UNIX Bourne Shell Programming Developed by: User Liaison Section, D-7131 Denver Office [Name and number removed at author's request] Revision Date: February 7, 1991 I. INTRODUCTION.............................................. v A. Audience............................................. v B. Course Objectives.................................... v C. Course Handout Conventions........................... vi 1. BOURNESHELL OVERVIEW...................................... 1 1.1 What is the BourneShell?............................ 2 1.2 Making a Bourne Shell Script Executable............. 3 1.3 Tracing Mechanisms.................................. 6 Workshop 1............................................... 9 2. USER, SHELL, AND READ-ONLY SHELL VARIABLES................ 11 2.1 User Variables...................................... 11 2.2 Shell Variables..................................... 14 2.2.1 HOME......................................... 14 2.2.2 IFS.......................................... 15 2.2.3 MAIL......................................... 15 2.2.4 MAILPATH..................................... 15 2.2.5 MAILCHECK.................................... 16 2.2.6 PATH......................................... 16 2.2.7 PS1.......................................... 17 2.2.8 PS2.......................................... 17 2.3 Read-Only User Variables............................ 18 2.4 Read-Only Shell Variables........................... 19 2.4.1 Name of the Calling Program.................. 19 2.4.2 Arguments.................................... 19 2.4.3 Shift........................................ 21 2.4.4 Set.......................................... 22 2.4.5 expr......................................... 23 Workshop 2............................................... 27 3. POSITIONAL PARAMETERS..................................... 33 3.1 Reading Input Into a Shell Variable................. 34 3.2 Command Substitution................................ 36 3.3 Comments in BourneShell Scripts..................... 38 3.4 BourneShell Environment - Exporting Variables....... 39 Workshop 3............................................... 41 4. CONTROL CONSTRUCTS:....................................... 45 4.1 Types of Tests Used with Control Constructs:........ 46 4.2 Test on Numeric Values.............................. 47 4.3 Test on Character Strings........................... 47 4.4 Test on File Types.................................. 49 4.5 if then............................................. 50 4.6 if then else........................................ 52 4.7 if then elif........................................ 54 4.8 for................................................. 55 4.9 while............................................... 57 4.10 until.............................................. 58 4.11 case............................................... 60 Workshop 4............................................... 63 5. COMPILING PROGRAMS IN UNIX................................. 67 5.1 "C": Sample Program with a Main and Two Functions in One ................................. 67 5.2 "C": Compiling a Program............................ 69 5.3 "C": Renaming the Executable Module................. 71 5.4 "C": Giving a Name to the Output File............... 72 5.5 "C": Producing an Assembly Listing.................. 73 5.6 "C": Main and Two Functions in Three Separate Source Files.......................................... 74 5.7 "C": Compiling but Not Producing an Executable Module............................................. 75 5.8 FORTRAN: Sample Program a Main and Two Subroutine... 76 5.9 FORTRAN: Compiling a Program........................ 77 5.10 FORTRAN: Renaming the Executable Module............ 79 5.11 FORTRAN: Giving a Name to the Output File.......... 80 5.12 FORTRAN: Producing an Assembly Listing............. 81 5.13 FORTRAN: Main and Two Subroutines in Three Separate Source Files.............................. 82 5.14 FORTRAN: Compiling But Not Producing an Executable Module.................................... 83 5.15 FORTRAN: Compiling Object Files to Produce an Executable Module............................. 84 5.16 COBOL: Sample Program with a Main and Two Subroutines................................. 85 5.17 COBOL: Compiling a Program......................... 86 5.18 COBOL: Running a Program........................... 87 Workshop 5............................................... 89 6. UNIX TOOLS................................................ 95 6.1 Processes........................................... 95 6.2 Executing a Command................................. 95 6.3 Process Identification.............................. 95 6.4 grep: A Pattern Matching Filter..................... 98 6.4.1 More on Regular Expressions.................. 99 6.4.2 Closure......................................103 6.4.3 Some Nice grep Options ......................104 6.4.4 Summary of Regular Expression Characters.....105 6.5 sed: Edit a File to Standard Output.................106 6.6 awk: A Pattern Matching Programming Language........110 6.7 sort: Sort a File...................................114 6.8 What Other Useful UNIX Tools are Available..........117 6.9 Archiver and Library Maintainer.....................118 6.9.1 ar: Creating an Archive File with Object Modules.....................................119 6.9.2 ar: Verifying the Contents of the Archive File.......................................119 6.9.3 ar: Removing Duplicate Object Files..........120 6.9.4 ar: Compiling Main and Archive Files.........120 Workshop 6...............................................121 7. VAX DCL TO UNIX SHELL SCRIPT CONVERSION...................125 7.1 Processes...........................................127 7.2 Pipes...............................................128 7.3 Input, Output, and Error Redirection................129 7.4 Command Structure and File Naming Conventions.......131 7.5 File Management Commands............................133 7.6 Metacharacters......................................135 7.7 Wildcards: Are They Really Wild?....................136 7.8 Summary.............................................137 Workshop 7...............................................139 8. ADVANCED FEATURES OF FTP..................................143 8.1 Initializing FTP on UMAX............................144 8.2 Multiple File Transfers.............................145 