Shell Scripting Tutorial for Beginners 5 - If Statement ( If then , If then else, If elif else)

Conditionals let us decide whether to perform an action or not, this decision is taken by evaluating an expression.

Expressions
An expression can be: String comparison, Numeric comparison, File operators and Logical operators and it is represented by [expression]:
String Comparisons:
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= compare if two strings are equal
!= compare if two strings are not equal
-n evaluate if string length is greater than zero
-z evaluate if string length is equal to zero

Examples:
[ s1 = s2 ] (true if s1 same as s2, else false)
[ s1 != s2 ] (true if s1 not same as s2, else false)
[ s1 ] (true if s1 is not empty, else false)
[ -n s1 ] (true if s1 has a length greater then 0, else false)
[ -z s2 ] (true if s2 has a length of 0, otherwise false)

Number Comparisons:
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-eq compare if two numbers are equal
-ge compare if one number is greater than or equal to a number
-le compare if one number is less than or equal to a number
-ne compare if two numbers are not equal
-gt compare if one number is greater than another number
-lt compare if one number is less than another number

Examples:
[ n1 -eq n2 ] (true if n1 same as n2, else false)
[ n1 -ge n2 ] (true if n1greater then or equal to n2, else false)
[ n1 -le n2 ] (true if n1 less then or equal to n2, else false)
[ n1 -ne n2 ] (true if n1 is not same as n2, else false)
[ n1 -gt n2 ] (true if n1 greater then n2, else false)
[ n1 -lt n2 ] (true if n1 less then n2, else false)

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