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Md. Iftiar Uddin Ahmed [011 122 084 ]
Md.Tanvir Hossain [ 011 101 035]
Rafid asrar prottoy [ 011 132 124 ]
Amit ghosh [011 132 134]](https://image.slidesharecdn.com/shell-160108104546/75/Shell-Shell-Script-1-2048.jpg)
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Shell & Shell Script
This document provides an overview of Linux, shells, and shell scripts. It begins with a short history of Linux and how it originated from efforts to create an affordable UNIX-like operating system that could run on personal computers. It then defines what a shell is and describes common shell commands. The remainder of the document explains what shell scripts are, how to write them, and includes examples of common scripting elements like variables, conditionals, loops, arithmetic, and file operations.
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Shell & Shell Script
- 1. Team Md. Iftiar UddinAhmed [011 122 084 ] Md.Tanvir Hossain [ 011 101 035] Rafid asrar prottoy [ 011 132 124 ] Amit ghosh [011 132 134]
- 2. About Shell |Shell Scripts
- 3. Overview A shorthistory of Linux About Shell About Shell Script
- 4. Linux Introduction In 80’s,Microsoft’s DOS was the dominated OS for PC Apple MAC was better, but expensive UNIX was much better, but much, much more expensive. Only for minicomputer for commercial applications People was looking for a UNIX based system, which is cheaper and can run on PC Both DOS, MAC and UNIX were proprietary, i.e., the source code of their kernel is protected No modification is possible without paying high license fees Before Linux
- 5. Beginning of Linux Afamous professor Andrew Tanenbaum developed Minix, a simplified version of UNIX that runs on PC Minix is for class teaching only. No intention for commercial use In Sept 1991, Linus Torvalds, a second year student of Computer Science at the University of Helsinki, developed the preliminary kernel of Linux, known as Linux version 0.0.1 Linux Introduction
- 6. Linux Introduction Linux hasbeen used for many computing platforms – PC, PDA, Supercomputer,… Not only character user interface but graphical user interface is available Commercial vendors moved in Linux itself to provide freely distributed code. They make their money by compiling up various software and gathering them in a distributable format – Red Hat, Slackware, etc Linux Today
- 7. Shell Shell is ancommand 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. Simply put, the shell is a program that takes your commands from the keyboard and gives them to the operating system to perform. What is Shell
- 8. Shell Commands Shell commandsare interpreted directly by the shell you specify. The commands are similar to the statement in some programming languages, such as C. Popular shells include: Enhanced C-shell tchs (csh+) Bourne-Again Shell, bash (sh+) Korn Shell (ksh)
- 9. Shells’ Features In particular: Pass arguments to your script Set and reference variables Use of control flow Interact with the user (read user input) Comments… Info on commands a given shell offers can be found in the man pages for that shell. There are many Linux/UNIX references that give detailed information and tips.
- 10. Shell Scripts Toautomate certain common activities an user performs routinely. They serve the same purpose as batch files in DOS/Windows. Example: rename 1000 files from upper case to lowercase What are they for?
- 11. What are Shell Scripts Just text/ASCII files with: a set of standard UNIX/Linux commands (ls, mv, cp, less, cat, etc.) along with flow of control some conditional logic and branching (if-then), loop structures (foreach, for, while), and I/O facilities (echo, print, set, ...). They allow use of variables. They are interpreted by a shell directly. Some of them (csh, tcsh) share some of C syntax. DOS/Win equivalent - batch files (.bat)
- 12. Why not use C/C++for that? C/C++ programming requires compilation and linkage, maybe libraries, which may not be available (production servers). For the typical tasks much faster in development, debugging, and maintenance (because they are interpreted and do not require compilation).
- 13. Shell Script Invocation Specifythe shell directly: % tcsh myshellscript % tcsh -v myshellscript (-v = verbose, useful for debugging) Make the shell an executable first and then run is a command (set up an execution permission): % chmod u+x myshellscript Then either this: % myshellscript (if the path variable has ‘.’ in it; security issue!) Or: % ./myshellscript (should always work)
- 14. Shell Script Invocation continue Ifyou get an error: “myshellscrip: command not found” The probably “.” is not in your path or there’s no execution bit set. When writing scripts, choose unique names, that preferably do not match system commands. Bad name would be test for example, since there are many shells with this internal command. To disambiguate, always precede the shell with “./” or absolute path in case you have to name your thing not very creatively.
- 15. Start Writing a ShellScript The very first line, often called 'shebang' (#!) should precede any other line, to assure that the right shell is invoked. Comments start with '#', with the exception of #!, $#, which are a special character sequences. Everything on a line after # is ignored if # is not a part of a quoted string or a special character sequence. #!/bin/tcsh #!/bin/bash # This is for tcsh # For Bourne-Again Shell #!/bin/sh # This is for Bourne Shell
- 16. Variables Variables startwith a $ sign when they are used. $x, $val There's no $ when a variable is declared. set x = 3 @ y = 1 set input = "$<" There are some system, predefined variables: $0, $1, $3 .... - argument references (arguments themselves) $* - all the arguments $< - user's input from STDIN $# - # of arguments passed to the script
- 17. Read To readuser input from keyboard and store it into a variable use read var1,var2,.....varn #!/bin/bash echo ‐n "Enter your name:” read name echo ‐n "Enter your student no:” read stdno echo "Your Name:$name” echo "Your Age:$stdno”
- 18. Shell Arithmetic The exprcommand evaluates its arguments as an expression. It is commonly used for simple arithmetic operations. #!/bin/bash expr 1 + 1 expr 1 ‐ 1 expr 1 * 1 expr 1 / 1 va r=`expr 1 + 1` x=1 x=`expr $x + 1`
- 19.
- 20. if if ( <expression>) then <statements> else if ( <another-expression> ) then <statements> else <statements> endif
- 21. foreach foreach var (<list-of-values> ) <statements> end
- 22. Switch switch ( string) case str1: <statements> breaksw ... default: <statements> breaksw endsw
- 23. while while ( <expression>) <statements> end
- 24. File Inquiry Operators: -op file rRead access w Write access xExecute access eExistence oOwnership z Zero size s Non-zero size F Plain file D Directory L Symbolic link B Block special file C Character special file P Named pipe (FIFO) S Socket special file
Editor's Notes
- #6 This is Linus Torvalds birthday