Unix Briefing

Variables

x=3

y='Hello'

Note: there should be no space in assigning variables.

Print Variables:

echo $x

echo $y

More accurate printing variables is double quote your variables

echo "$x"

echo "$y"

You can print literal also with variables.

echo "Value of variable x is $x"

If  else statement

x=5

y=7

if [[ $x -lt $y ]]; then

echo "true"

else

echo "false"

fi

Note : For good practice, put condition in [[...]]. There is semi-colon ; after condition end. fi command indicates closing of if else statements.

For comparison, use -lt,-gt, -le, -ge comparison parameters for good practice

if ((x>y)); then --- this also works -- Try yourself

Nested if else can be done with using elif command

x=cool

if [ "$x" = "cool" ]

then

    echo "Cool Beans"

elif [ "$x" = "neat" ]

then

    echo "Neato cool"

else

    echo "Not Cool Beans"

fi

Notice above if and else also works

For loop 

for <<counter_variable>> in <<input_variable>>

do

//perform action

Done

Few examples:

for i in 1 2 3 4 5

do

echo "$i"

done

x="foo boo coo doo moo"

for z in $x

do

echo "$z"

done

for i in {1..5}

do

echo "$i"

done

Note : For loop start with for command, then do and ends with done

While loop

a=0

while [ $a -lt 10 ]

do

echo $a

a=`expr $a + 1`

done

Note: Same as for loop. Action performed between do and done command

Function in Unix

hello()

{

echo “Hello, World”

}

hello    --- command to excute – just type function name

Output: Hello, World

If you have to pass arguments in Function and return the output.

Sum()

{

return `expr $1 + $2`

}

Sum 2 5  ---- arguments passed

echo “Result of sum function is $?”  -- value is captured by $?

Output:

Result of sum function is 7

Note :

Arguments pass after calling function and separated with spaces.

$1 $2 $3 indicates the arguments you pass to function

Return value is captured by $?

Run script and passing arguments to it

You can run script by following command

./script_name.sh

You can pass the arguments also to script while which can be access through $1 $2… $n value.

In Arguments, you can pass any literal value or file also.

./script_name.sh hello foo boo test.sh

Here

$1 = hello

$2 = foo

$3 = boo

$4 = test.sh

You can read file using below snippet or you can access by other means also.

echo $4

while read line

do

echo $line

done < $4

After reading file you can perform multiple operation


$# gives the no of arguments passed to the script.

In above example, $# gives "4" value

$@ give the list of all arguments passed to the script.

In above example, $@ gives "hello foo boo test.sh" value

$0 gives the path of the filename from where file got executed.

Suppose you have directory /tmp/examples/script

under script folder you have script test.sh

test.sh
echo $0
echo `dirname $0`

if you run this script from tmp folder
./examples/script/test.sh

Output:

./example/script/test.sh
./example/script

one more example : 

dir="/from/here/to/there.txt"
dir="$(dirname $dir)"   # Returns "/from/hear/to"

dir="$(basename $dir)"  # Returns just "to"

Create file through Touch command

Touch:
          If file is already existing, it will update the time stamp.
          If the file does not exist, then 'touch file_name' creates a new file with 0 KB file size. 

touch demo.sh   --- This will create file with name demo.sh

Once created, open in vi editor to write the code snippet using shift+I command.

vi demo.sh

Once done with changes. Press esc (it will come out of insert mode) and then type

:wq to save and exit from file.

Give permission to the file

Chmod 777 demo.sh

Run file

./demo.sh  (dot slash file name)

Delete the content of file

 > script_name.sh  

This will delete the contents of file

Refer one script file from another script file

Two script file : Square.sh and Circle.sh

Square.sh

Message= “ today is great day”

In order to refer Square file in Circle file, you can use below commands

. Square.sh   (dot space script_name)

source Sqaure.sh

Circle.sh

echo “ I am in circle file”

. Square.sh

echo “$Message”

or

Circle.sh

echo “ I am in circle file”

source  Square.sh

echo “$Message”

Run Circle file

./Circle.sh

Output :

I am in circle file

 today is great day

Note : even you can refer functions of Square file.