Linux-Shell-programming

1 Variable

in Shell, you can create a varibale with VARIABLE_NAME=value

by convention, use uppercase for variable name, and no space betwwen =.

when you need to output an varaible, put $ before variable name.

if you want to assign command output to an varibale, wrap command with $(command) or

VARIABLE_NAME=`command`

notice, when use variable inside a quote, use double quote "", single quote will ignore variable.

1.1 export and environment variable

if you want a variable to be accessible globally in the mcahine, export it as a environment variable in /etc/profile

export VARIABLE_NAME=value

1.2 positional parameter

positional paramters are arguments that follows the command/shell

such as command arg1 arg2

$0 refers to the command itsel

$1 refers to the first arg

$2 refers to the second arg

$* refers to all positional parameters

$@ refers to all positional params

$# refers to number of args

1.3 internal variables

$$ the PID of current process $! the PID of last background process $? the result of last bg process, if return 0, it means normal, not 0, something wrong.

1.4 expression in Shell

to declare an expression, there are three ways

$((expression))

$[expression]

expr num1 + num2

if you want to assign an expression to a variable, use ``

A=3
B=6
C=`$(($A+$B))`

echo $C

1.5 comparison operator

-eq = -ne != -gt > -ge >= -lt < -le <=

2. Conditions

2.1 if elif control

if [ condition ]
then command
elif [ condition ]
then command
else
command
fi

example

#!/bin/bash

if [ $1 -lt 60 ]
then
	echo "you failed your exam"
elif [ $1 -ge 60 ]
then
	echo "congratulations"
fi

2.2 case control

case  $variable-name  in
             pattern1|pattern2|pattern3)       
         command1
                 ...
                 ....
                 commandN
                 ;;
             pattern4|pattern5|pattern6)
         command1
                 ...
                 ....
                 commandN
                 ;;            
             pattern7|pattern8|patternN)       
         command1
                 ...
                 ....
                 commandN
                 ;;
             *)              
esac

#!/bin/bash

case $1 in
	1)
		echo "janurary"
		;;
	2)
		echo "february"
		;;
	3)
		echo "march"
		;;
	4)
		echo "April"
		;;
	*)
		echo "other month"
		;;
esac

2.3 for loop

for index in  something

do
  //code to run

done

#!/bin/bash

for i in $*
do
	echo $i
done

echo ========================================

for j in $@
do
	echo $j
done

echo ========================================

for k in "$*"
do
	echo $k
done

echo ========================================

for l in "$@"
do
	echo $l
done

or the (i =1; i < 100; i ++) style loop

#!/bin/bash

sum=0

for ((i=0; i <= $1; i++))
do
	sum=$(($sum + $i))
	echo "sum is now @ $sum"
done

2.4 while loop

#!/bin/bash

i=$1

while [ $i -ge 0 ]
do
	i=$(($i-1))
echo $i
done

2.5 The read prompt

read is kinda like prompt in javascript, -t for timeout, -p for message

read -p "the message you want to display" VARIABLE1

echo $VARIABLE1

read -t 10 -p "message to display" VARIABLE2

echo $VARIABLE2

#!/bin/bash

read -p "please enter the value for name  " NAME

echo "you name is $NAME"

read -t 10 -p "What is your age?  " AGE
echo "So you are $AGE years old?"

3. function

3.1 builtin function

basename will return the file name

basename /home/chao/Desktop/Linux-Shell-programming/position.sh
basename /home/chao/Desktop/Linux-Shell-programming/position.sh .sh

will respectively return postion.sh and psotion, as directory path is trimmed.

dirname will do the opposite

dirname /home/chao/Desktop/Linux-Shell-programming/position.sh

will return /home/chao/Desktop/Linux-Shell-programming/

if you want to define a function

function myFunc(){
  //code to
  return //something but optional
}

myFunc

note calling a function in shell don't need "()"

#!/bin/bash

read -p "enter the first nnumber" num1

read -p "enter the second number" num2

function myfunc(){
	sum=$(($num1+$num2))
	echo $sum
	return $sum

}

myfunc

echo $sum

if you want to display time

date +%y-%m-%d@%H:%M:%S

3.2 array in Shell

to create an array VARIABLE=(el1 el2 el3 ...)

to access an element inside an array ${array[index]}, index starting from 0

to access to all elements in an array, use ${array[*]} or ${array[@]}

to check the length of array ${#array[@]}

example array

#!/bin/bash

strangerThings=("Eleven" "Will" "Mike" "Dustin" "Lucas" "Steve" "Nancy" "Johnathon" "Joyce" "Hopper" "Max" "Billy" "Henry")

echo "the second element is ${strangerThings[1]}"

echo "we have ${strangerThings[@]} in the cast"

echo "there are ${#strangerThings[*]} in the array"

3.3 parameter expansion

3.3.1 empty parameter

mike=

echo ${mike:-"boy"}

echo $mike

"boy" was temporarily expanded into mike, but mike is still empty

mike=

echo ${mike:="boy"}

echo $mike

"boy" was expanded, and assigned to mike, therefore, mike now has a value of boy

3.3.2 non-empty parameter

will="boy"

echo ${will:+"man"}

echo $will

echo ${will:+"man"} will display man, while echo $will display boy, as value of $will is not changed, but temporarily replaced once.

3.3.3 check length of a string

str="In 1980s Indiana, a group of young friends witness supernatural forces and secret government exploits. As they search for answers, the children unravel a series of extraordinary mysteries."

echo ${#str}

echo ${#str} display 188, ${#variable} display string length

Chao216 / Linux-Shell-programming