How to write Shell script

How to write Shell script

Here you can get to know the basic grammar of shell quickly

I. Shell variable

i. Valid format of variable

NAME
LD_LIBTARY_PATH
_var
var2

ii. How to use the variable

var="this is a variable"
echo $var
#Set the var read only
readonly var
echo ${var}
#delete the var
unset var

It’s a good habit to have a pair of brace

II. Shell string

i. joint string

var1="ruboob"
var2='great'
greeting="${var1},${var2}"

ii. Operation string

string="abcdefg"
#get the length of the string
echo ${#string}
#get the four chars of string start from index 1, Attention:  index start from 0
echo ${string:1:4} #get "unoo"
var="This is a string"
echo ${string#*s}    #s is a string
echo ${string##*s}   #tring
echo ${string%s*}    #This is a 
echo ${string%%s*}   #Thi

Search the string

string="abcdefghijkl"
#print the first index of char 'i'
echo `expr index "${string} i"`

III. Shell array

i. Declare an array

#Attention : no comma inside
array_name1=(value0 value1 value2 value3)
#or
array_name2[0]=value0
array_name2[1]=value1
array_name2[2]=value2

ii. Operation array

array_value=${array_name1[n]}
#get all elements of the array
echo ${array_name1[@]}
#get the length of the array
length=${#array_name[@]}
length=${#array_name[*]}
#get the length of the first element of array
length=${#array_name[0]}

IV. Shell Comment

# This is a comment line

:<<EOF
This is a 
triple-line
comment
EOF
# 'EOF' here can be replaced by any other symbol (like ', !)

V. Shell parameter

$ bash ./shell.sh 1 2 3
$0 : the file name of shell
$1 : 1
$2 : 2
$3 : 3
$# : 3 # the number the parameter
$* : show all the parameters as a string
$@ : some as $*
$? : show the status of the last command

VI. Shell Operator

var=`expr 2 + 2`
var=$(expr 2 + 2)
var=`expr 2 \* 3`
a=10
b=20
if [ $a != $b -a $b -gt $(expr $a - $b) ];then
 	echo "hi"
fi
string=""
if [ -n "$string" ]
	echo "The length of the string is 0"
fi
[[ && || > < ]]