shell if statement

reference: https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php

if [ <some test> ]
then
<commands>
fi

================== compare number ================
#!/bin/bash
# Basic if statement
if [ $1 -gt 100 ]
then
    echo Hey that\'s a large number.
    pwd
fi
================== compare string ================
#!/bin/sh
if [ -z $1 ]
then
   echo "no input string given"
   return
fi
==============================================

Operator	Description
! EXPRESSION	The EXPRESSION is false.
-n STRING	The length of STRING is greater than zero.
-z STRING	The lengh of STRING is zero (ie it is empty).
STRING1 = STRING2	STRING1 is equal to STRING2
STRING1 != STRING2	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is numerically less than INTEGER2
-d FILE	FILE exists and is a directory.
-e FILE	FILE exists.
-r FILE	FILE exists and the read permission is granted.
-s FILE	FILE exists and it's size is greater than zero (ie. it is not empty).
-w FILE	FILE exists and the write permission is granted.
-x FILE	FILE exists and the execute permission is granted.

= is slightly different to -eq. [ 001 = 1 ] will return false as = does a string comparison (ie. character for character the same) whereas -eq does a numerical comparison meaning [ 001 -eq 1 ] will return true.

When we refer to FILE above we are actually meaning a path. Remember that a path may be absolute or relative and may refer to a file or a directory.

bash> test 001 = 1

bash> echo $?

bash> 1

bash> test 001 -eq 1

bash> echo $?

bash> 0

bash> touch myfile

bash> test -s myfile

bash> echo $?

bash> 1

bash> ls /etc > myfile

bash> test -s myfile

bash> echo $?

bash> 0