flow control: if, and the 'test expression'

if [ commands/test expression ]; then

    commands

elif [ commands /test expression ]; then

    commands

else

    commands

fi

Test Expression in IF : is envaluated either true or false; if true, execute following commands, then exit 0; elif false, exit 1 directly, no any commands executed;

1. String Test Expression

string test expressions

2. File Test Expression

file test expressions

Integer expressions

Advanced Test: 

1).  syntax [[  expression  ]] , this advanced test supports a new string expression:

    string1 =~ regex

### the string operator ' =~ ' means ' string1 matchs the regular expression regex';

this '[[   ]]' still supports 'pattern matching' with operator '==', examples:

Sting [[ ]]

Advanced test for integer:  synx  ((   )), used to perform arithmetic truth test; it's Truth if the expression result is non-zero.

a=1

(( b = a + 1 )) 

echo $b   ### 2