Linux shell variable

Variable type
I always thought that the bash shell variables are non-typed and can also be specified.

Administrator@DADI ~$ declare -i num=10Administrator@DADI ~$ echo $num10Administrator@DADI ~$ num='iamaint'Administrator@DADI ~$ echo $num0

After the type is forcibly specified using the declare method, the variable does not receive non- Int.
Constant definition Syntax:

readonly OPTION VARIABLE(s)

After definition, the variable cannot be re-assigned or unset

Administrator@DADI ~$ readonly MY_NAME='Administrator'Administrator@DADI ~$ MY_NAME='ANOTHERNAME'-bash: MY_NAME: readonly variableAdministrator@DADI ~$ unset MY_NAME-bash: unset: MY_NAME: cannot unset: readonly variable

Two definition methods for Array Operations

ARRAY=(value1 value2 ... valueN)ARRAY[0]="no.1 element"

Element

myarray=(1 2 3 4 5 6 7 8)echo ${myarray[0]}myarray[0]="new value 1"

Number of array elements

echo ${#myarray[*]}

Do not confuse with the following. Only the length of the first element of myarray is returned.

echo ${#myarray}

Traversal

echo ${myarray[*]}for i in ${myarray[*]} ; do echo $i;done

Continuous length can also be used

array_len=${#myarray[*]}i=1while [ $i -lt $array_len ]do  echo ${myarray[$i]};  i=$[ $i + 1 ];donedone;

String operation substring

${VAR:OFFSET:LENGTH}

Administrator@DADI ~$ mystring="123456789"Administrator@DADI ~$Administrator@DADI ~$ echo ${mystring:2:2}34Administrator@DADI ~$ echo ${mystring:2:4}3456

Replace

${VAR/PATTERN/STRING}

$ echo ${mystring/2/new2}1new23456789