use a for loop beginning for x in `seq 1 10` to have the loop execute 10 times, with the value of x incrementing with each iteration.

Front 44

while loop

Back 44

while [ condition ]
do
commands
done

The until loop is similar in form, but it continues execution for as long as its condition
is false—that is, until the condition becomes true.

Front 45

function is a part of a script that performs a specific sub-task and that can be called by name from other parts of the script. How you define func?

Back 45

myfn() {
commands
}

keyword function may optionally precede the function name

Front 46

demonstrates the use of functions in a simple program that copies a file but aborts with an error message if the target file already exists. This script accepts a target and a destination filename and must pass those filenames
to the functions

Back 46

#/bin/bash
doit() {
cp $1 $2
}

function check() {
if [ -s $2 ]
then
echo “Target file exists! Exiting!”
exit
fi
}
check $1 $2
doit $1 $2

Front 47

The mail server holds incoming messages for each user, typically in a file in

Back 47

/var/spool/mail

Front 48

what folders howl mail for the user benf

Back 48

/var/spool/mail/benf

Front 49

mail spool def

Back 49

Some e‑mail servers store incoming mail in subdirectories of the users’ home directories, though. This incoming mail file or directory is referred to as the user’s mail spool.

Front 50

sendmail prog

Back 50

sendmail program was for many years the dominant e‑mail server package on the Internet

Front 51

Postfix prog

Back 51

Postfix was designed as a modular replacement for sendmail.
Uses multiple programs each with its own specific task

Front 52

Exim prog

Back 52

is a monolithic server, like sendmail, it has a much simpler configuration file format and so is easier to configure. A few Linux distributions use Exim as the default e‑mail server.

Front 53

qmail prog

Back 53

Like Postfix and Exim, qmail is easier to configure than sendmail. It’s not the standard e‑mail server in any Linux distribution because its license is a bit strange and complicates qmail distribution with Linux; however, many system administrators like qmail enough that they replace their distributions’ standard e‑mail
servers with qmail.

Front 54

You can learn which e‑mail server your Linux distribution runs in several ways. The two most reliable are to use ps

Back 54

$ ps ax |grep send
31129 pts/2 R+ 0:00 grep send
$ ps ax | grep post
7778 ? Ss 0:45 /usr/lib/postfix/master
31132 pts/2 S+ 0:00 grep post

Front 55

Pull mail servers

Back 55

Two pull mail protocols, POP and IMAP, are popular. If a Linux system should function as a mail server from which users can read their e‑mail remotely, chances are you’ll install a POP or an IMAP server package, such as Cyrus IMAP or Dovecot.

Front 56

Fetchmail prog

Back 56

If you run a small site that relies on an external ISP for e‑mail delivery, chances are the ISP supports only POP or IMAP. If you want to use a variety of e‑mail clients, you may want to run your own SMTP server, and perhaps your own POP or IMAP server, to deliver mail locally. To do this, you need a program that pulls mail using POP or IMAP and then injects it into a local SMTP mail queue. This is the job of Fetchmail

Front 57

nail cmd

Back 57

some linux distro ship with the nail cmd which is similar to mail but supports more features

Front 58

mail program is intended to be used on the command line to send or receive messages.
The basic syntax for mail

Back 58

mail [-v] [-s subject] [-c cc-addr] [-b bcc-addr] to-addr
mail [-v] [-f [name] | -u user]

$ mail -s “Meeting reminder” -c [e-mail] [e-mail]
Remember the meeting at 4:00 today!

Front 59

This line, if included in a script, sends the contents of /tmp/alert.txt to [e-mail] with the specified subject.

Back 59

mail -s “Automated alert!” < /tmp/alert.txt [e-mail]

Front 60

main tool to help in e‑mail queue management

Back 60

mailq program is the main tool to help in e-mail queue management without any options, shows the contents of the e‑mail queue on all systems:
$ mailq
-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
5B42F963F* 440 Sat Aug 23 13:58:19 [e-mail]
[e-mail]
-- 0 Kbytes in 1 Request.

Front 61

you want to clear the mail queue immediately what cmd will you use

Back 61

sendmail -q

Front 62

postmaster account usage

Back 62

all e‑mail servers are supposed to maintain an account called postmaster. E‑mail to this account should be read by somebody who’s responsible for maintaining the system. One way to do this is to set
up an alias linking the postmaster name to the name of a real account. You can do this by editing the aliases file, which usually resides in /etc or sometimes in /etc/mail

Front 63

aliases file format

Back 63

used in email server to set up email forwarding

comment line #

name: addr1[,addr2[,...]]

a typical config includes a few map aliases to root.
Reading emails through root is not advisable

Front 64

CMD to learn what databases are defined in you mysql

CMD to see all tables in a DB called test

Back 64

SHOW DATABASE;

cmds are terminated by semicolons
------------------------------------------------
USE test;
SHOW TABLES;

Front 65

CMD to create a DB called test in mysql

Back 65

CREATE DATABASE test;