shell scripting

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:

--------------------------------- = 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
= is equal to - if [ "$a"= "$b" ]
== is equal to -f if [ "$a"=="$b" ]
!= is not equal to if [ "$a" !="$b" ] < is less then ,in ASCII alphabetical order if [["$a" < "$b"]]
> is greater than, in ASCII alphabetical order if [[ "$a" > "$b"]]
-z string is null, that is has zero length


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: ------------------------------------

-eq is equal to if [ "$a" -eq "$b" ]
-ne is not equal to if [ "$a" -ne "$b" ]
-gt is greater than if [ "$a" -gt "$b" ]
-ge is greater than or equal to if [ "$a" -ge "$b" ]
-lt is less than if [ "$a" -lt "$b" ]
-le is less than or equal to if [ "$a" -le "$b" ] < is less than (("$a"<"$b" ))
<= is less than or equal to (("$a" <= "$b"))
> is greater than ["$a" > "$b"]
>= is greater than equal to ["$a" >= "$b"]


-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)

BASH command output to the variable

Command Substitution Syntax:

variable=$(command)
variable=$(command [option…] argument1 arguments2 …)
variable=$(/path/to/command)

OR

variable=`command`
variable=`command [option…] argument1 arguments2 …`
variable=`/path/to/command`

***Note: Don’t use any space before and after the equal sign when using the above commands.