Bash Shell Scripting Guide

1. Basics
a. The first line of the script must be
#!/bin/bash
b. Make the script exicutable
chmod +x test.sh

2. Conditional Statement
2.1. if statement
syntax:
if [ condition ]
then
commands
fi

2.2. if else statement
syntax:
if [ condition ]
then
commands
else
commands
fi

2.3. if elif statement
syntax:
if [ condition1 ]
then
commands
elif [ condition2]
then
commands
else
commands
fi

3.1 Relational operators
-eg Equal to
-lt Less than
-gt Greater than
-ge Greater than or equal to
-le Less than or equel to

3.2 File Related Tests
-f file True if file exists and is a regular file
-r file True if file exists and is readable
-w file True if file exists and is writable
-x file True if file exists and is executable
-d file True if file exists and is a directory
-s file True if file exists and its size is greater than zero

3.3 String tests
-n str True if string str is not a null string
-z str True if string str is a null string
str1 == str2 True if both the strings are equel
str1 != str2 True if both the strings are not equel
str True if str is assigned a value and is not null

3.4 Multiple conditions
-a Performs the AND function
-o Performs the OR function

4. Case Statement
Syntax:
case expression in
pattert1) execute commands;;
pattert2) execute commands;;
pattert3) execute commands;;
....
esac

Example:
#!/bin/bash
case `datacut -d" " -f1` in
Mon) commands;;
Tue) commands;;
Wen) commands;;
....
esac

5. Looping Statements
5.1 while loop
syntax:
while [ condtion_is_true ]
do
execute commands
....
done

Example:
while [ $NUM -gt 100 ]
do
sleep
done

5.2 until loop
Syntax:
until [ condition_is_false ]
do
execute commands
done

Example:
until [ -f $FILE ]
do
sleep 5
done

5.3 for loop
Syntax:
for variable in list
do
execute commands
done

Example:
for I in 1 2 3 4 5
do
echo "The value of I is $I";
done

Example 2:
#!/bin/bash
LIMIT=10
for ((a=1;a<=$LIMIT;a++)) do echo "$a" done
6. Special symbols
$0 Name of the shell script being executed
$1 First parameter passed to the script
$* All the paramerts passes to the script
$# Number of parameters passed to the script
$? Exit status of the last command

7. Read statement
#!/bin/bash
echo "Enter your name:"
read NAME
echo "Hello $NAME, Have a nice day."

8. Functions
Syntax:
Function_name ()
{
statements
}

Example:
#!/bin/bash
sumcalc ()
{
SUM= $[ $1 + $2 ]
echo "Result = $SUM"
}

echo -e "Enter the first number:\c"
read NUM1
echo -e "Enter the second number:\c"
read NUM2
read NUM2

# Call the function
sumcal $NUM1 $NUM2