#** snippets

#Core concepts

############ ##############

bash_profile --gets executed only when you log in

bashrc -- runs when a new shell is initiated likes of opening a new terminal.

export PATH= $PATH:$HOME/Desktop -- Appending new path to $PATH variable.

alias lZ='ls -alZ | more' --sets alias

bash -- creates a new shell , runs bashrc. { exec bash will consume. the same process ID and do the same thing}

bash_history -- history of what we typed as the current user.

HISTCONTROL-- variable to change the behaviour of bash_history

export HISTCONTROL=$HISTCONTROL:ignorespace --- once set , any command written in the shell after the space will not be tracked by bash_history

bash_logout -- last thing that happens when we issue the log out command . works when we do a clean exit .

#!/bin/bash --simply means that the script needs to be executed in a bash shell

`` -- commands enclosed in back ticks gets executed for eg "TODAYDATE=`date' -- will execute command date and store the value in TODAYDATE."

shopt -s expand_aliases --- turns on shell scripting expanding aliases with in the subshell.

exit status 0 -- good

exit status non-zero -- bad

echo $? -- shows the last exit status .

set -e -- lets the shell know to exit the moment encountered with an error.

expr -- evaluate mathematic expression . expr 2 + 2 gives the value 4.

expr 2 + 2 --addition

expr 10 / 2 -- division

expr 2 \* 4 --- multipication

expr 4 - 2 ---subtraction.

expr \( 2 + 2 \) \* 4 -- grouping to set order of precedence and escaping the brackets using \.

env -- gives the list of all the global variables

set -- gives the list of local variable to shell.

escape sequence -- \ (back slash)

/dev/null redirects the command standard output to the null device, which is a special device which discards the information written to it.

read "variable name " --- reads the interactive input and stores it in the variable.

echo ${master=prabin} -- sets value to the variable and dispalys at the same time in the command line

declare -i NEWVAR=10 --explicitly defines NEWVAR as an integer.

declare -r VAR="this is a string" --- explicity defines the varibale as read only.

**There is a distinction in Linux between shell variables, and environment variables. In Linux, shell variables are only in the current shell, and Environment variables, are in that shell and all child shells.

shell variables are set with e.g. just a=5

environment variables are set with export, export also sets the shell variable**

**Files and directories whose names begin with a dot (.) by default are not displayed in directory listings by the standard command ls. Therefore, they are traditionally used to store settings, preferences, etc..There are also two special directory names in this class: the directory named simply . is an alias for the same directory in which it appears (a self reference), and the directory named .. refers to the parent directory of **

**Best practice make your variables caps and put the values in double quotes and no space between variable and value**

**Enclosing characters in single quotes (') preserves the literal value of each character within the quotes.**

**Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !**

#Array

#############################

Array elements are space seperated

MYARRAY=("first" "second" "third") --- array

echo ${MYARRAY[0]} --- dispaly an array value indexed at first posiiton.

echo ${MYARRAY[*]} --- displays everything out in thte array.

MYARRAY[3]="Fourth" -- appends the array with the new value indexed in the number mentioned.

**code snippet array iteration***

ARRAY=("web1" "web2" "web3")

for INDEX in ${ARRAY[@]} ; do

echo "the server is $INDEX "

done

**code snippet array iteration using count***

NEWARRAY=("prema" "benny" "bittu" "bikku")

COUNT=0

for INDEX in ${NEWARRAY[@]}; do

echo " The name is ${NEWARRAY[$COUNT]}"

#COUNT="$COUNT + 1" --- works as well.

COUNT=`expr $COUNT + 1`

done

#Passing varibale to script at the command line

#####################################################

** code snippet for passing varibales**

USERNAME=$1

PASSWORD=$2

echo "The user name is $USERNAME and password is $PASSWORD"

**

variales passed in from the command line from the first and second position delimited by space will be stored in $1 and $2 respectively.

