Bash Scripting Cheatsheat

bashscriptingcheatsheat.txt

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##  This is a list of commands to aid in scripting for the bash shell.  ###
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#Arithmetic Operators |
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+ (Addition)	Adds values on either side of the operator	`expr $a + $b` will give 30
- (Subtraction)	Subtracts right hand operand from left hand operand	`expr $a - $b` will give -10
* (Multiplication)	Multiplies values on either side of the operator	`expr $a \* $b` will give 200
/ (Division)	Divides left hand operand by right hand operand	`expr $b / $a` will give 2
% (Modulus)	Divides left hand operand by right hand operand and returns remainder	`expr $b % $a` will give 0
= (Assignment)	Assigns right operand in left operand	a = $b would assign value of b into a
== (Equality)	Compares two numbers, if both are same then returns true.	[ $a == $b ] would return false.
!= (Not Equality)	Compares two numbers, if both are different then returns true.	[ $a != $b ] would return true.

** It is very important to understand that all the conditional expressions should be inside square braces with spaces around them, for example [ $a == $b ] is correct whereas, [$a==$b] is incorrect.

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#..Relational Operators
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#/ Assume variable a holds 10 and variable b holds 20 then

-eq	Checks if the value of two operands are equal or not; if yes, then the condition becomes true.	[ $a -eq $b ] is not true.
-ne	Checks if the value of two operands are equal or not; if values are not equal, then the condition becomes true.	[ $a -ne $b ] is true.
-gt	Checks if the value of left operand is greater than the value of right operand; if yes, then the condition becomes true.	[ $a -gt $b ] is not true.
-lt	Checks if the value of left operand is less than the value of right operand; if yes, then the condition becomes true.	[ $a -lt $b ] is true.
-ge	Checks if the value of left operand is greater than or equal to the value of right operand; if yes, then the condition becomes true.	[ $a -ge $b ] is not true.
-le	Checks if the value of left operand is less than or equal to the value of right operand; if yes, then the condition becomes true.	[ $a -le $b ] is true.
It is very important to understand that all the conditional expressions should be placed inside square braces with spaces around them. For example, [ $a <= $b ] is correct whereas, [$a <= $b] is incorrect.

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#   Boolean Operators
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Assume variable a holds 10 and variable b holds 20 then −

!	This is logical negation. This inverts a true condition into false and vice versa.	[ ! false ] is true.
-o	This is logical OR. If one of the operands is true, then the condition becomes true.	[ $a -lt 20 -o $b -gt 100 ] is true.
-a	This is logical AND. If both the operands are true, then the condition becomes true otherwise false.	[ $a -lt 20 -a $b -gt 100 ] is false.

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# String Operators
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The following string operators are supported by Bourne Shell.

Assume variable a holds "abc" and variable b holds "efg" then −

=	Checks if the value of two operands are equal or not; if yes, then the condition becomes true.	[ $a = $b ] is not true.
!=	Checks if the value of two operands are equal or not; if values are not equal then the condition becomes true.	[ $a != $b ] is true.
-z	Checks if the given string operand size is zero; if it is zero length, then it returns true.	[ -z $a ] is not true.
-n	Checks if the given string operand size is non-zero; if it is nonzero length, then it returns true.	[ -n $a ] is not false.
str	Checks if str is not the empty string; if it is empty, then it returns false.	[ $a ] is not false.

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#File Test Operators
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Operator	Description	Example
-b file	Checks if file is a block special file; if yes, then the condition becomes true.	[ -b $file ] is false.
-c file	Checks if file is a character special file; if yes, then the condition becomes true.	[ -c $file ] is false.
-d file	Checks if file is a directory; if yes, then the condition becomes true.	[ -d $file ] is not true.
-f file	Checks if file is an ordinary file as opposed to a directory or special file; if yes, then the condition becomes true.	[ -f $file ] is true.
-s file	Checks if file has size greater than 0; if yes, then condition becomes true.	[ -s $file ] is true.
-e file	Checks if file exists; is true even if file is a directory but exists.	[ -e $file ] is true.


These are ones I find most valuable compiled from Sources: 
https://www.tutorialspoint.com/unix/unix-basic-operators.htm