8.3 Auto Login Feature..................................146 8.4 Macros..............................................148 8.5 Filename Translation................................149 8.6 Aborting Transfers..................................150 8.7 More Remote Computer Commands.......................151 Workshop 8...............................................153 9. OPTIONAL CHAPTER - KORNSHELL PROGRAMMING..................155 9.1 KornShell Variables.................................155 9.2 User Defined Variables..............................157 9.3 Values of Variables Between Child and Parent Processes...........................................158 9.4 ksh: Aliases........................................159 9.5 ksh: Command Line Editing...........................161 9.6 ksh: Interactive Command Line Editing...............162 9.7 ksh: Functions......................................164 9.8 ksh: The Select Construct...........................166 9.9 ksh: Tracing and Conditional Execution..............168 Workshop 9...............................................169 APPENDIX A - sh...............................................173 APPENDIX B - test.............................................189 APPENDIX C - expr.............................................193 APPENDIX D - ftp..............................................195 APPENDIX E - cc...............................................209 APPENDIX F - f77..............................................219 APPENDIX G - lint.............................................231 APPENDIX H - cb...............................................235 APPENDIX I - ar...............................................237 APPENDIX J - time.............................................243 APPENDIX K - ksh..............................................245 INDEX.........................................................279 I. INTRODUCTION A. Audience This course is for individuals who have completed "UNIX for Beginning Users" (or equivalent experience) and want to write UNIX BourneShell script files. A script file contains a sequence of UNIX commands which can be executed by entering one command. It is assumed that the student already has a good understanding of the UNIX operating system, be able to use a UNIX editor, and be familiar with a computer terminal or typewriter keyboard. B. Course Objectives Upon successful completion of this course the student will be able to: 1. Write moderately complex BourneShell scripts. 2. Make a BourneShell script executable. 3. Demonstrate how to use the following BourneShell commands: shift, exit, expr, test, if then, if then else, if then elif, for, while, until, and case. 4. Use the following BourneShell constructs: tracing mechanisms (for debugging), user variables, BourneShell variables, read-only variables, positional parameters, reading input to a BourneShell script, command substitution, comments, and exporting variables. In addition, test on numeric values, test on file type, and test on character strings are covered. 6. Create a ".profile" script to customize the user environment. 7. Use advanced features of File Transfer Protocol (FTP) 8. Compile source code into object and executable modules. 9. Optional: KornShell programming. This is of primary interest to programmers. 10. Convert VMS DCL command files to UNIX Shell. C. Course Handout Conventions There are several conventions used in this handout for consistency and easier interpretation: 1. Samples of actual terminal sessions are single-lined boxed. 2. User entries are shown in bold print and are underlined. exit 3. All keyboard functions in the text will be bold. (Ret) Backspace Tab Ctrl-F6 Print (Shift-F7) Go to DOS (1) NOTE: (Ret) indicates the Return or Enter key located above the right Shift key. 4. Examples of user entries not showing the computer's response are in dotted-lined boxes. 5. Command formats are double-lined boxed. 6. Three dots either in vertical or horizontal alignment mean continuation or that data is missing from diagram. 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � � � Multimax, Nanobus, and UMAX are trademarks of � � Encore Computer Corporation. � � � � � � Annex is a trademark of XYLOGICS, Inc. � � � � � � UNIX and Teletype are registered trademarks of � � AT&T Bell Laboratories � � � � � � Ethernet is a trademark of Xerox Corporation � � � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� NOTES 複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複� 1. BOURNESHELL OVERVIEW The BourneShell is both a command-line interpreter and a high- level programming language. When it is acting as a command-line interpreter, it processes commands as you enter them at the command prompt. When you use it as a programming language, it processes commands that are stored in files known as BourneShell scripts. This course will show you how to create and execute BourneShell scripts. We will explore BourneShell programming including such features as variables, control structures, processes, and executable files. The BourneShell is one of three shells available on most UNIX systems. Bourne is the accepted standard for System V UNIX. The other shells are being used more and more. The other shells are the CShell and the KornShell. The CShell is BSD (Berkeley Software Distribution) UNIX. BSD was developed at the University of California at Berkeley, California. Most of the features found in the BourneShell are also found in the other shells; there are differences, however. The CShell and KornShell are not standard on UNIX System V but are generally available. BourneShell scripts allow you to group command lines together and execute them by entering a single command at the command line. This allows complex functions to be completed by any user, and repetitive functions can be completed easily. Input and output can also be redirected from a BourneShell script. 1.1 What is the BourneShell? BourneShell is a high level programming language and a command line interpreter. The command to invoke the BourneShell is: �袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴敲 � Command Format: sh [-acefhiknrstuvx] [args] � � � � (See Appendix A for a complete list of options etc) � 훤袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴� A Shell script is an executable plain file that contains UNIX and shell commands. To execute the shell script type the name of the script at the prompt. A simple shell script called shell_ex is shown in the following example. The output from the execution of the shell is also shown. