Shell Programming:DECISION-MAKING & LOOP CONSTRUCTS

DECISION-MAKING & LOOP CONSTRUCTS: * Shell programs can perform conditional tests on their arguments and variables and execute different commands based on the results. For example: if [ "$1" = "hyena" ] then echo "Sorry, hyenas not allowed." exit elif [ "$1" = "jackal" ] then echo "Jackals not welcome." exit else echo "Welcome to Bongo Congo." fi echo "Do you have anything to declare?" -- checks the command line to see if the first argument is "hyena" or "jackal" and bails out, using...

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Shell Programming:COMMAND-LINE ARGUMENTS

COMMAND-LINE ARGUMENTS: * In general, shell programs operate in a "batch" mode, that is, without interaction from the user, and so most of their parameters are obtained on the command line. Each argument on the command line can be seen inside the shell program as a shell variable of the form "$1", "$2", "$3", and so on, with "$1" corresponding to the first argument, "$2" the second, "$3" the third, and so on. There is also a "special" argument variable, "$0", that gives the name of the shell program itself. Other special variables include...

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Shell Programming:COMMAND SUBSTITUTION

COMMAND SUBSTITUTION: * The next step is to consider shell command substitution. Like any programming language, the shell does exactly what it is told to do, and so it is important to be very specific when telling it to do something. As an example, consider the "fgrep" command, which searches a file for a string. For example, to search a file named "source.txt" for the string "Coyote", enter: fgrep Coyote source.txt -- and it would print out the matching lines. However, suppose we wanted to search for "Wile E. Coyote". If we did this...

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Shell Programming:SHELL VARIABLES

SHELL VARIABLES: * The first useful command to know about in building shell programs is "echo", which can be used to produce output from a shell program: echo "This is a test!" This sends the string "This is a test!" to standard output. It is recommended to write shell programs that generate some output to inform the user of what they are doing. The shell allows variables to be defined to store values. It's simple, just declare a variable is assign a value to it: shvar="This is a test!" The string is enclosed in double-quotes...

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Shell Programming:GETTING STARTED

GETTING STARTED: * The first thing to do in understanding shell programs is to understand the elementary system commands that can be used in them. A list of fundamental UNIX system commands follows: ls # Give a simple listing of files. cp # Copy files. mv # Move or rename files. rm # Remove files. rm -r # Remove entire directory subtree. cd # Change directories. pwd # Print working directory. cat # Lists a file or files sequentially. more # Displays a...

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Shell Programming:Introduction

* The UNIX operating system provides a flexible set of simple tools to perform a wide variety of system-management, text-processing, and general-purpose tasks. These simple tools can be used in very powerful ways by tying them together programmatically, using "shell scripts" or "shell programs". The UNIX "shell" itself is a user-interface program that accepts commands from the user and executes them. It can also accept the same commands written as a list in a file, along with various other statements that the shell can interpret to provide...

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Visuel Basic:Loop

Do...Loop: Used to execute a block of statements an unspecified number of times. Do While condition      statements Loop First, the condition is tested; if condition is True, then the statements are executed. When it gets to the Loop it goes back to the Do and tests condition again. If condition is False on the first pass, the statements are never executed. For...Next: When the number of iterations of the loop is known, it is better to use the For...Next rather than the Do...Loop. For counter...

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