Shell Scripting Tuts

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Shell Scripting
1. Use any scripting editor, i will use VIM

2. After the shell script is complete set execute permission to the script for the script to execute. The permissions can be changed by the command chmod.
    Syntax is
    chmod permission your-script name
    e.g
    $ chmod +x script
    $ chmod 755 script
    755 sets permission for user (read 4 write 2 execute 1) group and others (read 4 execute 1)

3. Executing your script : There are three ways of executing a script:
    Syntax:
        bash script-name
        sh script-name
        ./script-name
    E.g
        $ bash script
        $ sh script
        $ ./script
    Note: ./means current directory. But only .(dot) means execute a given command in the current shell without starting a new copy of the shell. The syntax for .(dot) command is . command-name e.g $ . foo
# This is my 1st shell program
clear
echo "Knowledge is Power"
Anything after "#" is a comment
clear - this will clear the screen
echo - prints the message

4. The computer memory is divided into small locations and each location is called as a memory address. These memory addresses hold data in them. A programmer can give a name to these memory locations. This is called as variable. These variable can take values.
Linux has two types of variables:
i. System Variables (Maintained by Linux system, seen in caps case)
ii. User defined varibales (User defined, small case)
Variable declaration:
n=10
Dont put spaces on either side of the equals sign that should give an error. Variables are case sensitive. While printing the values of the variables use the '$' symbol, e.g echo $n

5. Shell Arithmetic
The term expr is used to perform arithmetic operations in shell.
Syntax : expr op1 operator op2
e.g
$ expr 1 + 3
$ expr 2 - 1
$ expr 10 / 2
$ expr 20 % 3
$ expr 10 \* 3
$ echo `expr 6 + 3`

Note : Backticks ` is called backtick and this is mean to execute any command in the script e.g echo "today is `date`"

6. Exit status
By default in Linux if any command is executed, it would return two types of values, 1. 0 (ZERO) 2. Non ZERO
zero is success while non zero is not
This is called as the exit status of the command. By default to find the exist status of any command. Just do a $? will give you the exit status
e.g In the terminal try removing a file that does not exist rm newfile, Now do an echo $? it should give you a non zero value. Now do an ls and again echo $? it should give you a 0

7. User input
to accept an input from the user, use the command 'read'
read var1, var2, var3

8. WildCards
There are three wildcards in shell
?, *, [...]
? - will Match a single character
* - will Match a group of characters
[...]* - will match a group of characters in the mentioned set
e.g
[abc]* will look for all the matches starting with a,b and c
[!abcd] or [!a-d] everything except a,b,c,d

Note: If you want to execute two or more commands in one line, use a semicolon
e.g ls; who; date; cal

9. Command line arguments
Some commands need arguments to be passed. e.g if you do an rm, you would need to specify the file you want to remove. The specifying of the file is the command line argument to that command. For every shell script there is a command line argument that cane be specified by $<number>. $0 is the name of the script. $1 is the 1st argument. $2 is the 2nd argument and so on...

10. Redirection of Standard input output
Three symbols are used for redirection'
> , < , >>
> : command > file : this will write to the file provided
< : command < file : take input from the file provided
>> : command >> file : will append input to the file if file already exist

11. Pipes
pipe is a utility to connect the output of one program to the input of another program without any temporary file. Pipe is a temporary storage place, where the output of one command is stored, and then passed to the input of another command. Pipes are used to run more than two commands for the same command line.
$ ls | more
$ who | sort
$ who | sort > user_list
$ who | wc -l
$ ls -l | wc  -l
$ who | grep raju

12. Filters
If a Linux command accepts its input from the standard input and produces its output on standard output is know as a filter. A filter performs some kind of process on the input and gives output. For e.g.. Suppose you have file called 'hotel.txt' with 100 lines data, And from 'hotel.txt' you would like to print contains from line number 20 to line number 30 and store this result to file called 'hlist' then give command:
$ tail +20 < hotel.txt | head -n30 >hlist

13. If Condition
if is used for decision making. If a particular condition is true command1 is executed.
Syntax:
if condition
then
  command1
fi

condition is comparison between values, comparison can be done using the expr operator or the by cheking the exit status.
#!/bin/bash
if cat $1
then
  echo "$1 file found on this location"
fi

if [ $2 -gt 0 ]
then
  echo "$2 positive number"
fi

if [ -f $3 ]
then
  echo "$3 file exists"
fi

if...else...if
#!/bin/bash
if [ -f $1 ]
then
  echo "$1 exist"
else
  if [ -f $2 ]
  then
    echo "$2 exist"
  else
    if [ -f $3 ]
    then
      echo "$3 exist"
    fi 
  fi 
fi
execution
$ sh -x shell3.sh asdff adfdag shell1.sh
+ [ -f asdff ]
+ [ -f adfdag ]
+ [ -f shell1.sh ]
+ echo shell1.sh exist
shell1.sh exist
$ sh -x shell3.sh asdff shell2.sh shell1.sh
+ [ -f asdff ]
+ [ -f shell2.sh ]
+ echo shell2.sh exist
shell2.sh exist
$ sh -x shell3.sh shell3.sh shell2.sh shell1.sh
+ [ -f shell3.sh ]
+ echo shell3.sh exist
shell3.sh exist

#!/bin/bash
echo "1. Linux \n 2. Windows \n 3. None"
read ch
if [ $ch -eq 1 ]
then
  echo "You seem like a geek"
else
  echo "You are good"
  if [ $ch -eq 3 ]
  then
    echo " But, I dont know, what you do"
  fi 
fi

#!/bin/bash
echo "1. student\n2.work\n3.none"
read ch
if [ $ch -eq 1 ]
then
  echo "Happy Studying"
elif [ $ch -eq 2 ]
then
  echo "You should study too"
elif [ $ch -eq 3 ]
then
  echo "Study an get a job"
fi



14. Looping in Shell
Looping is execution of a particular expression time and again until a particular condition is satisfied.
bash supports for loops and while loops
looping needs 1. initializing of a variable, 2. test condition in beginning, 3. body of loop at the end modifies the initialized variable

15. For Loop

Syntax:

for { variable name } in { list }
do
execute one for each item in the list until the list is
not finished (And repeat all statement between do and done)
done

  e.g

for i in 1 2 3 4 5
do
echo "Welcome $i times"
done

#!/bin/bash
for i in `seq 1 10`
do
  echo "Welcome $i times"
done


16. While loops
 Syntax:

while [ condition ]
  do
     command1
     command2
     command3
  done

To be contd...