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat shell_ex � � echo "This is a very simple shell procedure " � � echo "created with the basic echo command " � � echo "and three other very basic commands " � � echo � � ps � � echo � � who � � echo � � ls � � $sh shell_ex � � This is a very simple shell procedure � � created with the very basic echo command � � and three other very basic commands � � � � PID TTY TIME COMMAND � � 10443 rt02120 0:01 sh � � 10427 rt02120 0:04 ksh � � � � sgavlick rt021e0 Sep 7 13:26 � � teacher rt021b0 Sep 7 14:39 � � � � memo � � class_notes � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 1.2 Making a Bourne Shell Script Executable A BourneShell script is an ordinary file that contains commands which can be executed in sequence by entering one command at the BourneShell prompt. In order for a script to be executed, it must first be executable. This is done with the chmod command. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat shell_ex � � echo "This is a very simple shell procedure " � � echo "created with the basic echo command " � � echo "and three other very basic commands " � � echo � � ps � � echo � � who � � echo � � ls � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� If the ls -l shell_ex command were entered, we would see the protections assigned to this file. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $ls -l shell_ex � � -rw-r--r-- 1 teacher class 66 Sep 7 10:24 shell_ex � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The character in column one is the type of file. - = ordinary (plain) disk file d = directory b = block special file c = character special file p = fifo file ("named pipe") special file l = symbolic link Notice that the script file in the previous sample session has the following file protections: User - Read and Write Group - Read Other - Read No execute permissions have been granted for user, group, or other. If we try to execute this script by typing its name, the following would result. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $shell_ex � � shell_ex: execute permission denied � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� This error message would indicate that execute permission was denied. The BourneShell script could not be executed. To change the permissions for the BourneShell script, use the chmod command. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $chmod 755 shell_ex � � $ls -l shell_ex � � -rwxr-xr-x 1 teacher class 66 Sep 7 10:26 shell_ex � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Now that the permissions have been changed to allow user, group, and others to execute the file, it will execute properly. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $shell_ex � � This is a very simple shell procedure � � created with the basic echo command � � and three other very basic commands � � � � PID TTY TIME COMMAND � � 10443 rt02120 0:01 sh � � 10427 rt02120 0:04 ksh � � � � sgavlick rt021e0 Sep 7 13:26 � . teacher rt021b0 Sep 7 14:39 . . . . . The protections will work as you expect. Execute permission for the user will allow you (the owner) to run the BourneShell script. Group permissions allow anyone in your group to execute the script, and other permission allows anyone on the system to execute the script. 1.3 Tracing Mechanisms It is possible to have a trace made of the BourneShell script as it executes. This is invaluable for debugging purposes. All that is required is to give an option to the BourneShell. This is done by including an option on the call to "sh". The command to do this is: �袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴敲 � Command Format: sh [-acefhiknrstuvx] [args] � � � � See Appendix A for a complete list of options etc � � � 훤袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴� The option to turn on tracing is -x. For an example, let's trace the execution of the simple script shell_ex. Sample session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat shell_ex � � echo "This is a very simple shell procedure " � � echo "created with the basic echo command " � � echo "and three other very basic commands " � � echo � � ps � � echo � � who � � echo � � ls � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Execute the BourneShell script using the -x option on the call to the shell. The following sample session shows how to do this and it shows the results of the trace. Sample session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $sh -x shell_ex � � + echo This is a very simple shell procedure � � This is a very simple shell procedure � � + echo created with the basic echo command � � created with the basic echo command � � + echo and three other very basic commands � � and three other very basic commands � � + echo � � � � + ps � � PID TTY TIME COMMAND � � 10443 rt01120 0:01 sh � � 10427 rt02120 0:04 ksh � � + echo � � � � + who � � sgavlick rt021e0 Sep 7 13:26 � � teacher rt02120 Sep 7 14:39 � � + echo � � � � + ls � � memo � � class_notes � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The commands as read from the BourneShell script are indicated by the plus sign (+). The next line or lines are the results of the execution of the command. Using this tracing option allows you to se the execution of each command in the script and see the results of that execution. NOTES 複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複 Workshop 1 This workshop will reinforce your understanding of the ideas presented in Chapter 1. Each student is to complete the entire workshop. DESK EXERCISES 1. The BourneShell can act as a command line or a high level . 2. The BourneShell is one of three shells generally available. What are the other two? 3. One advantage of using a shell script is . 4. The command to call the BourneShell is: a. bourne b. ksh c. b d. sh 5. Why would you use tracing? 