Shell is a command language interpreter that executes commands read from the standard input device (keyboard) or from a file. Shell is not part of system kernel, but uses the system kernel to execute programs, create files etc.
So we can say “Shell” is a user program or it’s environment provided for user interaction.
There are many type shell available with Linux UNIX-Like System
- BASH ( Bourne-Again SHell )– Developed by “Brian Fox and Chet Ramey “
- C SH ( C SHell)– Developed by “Bill Joy“
- K SH (Korn SHeel)– Developed By “ David Korn “
Note: For Knowing all available shell in your system type the command ” # Cat /etc/shells” And that each shell does the same job, but each understand a different command syntax and provides different built-in functions.
Also you can check, what is your current shell where you are working just type the command #echo $SHELL
Linux Shell Scripting
In simple way, A Shell Scripts is a text file that contains one or more no. of commands for reboot a task, it helps to System Administrator or DevOps to make his task easier and automated. And the file extension of any Shell scripts always should be ” <name>.sh ” and execute (x) Permission. Shell is a interface between user and the kernel. Even though there can be only one kernel a system can have many shell running simultaneously . Whenever a user enters a command through keyboard the shell communicates with the kernel to execute it and then display the output to the user.
Lets have a example:
- Create a script file and write your script_code in file and save it.
- Now make it executable with Excute Permission.
$ sudo vim guarav.sh #! /bin/bash ~ ~ (Your code/command that what do you want to do with this script) ~ :wq $ sudo chmod +x gaurav.sh
Here as we can see the first line of script’s file is started with #! /bin/bash it means that #! – It indicates that the file is an executable shell scripts. its Know as Command Interpreter.
Some Advantages of BASH Shell
- Shell Script can take an input from user, file and output them on screen.
- Use full to create our own commands and easy to schedule with cron utils.
- To automate some task day today life.
- System Administrator’s tasks can also be easy.
Loops in Bash Shell Scripting:
“A group of instruction that is executed repeatedly is called a loop.” Loops are very useful for Bash Shell scripting, it allows us to take a series of commands and keep running them until a particular situation are reached or condition is matched. Loops are useful for automating repetitive tasks. There are different type of loop
So, here below some points are important in loops
- The variable must be initialized which is used in loop, then execution of loop begins.
- A condition (test) is made at the beginning of each iteration.
- The body of loop ends with a statement that modifies the value of condition (test) variable.
Lets have an example of “for loop” to make a script of ‘Table’s 2‘
$ vim test.sh #!/bin/bash # BASH Shell script of table '2' echo "Table of 2 " for i in 1 2 3 4 5 6 7 8 9 10 do echo "Gaurav $((i+i)) " done ~ :wq $ chmod +x test.sh $ ./test.sh Table of 2 Gaurav 2 Gaurav 4 Gaurav 6 Gaurav 8 Gaurav 10 Gaurav 12 Gaurav 14 Gaurav 16 Gaurav 18 Gaurav 20 $_