Bash Programming. Student Workbook

Transcription

1 Student Workbook

2 Bash Programming Published by ITCourseware, LLC, 7245 South Havana Street, Suite 100, Englewood, CO Contributing Authors: Julie Johnson, Rob Roselius Editor: Jeff Howell Special thanks to: Many Shell instructors whose ideas and careful review have contributed to the quality of this workbook, including Brandon Caldwell, Roger Jones, Jim McNally, and Bill Parette, and the many students who have offered comments, suggestions, criticisms, and insights. Copyright 2002 by ITCourseware, LLC. All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by an information storage retrieval system, without permission in writing from the publisher. Inquiries should be addressed to ITCourseware, LLC, 7245 South Havana Street, Suite 100, Englewood, CO (303) All brand names, product names, trademarks, and registered trademarks are the property of their respective owners. Page ii Rev ITCourseware, LLC

3 Contents Chapter 1 - Course Introduction... 7 Course Objectives... 8 Course Overview Using the Workbook Suggested References and Reading Chapter 2 - UNIX Processes What is a Process? Process Structure The ps Utility Options to the ps Utility Background Commands (&) Killing Background Processes Redirecting the Standard Error Labs Chapter 3 - Getting Started What is a Shell?...34 Running Scripts Specifying the Script's Interpreter The PATH Environment Variable Sub-shells Labs Chapter 4 - Variables Shell Variables The read Command The export Command The Shell Environment Parameter Expansion Command Substitution Labs ITCourseware, LLC Rev Page iii

4 Chapter 5 - The Login Process The Login Process The System Profile Script Your.bash_profile Script The. Command...70 Labs Chapter 6 - Conditional Statements The Exit Status of Commands Command Line Examples The test Command The if-then-else Construct The elif Construct case Statements...86 Labs Chapter 7 - Loops The for Loop The while Loop break and continue Reading Lines From Files Using Arrays with Loops Labs Chapter 8 - Special Variables $$ - PID of Shell Command-Line Arguments $# - Number of Arguments $* - All Arguments The shift Command The set Command Getting Options Labs Page iv Rev ITCourseware, LLC

5 Chapter 9 - Quoting Mechanisms Single vs. Double Quotes What is a Here Document? Using a Here Document Here Document Quoting Ignoring Leading Tabs Labs Chapter 10 - Functions Shell Functions Passing Arguments to Functions Returning Values from Functions Function Declarations Labs Chapter 11 - Advanced Programming Shell Arithmetic The select Statement Terminal Independence in Scripts The eval Command Labs Chapter 12 - Debugging Techniques Using echo Using Standard Error Script Tracing Options for Debugging Conditional Debugging Labs Solutions - Bash Programming Index ITCourseware, LLC Rev Page v

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7 Chapter 4 Variables Chapter 4 - Variables Objectives Create and use shell variables on the command line and in scripts. Write shell scripts that interact with users by displaying prompts and reading their response. Pass variables from parent to child scripts using environment variables. Capture the output of a command and store it in a variable or use it as parameters to another command. Use variable substitution ITCourseware, LLC Rev Page 47

8 Shell Variables Variables (also called parameters) are set with the equal sign binary operator. The value of a variable is retrieved with the dollar sign unary operator. Command line example: myname=bobo echo $myname bobo echo myname myname # Same as: echo bobo $name dereferences (i.e., returns the value of) the variable name. Don t put spaces or tabs before or after the equal sign in name=value. Page 48 Rev ITCourseware, LLC

9 Chapter 4 Variables Shell variables (called parameters in the manual) are variables much like those found in any programming language. Their values are always stored as strings, but there are mathematical operators in the shell language that will convert variables to numbers for calculations. Nearly all scripts use variables. The term binary operator on the facing page means that the equal sign operator takes two operands: the name of a variable on the left, and the variable s intended value on the right. The term unary operator means that the dollar sign takes only one operand, the name of a variable. Consider once more the following: echo $myname bobo What really happens is that the shell substitutes the value of myname for $myname right on the command line, before executing the echo command. The dollar sign tells the shell to look at the value of the variable myname and then replace $myname with bobo. That is to say, change to echo $myname echo bobo before executing the echo command. This is known parsing the command line. The echo command never saw a $, never saw myname, never saw $myname. All echo saw was bobo ITCourseware, LLC Rev Page 49

10 The read Command The read command allows you to prompt for input and store it in a variable. nameit2 echo -n "Enter your name: " read name echo "Did you parents actually pick $name for you?" del echo -n "Enter name of file to delete: " read file echo "Type 'y' to remove it, 'n' to change your mind..." rm -i $file echo "That was YOUR decision!" The -n option to echo omits the final newline character. Page 50 Rev ITCourseware, LLC

11 Chapter 4 Variables When it executes a read command, the shell script will pause, waiting for the user to type something on the keyboard. When the user types something and then presses <Enter>, whatever typed goes into the variable whose name appears to the right of read. When assigning a value to a variable with the read command, notice that the $ is not used. You only use a $ when you wish to substitute the value of a variable on a line, not when assigning it with read or =. Like this: read name NOT like this: read $name < Don t attempt to read into variables like this!!! Try it: echo "Line 1 here\n and line 2 here\n" echo "\n\n\n\n\n\n" 2002 ITCourseware, LLC Rev Page 51

