Bash-Beginners-Guide

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1.5.1. Properties of good scripts This guide is mainly about the last shell building block, scripts. Some general considerations before we continue: 1. A script should run without errors. 2. It should perform the task for which it is intended. 3. Program logic is clearly defined and apparent. 4. A script does not do unnecessary work. 5. Scripts should be reusable. 1.5.2. Structure The structure of a shell script is very flexible. Even though in Bash a lot of freedom is granted, you must ensure correct logic, flow control and efficiency so that users executing the script can do so easily and correctly. When starting on a new script, ask yourself the following questions: • Will I be needing any information from the user or from the user's environment? • How will I store that information? • Are there any files that need to be created? Where and with which permissions and ownerships? • What commands will I use? When using the script on different systems, do all these systems have these commands in the required versions? • Does the user need any notifications? When and why? 1.5.3. Terminology Bash Guide for Beginners The table below gives an overview of programming terms that you need to be familiar with: Table 1-1. Overview of programming terms Term Command control Conditional branch Logic flow Loop User input What is it? Testing exit status of a command in order to determine whether a portion of the program should be executed. Logical point in the program when a condition determines what happens next. The overall design of the program. Determines logical sequence of tasks so that the result is successful and controlled. Part of the program that is performed zero or more times. Information provided by an external source while the program is running, can be stored and recalled when needed. 1.5.4. A word on order and logic In order to speed up the developing process, the logical order of a program should be thought over in advance. This is your first step when developing a script. Chapter 1. Bash and Bash scripts 16
A number of methods can be used; one of the most common is working with lists. Itemizing the list of tasks involved in a program allows you to describe each process. Individual tasks can be referenced by their item number. Using your own spoken language to pin down the tasks to be executed by your program will help you to create an understandable form of your program. Later, you can replace the everyday language statements with shell language words and constructs. The example below shows such a logic flow design. It describes the rotation of log files. This example shows a possible repetitive loop, controlled by the number of base log files you want to rotate: 1. Do you want to rotate logs? 2. a. If yes: Bash Guide for Beginners i. Enter directory name containing the logs to be rotated. ii. Enter base name of the log file. iii. Enter number of days logs should be kept. iv. Make settings permanent in user's crontab file. b. If no, go to step 3. Do you want to rotate another set of logs? 3. Exit a. If yes: repeat step 1. b. If no: go to step 3. The user should provide information for the program to do something. Input from the user must be obtained and stored. The user should be notified that his crontab will change. 1.5.5. An example Bash script: mysystem.sh The mysystem.sh script below executes some well-known commands (date, w, uname, uptime) to display information about you and your machine. tom:~> cat -n mysystem.sh 1 #!/bin/bash 2 clear 3 echo "This is information provided by mysystem.sh. Program starts now." 4 5 echo "Hello, $USER" 6 echo 7 8 echo "Today's date is `date`, this is week `date +"%V"`." 9 echo 10 11 echo "These users are currently connected:" 12 w | cut -d " " -f 1 - | grep -v USER | sort -u 13 echo 14 15 echo "This is `uname -s` running on a `uname -m` processor." 16 echo 17 18 echo "This is the uptime information:" 19 uptime Chapter 1. Bash and Bash scripts 17
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Bash Guide for Beginners # This is
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Chapter 6. The GNU awk programming
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Bash Guide for Beginners 121script.
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Bash Guide for Beginners amd.conf a
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Chapter 7. Conditional statements I
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[root@elegance root]# We switch to
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Bash Guide for Beginners echo "Prop
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Bash Guide for Beginners We use the
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Bash Guide for Beginners case EXPRE
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Upon successful condition testing,
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Bash Guide for Beginners bit better
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8.2. Catching user input 8.2.1. Usi
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Bash Guide for Beginners michel ~/t
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Bash Guide for Beginners # DO NOT E
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File descriptor 5 Bash Guide for Be
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Bash Guide for Beginners The result
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Bash Guide for Beginners ♦ Compre
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Bash Guide for Beginners for i in `
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touch pic-`date +%s`.jpg sleep 300
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DISKFUL=$(df -h $WEBDIR | grep -v F
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;; 2) $FORTUNE startrek ;; 3) $FORT
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[carol@octarine ~/test] ls -a ./ ..
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Chapter 10. More on variables In th
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Bash Guide for Beginners [bob in ~]
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if [ -z $1 ]; then echo "Usage: $0
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10.3.3. Transformations of variable
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Bash Guide for Beginners The opposi
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Chapter 11. Functions In this chapt
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The commands after && are only exec
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zip2 "$TAR" echo "...done." >> "$LO
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Chapter 12. Catching signals In thi
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Bash Guide for Beginners maud: ~> k
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Bash Guide for Beginners 2. Write a
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Bash Guide for Beginners jobs kill
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if [ $i -eq 5 ] ArrowUp+Enter or !!
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cd Change directory. cdp/cdplay An
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Bash Guide for Beginners gimp grep
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Send and receive mail. man Read man
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Bash Guide for Beginners rsh RPM Pa
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W Bash Guide for Beginners w Show w
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Index A aliases Section 3.5.1 ANSI-
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Bash Guide for Beginners Section 3.
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J Bash Guide for Beginners K kill k
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eadonly Section 10.1.3 regular expr
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Bash Guide for Beginners vi(m) Sect
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