#The if statement

######################################################

**code snippet guess number

echo "Enter a number between 1 and 5"

read NUMBER

if [ $NUMBER -eq 3 ] ; then

echo "You have guessed the correct number : $NUMBER"

fi

**

**cide snippet for fileexiistance and readable check

FILENAME=$1

if [ -f $FILENAME ] && [ -r $FILENAME ] ; then

echo " the file $FILENAME exist and is readable "

fi

**

*** code snippets for nested if ***

echo "Guess a number between 1 and 3"

read NUMBER

if [ $NUMBER -eq 1 ] 2>/dev/null; then

echo "You have entered #1"

elif [ $NUMBER -eq 2 ] 2>/dev/null; then

echo "You have entered #2"

elif [ $NUMBER -eq 3 ] 2>/dev/null; then

echo "You have entered #3"

else

echo "You havent followed the directions"

fi

**

** 2>/dev/null -- dumps the error to the blackhole.

#Forloop

###################################################

** code snippet for iterating through a listing and cating the output to a file

LISTING=`ls *.sh`

for ITEMS in $LISTING ; do

echo $ITEMS

`cat $ITEMS >> /home/bernard/Scripts/dump.txt`

done

#while loop

**code snippet for display the number of times hello world.

COUNT=1

echo "Enter the dispaly number"

read DISPLAYNUMBER

while [ $COUNT -le $DISPLAYNUMBER ] ; do

echo "Hello World : $COUNT"

COUNT="`expr $COUNT + 1`"

done

#Execution Operator

#################################################

2>/dev/null -- dumps the error to the blackhole.

rm master.txt 2> /dev/null && echo "File exist and was removed" || echo "File doesnt exist and cant be removed" --- the 'and' is executed only when the remove command is true else 'or' is executed

** code snippet demonstrating execution operators.

echo "Enter a number between 1 and 5"

read NUMBER

if [ $NUMBER -eq 1 ] || [ $NUMBER -eq 3 ] || [ $NUMBER -eq 5 ] 2> /dev/null ; then

echo "You have entered an OOD NUMBER "

elif [ $NUMBER -lt 3 ] && [ $NUMBER -gt 1 ] 2> /dev/null ; then

echo "You have entered my favourite number"

fi

#Reading files

#################################

**code snippet for reading file using while

FILENAME=$1

while read -r line ; do

echo " Names : $line"

done < "$FILENAME"

**code snippet to reading file using for

FILENAME=`cat list`

for ITEM in $FILENAME ;do

echo "Name : $ITEM"

done

#File descriptors

##################################

** code snippet for reading and writing using file descriptors

FILENAMEREAD=$1

FILENAMEWRITE=$2

exec 5<$FILENAMEREAD

exec 6>$FILENAMEWRITE

while read -r LINE ; do

if [[ $LINE =~ bittu ]] ; then

echo "i love $LINE " >&6

fi

done <&5

exec 5>&-

exec 6>&-

#IFS and Delimiting

IFS -- internal field seperator ---

** code snippet using IFS

FILENAME=$1

IFS=' '

while read -r processor memory cpu company; do

echo "processor : $processor"

echo "memory : $memory"

echo "cpu : $cpu"

echo "company : $company"

done < $FILENAME

# trap and signals

##########################

trap is a command which helps us to look for the occurance of something likes of events , signals

** code snippet for reading and writing using file descriptors

trap 'echo " - Please press Q to exit"' SIGINT SIGTERM SIGTSTP

while [ "$CHOICE" != "Q" ] && [ "$CHOICE" != "q" ] ; do

echo "MAIN MENU"

echo "---------"

echo "1)DEV"

echo "2)TEST"

echo "3)UAT"

echo "4)PROD"

echo "Q)EXIT"

echo "--------"

echo "Enter your Choice:"

read CHOICE

clear

done

#Debugging

#############################

set -x --start debug

set +x --stop debug

#Error Handling

#################

#!/bin/bash

cd master 2> /dev/null

if [ $? = 0 ] ; then

ls -al

else

echo "Directory doesnt exist"

fi

#Simple functions

####################

Funcions are way of grouping like commands to a single referable name

: -- null command

fucntion () {

echo "Hello World"

}

fucntion

#Function return and exit

** code snippet to show funciton return and exit.

YES=0

NO=1

FIRST=$1

SECOND=$2

THIRD=$3

function complex () {

if [ ! -z "$THIRD" ] ; then

return "$YES"

else

return "$NO"

fi

}

complex

RETURN_VAL=$?

echo "$RETURN_VAL"

if [ $RETURN_VAL -eq 0 ] ; then

echo "We received all the parameters"

else

echo "Usage: complexfucntion.sh [param1] [param2] [param3]"

fi