# shell Programming

* [Tutorial](https://www.tutorialspoint.com/unix/shell_scripting.htm)

```sh
#!/bin/sh

echo "File Name: $0"
echo "First Parameter : $1"
echo "Second Parameter : $2"
echo "Quoted Values: $@"
echo "Quoted Values: $*"
echo "Total Number of Parameters : $#"
```



* while loop

```sh
#!/bin/sh

a=0
while [ "$a" -lt 10 ]    # this is loop1
do
   b="$a"
   while [ "$b" -ge 0 ]  # this is loop2
   do
      echo -n "$b "
      b=`expr $b - 1`
   done
   echo
   a=`expr $a + 1`
done
```



* Assume variable **a** holds 10 and variable **b** holds 20 then 

| Operator |                         Description                          |          Example           |
| :------: | :----------------------------------------------------------: | :------------------------: |
| **-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.   |



| Operator |                         Description                          |                Example                |
| :------: | :----------------------------------------------------------: | :-----------------------------------: |
|  **!**   | 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. |

* Other [Operator](https://www.tutorialspoint.com/unix/unix-basic-operators.htm)