12 The export Command The export command puts a variable into the environment so it will be accessible to child processes. Try this on the command line: x=hello sh echo $x exit export x sh echo $x hello exit # Run a child shell. # Nothing in x. # Return to parent. # It's there. If the child modifies x, it will not modify the parent s original value. Verify this by doing the above commands again, but this time change x in the child and then display it when back in the parent. By convention, exported variables are in all CAPS. Page 52 Rev ITCourseware, LLC

13 Chapter 4 Variables Since UNIX is a multitasking operating system, many processes (jobs) can execute concurrently. It is very common to write shell scripts that fire off children to do specific tasks in parallel. A primary mechanism for communicating information from the parent to the child is through environment variables. This allows parameterization of children ITCourseware, LLC Rev Page 53

14 The Shell Environment The environment is a list of name-value pairs that is passed to the child program. Variables that are not in the environment are not passed to children. The export command puts variables into the environment. set... # Show values of all variables. env # Show environment variables only.... myname=mary export myname env # Look at environment now.... Page 54 Rev ITCourseware, LLC

15 Chapter 4 Variables When you run set or env, you see several variables that you know you didn t set. So who did? Answer: the shell. There are several predefined variables that the shell creates and sets when you log in. They are used by the shell and can also be used by you and your shell scripts. A few of them are shown here (see ksh for more): HOME PATH The default argument (home directory) for cd. The search path for commands. It is a colon-separated list of directories that are searched when you type a command. PS1 Primary prompt string, whose default is $. PS1 means Prompt String 1. PS2 Secondary prompt string, whose default is >. SECONDS BASH Number of seconds since the shell was invoked. Pathname of instance of shell you are running. Try it: echo $HOME 2002 ITCourseware, LLC Rev Page 55

16 Parameter Expansion Parameters can be tested and manipulated with ${...} operations. Following are several substitution mechanisms: ${#parameter} ${parameter:-word} ${parameter:=word} ${parameter:?word} ${parameter:+word} ${parameter#pattern} ${parameter##pattern} ${parameter%pattern} ${parameter%%pattern} Length of parameter. If parameter is set and non-null, substitute it; else substitute word. If parameter is not set or is null, set it to word; substitute value of parameter. If parameter is set and non-null, substitute it; else print word and exit shell. If parameter is set and non-null, substitute word; else substitute nothing. See bash(1). Page 56 Rev ITCourseware, LLC

17 Chapter 4 Variables These parameter substitution mechanisms are very powerful and can make your code quite compact. Several of them are very useful for setting default values or printing error messages. Don t worry about using them a lot right now; you may find it easier to write the same functionality in your scripts with more conventional (although lengthier) if then else constructs. But even if you don t use these parameter substitution mechanisms yourself, you will run into existing scripts that do use them, and you need to be able to understand what they do. Example scripts: 1. # If the variable BACKUPPATH is defined in the # environment, use it, otherwise use $HOME/.backup Backup_path=${BACKUPPATH:-$HOME/.backup} cp -r * $Backup_path 2. # Extract the name of file $1 minus the. extension File=${1%.*} # $1 is positional arg 1 in a script 2002 ITCourseware, LLC Rev Page 57

18 Command Substitution The stdout of a command can be captured and stored in a variable by using command substitution. When you enclose a command in grave accent marks (a.k.a. back-ticks), the shell replaces the command (and the graves) with the command s output. Example 1: here=`pwd` #Graves, not single quotes. cd /a/long/way/away pwd /a/long/way/away echo $here /home/s3/scripts cd $here pwd /home/s3/scripts Example 2: num=`who wc -l` echo $num 7 Page 58 Rev ITCourseware, LLC

19 Chapter 4 Variables You may also perform command substitution with $( ). Example: here=$(pwd) echo $here num=$(who wc - l) echo $num 2002 ITCourseware, LLC Rev Page 59

20 Labs Write a script that prompts for your first name and reads it into a variable, then prompts for your last name and reads it into another variable, then displays your name in the format last, first. (Solution: showname) If your PS1 environment variable is set to PS1='${PWD#${HOME}/}-->', how would it affect your prompt? (Solution: PS1_ques) Write a script that tells how many people are logged on. The output should look something like this: There are 5 users currently logged on. (Solution: logger1) Make a directory in your home directory called bin. Move one of your executable shell scripts into your bin directory. Now edit.profile so that your bin directory (e.g., $HOME/bin) is added to your PATH variable. Log out and back in, then while still in your home directory, run the script that is in the bin directory. (Solution: bin_ex) What happens (if anything) when you set your environment variable HOME to /tmp, then do a cd? (Solution: HOME_ex) What happens (if anything) when you set your environment variable PATH to /tmp, then do an ls? (Solution: PATH_ex) Page 60 Rev ITCourseware, LLC

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