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Linux Shell Scripting Tutorial Ver. Written by Vivek G Gite. I N D E X. Introduction. Kernel r. Shell r. How to use Shell r. Common Linux Command Introduction. Introduction. Time is precious. It is non-sense-ical to waste time typing a frequently used sequence of commands at a command prompt, more especially if they. You can now download this tutorial as a PDF for $5! to help people understand some of the basics of shell script programming (aka shell scripting).
Introduction Purpose Of This Tutorial This tutorial is written to help people understand some of the basics of shell script programming aka shell scripting , and hopefully to introduce some of the possibilities of simple but powerful programming available under the Bourne shell. As such, it has been written as a basis for one-on-one or group tutorials and exercises, and as a reference for subsequent use. Always check there for the latest copy. If you are reading this at some different address, it is probably a copy of the real site, and therefore may be out of date. Many other shells have been written; this particular tutorial concentrates on the Bourne and the Bourne Again shells. Other shells include the Korn Shell ksh , the C Shell csh , and variations such as tcsh.
Case-Control Instruction — This is used when we need to select from several alternatives. Q 35 What are Loops and explain three different methods of loops in brief?
For loop allows specifying a list of values which the control variable in the loop can take. The loop is then executed for each value mentioned in the list. While loop — This is used in a program when we want to do something for a fixed number of times.
While loop gets executed till it returns a zero value. Until loop — This is similar to while loop except that the loop executes until the condition is true.
Until loop gets executed at least once till it returns a non-zero value. Q 36 What is IFS? And it is one of the system variables. By default, its value is space, tab, and a new line.
It signifies that in a line where one field or word ends and another begins. Q 37 What is a Break statement and what is it used for? Ans: The break is a keyword and is used whenever we want to jump out of a loop instantly without waiting to get back to the control command.
When the keyword break is encountered inside any loop in the program, control will get passed automatically to the first statement after a loop. A break is generally associated with an if. Q 38 What is Continue statement and what is it used for? When the keyword continue is encountered inside any loop in the program, control automatically passes to the beginning of a loop.
Continue is generally associated with an if. Q 39 What are Metacharacters in a shell? Explain with some examples. Matches any one of the enclosed characters Examples will show all files will show all files whose first name is starting with letter 'a' will show all files having extension. Now it creates one question What are commands? The first word on command line is, ls - which is name of the command to be executed.
Everything else on command line is taken as arguments to this command. Why Command Line arguments required 1. Let's take rm command, which is used to remove file, but which file you want to remove and how you will tail this to rm command even rm command don't ask you name of file that you would like to remove.
This way you tail rm command which file you would like to remove. So we are doing one way communication with our command by specifying file-name. Also you can pass command line arguments to your script to make it more users friendly. But how we access command line argument in our script.
It means put output of ls command to filename. Note that If file already exist, it will be overwritten else new file is created. Now issue following command.
A pipe is nothing but a temporary storage place where the output of one command is stored and then passed as the input for second command. Pipes are used to run more than two commands Multiple commands from same command line. A filter performs some kind of process on the input and gives output.
Suppose we have file called 'hotel. What is Processes Process is kind of program or task carried out by your PC. It is a process. A process is program command given by user to perform specific Job. Why Process required Linux is multi-user, multitasking Os. It means you can run more than two process simultaneously if you wish. An instance of running command is called process and the number printed by shell is called process-id PID , this PID can be use to refer specific running process.
For Answer click here. Syntax: test expression OR [ expression ] Now will write script that determine whether given argument number is positive.
If it is true 0 then test will return 0 and output will printed as 5 number is positive but for argument there is no output because our condition is not true 0 no is not greater than 0 hence echo statement is skipped. And for last statement we have not supplied any argument hence error. Integer Number without decimal point 2.
File types 3. Syntax: if condition then command1 if condition is true or if exit status of condition is 0 zero The echo command i. Multilevel if-then-else Syntax: if condition then condition is zero true - 0 execute all commands up to elif statement elif condition1 condition1 is zero true - 0 execute all commands up to elif statement elif condition2 condition2 is zero true - 0 execute all commands up to elif statement else None of the above condtion,condtion1,condtion2 are true i.
Loops in Shell Scripts Computer can repeat particular instruction again and again, until particular condition satisfies. A group of instruction that is executed repeatedly is called a loop. This is usually know as iteration This process will continue until all the items in the list were not finished, because of this it will repeat 5 echo statements. The shell then executes all the statements up to the two semicolons that are next to each other. For eg.
Here for first run its match with van and it will show output For van Rs. For second run it print, "For car Rs. And for last run, there is no match for Maruti, hence default i. Note that esac is always required to indicate end of case statement. How to de-bug the shell script? While programming shell sometimes you need to find the errors bugs in shell script and correct the errors remove errors - debug. For this purpose you can use -v and -x option with sh or bash command to debug the shell script.
The files in this directory mostly represent peripheral devices such disks like floppy disk, sound card, line printers etc. Local and Global Shell variable export command Normally all our variables are local. To do this use export command Syntax: export variable1, variable2, The syntax for OR list as follows Syntax: command1 command2 Here command2 is executed if and only if command1 returns a non-zero exit status. This is called input redirection.
Following are name of such files Standard File stdin stdout stderr File Descriptors number 0 1 2 Use as Standard input as Standard output as Standard error Example Keyboard Screen Screen By default in Linux every program has three files associated with it, when we start our program these three files are automatically opened by your shell The use of first two files i.
The last file stderr numbered as 2 is used by our program to print error on screen.
Now if we try to redirect this error-output to file, it can not be send to file. For second sample run, we have redirect output of our script to file, since it's error we have to show it to user, It means we have to print our error message on stderr not on stdout.
Function performs particular activity in shell. A return statement will terminate the function. This way you can call function. Note that after restarting your computer you will loss this SayHello function, since its created for current session only. Note that to add function to this file you must logon as root.
Putting PS1 here ensures that it gets loaded every time. Life never be the same, until you login again! Tip: If you want to show some message or want to perform some action when you logout, Open file. When ever I logout, I want to show message download!
Then open your. You can accomplish this as follows: 1 Use command line arguments args to script when you want interaction i. Please select choice 1,2,3,4, or 5"; echo "Press a key. In Linux shell there is no built-in facility available to create such user interface, But there is one utility supplied with Red Hat Linux version 6.