6. What UNIX command do you enter to make a BourneShell script executable? That's all NOTES 複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複� 2. USER, SHELL, AND READ-ONLY SHELL VARIABLES The BourneShell has no true numeric variables. It uses string variables to represent numbers, as well as text. String variables are able to take on the value of a string of characters. There are three types of variables in the BourneShell. They are user variables, BourneShell variables, and Read-only BourneShell variables. You can declare, initialize, read, and modify user variables from a BourneShell script or from the command line. The BourneShell itself declares and initializes shell variables, but you can read and modify them. The BourneShell also initializes the read-only shell variables, and you can read but not modify them. 2.1 User Variables It is legal to assign any sequence of non-blank characters as the name of a variable. The sample session below creates a variable called person and initializes it with the string Richard. It is important to note that you must NOT precede or follow the equal sign with a space or TAB character. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $person=Richard � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� This sample session indicates that person does not represent the string Richard. The string person is echoed as person. The BourneShell will only do the substitution of the value of the variable when the name of the variable is preceded with a dollar sign ($). Sample Sesssion: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo person � � person � � $echo $person � � Richard � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� If you want to have imbedded spaces in a variable, it is necessary to quote the string. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $person='Richard and Kathleen' � � $echo $person � � Richard and Kathleen � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The echo utility copies its arguments to the standard output. The command echo $person displays the value of the variable person. It will not display $person because the BourneShell doesn't pass $person as an argument. The leading dollar sign ($) causes the BourneShell to substitute the value of the variable and then passes that value to the utility. The echo utility then displays the value of the variable, not its name, never knowing that you called it with a variable. The BourneShell passed the same command line as if you had typed in echo Richard and Kathleen. The BourneShell can be prevented from doing this substitution by entering one of the following: Sample Sessions: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo $person � � Richard and Kathleen � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� In this session the contents of the variable person are displayed. The BourneShell made the substitution because the variable name person is preceded by a dollar sign ($). 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo \$person � � $person � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� In the above example the variable person is preceded by a dollar sign ($) but the dollar sign has a backslash (\) ahead of it. The backslash has the effect of cancelling the special meaning of the character following the backslash. In this case, the special meaning of the dollar sign is ignored and the substitution is not done. 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo '$person' � � $person � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The single quote marks (') causes the characters between the marks to be taken as literal. The shell makes no attempt to interpret the meanings of these characters. The shell passes these characters on with no substitution. 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo "$person" � � Richard and Kathleen � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The double quote marks do not prevent the shell from making substitution; and the value of the variable will be displayed by the utility. 2.2 Shell Variables The BourneShell declares and initializes variables that determine such things as your home directory, what directories the shell will look in when you give commands, how often to look for mail, your prompt, and many other things. We will look at several of these BourneShell variables and their functions. You can assign new values to these variables from the command line or from the execution of the .profile file in your home directory. 2.2.1 HOME The first BourneShell variable that we will look at is the HOME variable. By default, the home directory is the current working directory after you login. The system administrator determines your home directory when you establish an account and places that information in the /etc/passwd file. When you login, the BourneShell gets that pathname and assigns it to the HOME variable. When you enter a cd command with no argument, the utility takes the name of the directory from the HOME variable and makes it the current working directory. If you change the HOME variable to another directory pathname, the utility will make the new directory the current working directory. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo $HOME � � /user0/rharding � � $cd � � $pwd � � /user0/rharding � � $HOME=/user0/rharding/eng � � $cd � � $pwd � � /user0/rharding/eng � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� This example shows how the value of the HOME variable affects the cd utility. The cd command will use the value of the HOME variable as the pathname for the current working directory. 2.2.2 IFS This is the internal-field separator BourneShell variable. You can always use a space or tab to separate characters on the command line. When you assign the IFS variable to another character, you can also use this character as the field separator. Example: ................................................................. . $num_args a:b:c:d . ................................................................. This example shows only one argument, namely a:b:c:d. ................................................................. . $IFS=: . . $num_args a:b:c:d . ................................................................. This example now shows four different arguments; each being separated by the new IFS, (:). 2.2.3 MAIL The MAIL variable contains the name of the file that the mail (and mailx) utilities use to store your mail. Usually, the absolute pathname of this file is /usr/mail/name, where name is your login name. Example: ................................................................. . $MAIL=/usr/mail/rharding . ................................................................. 2.2.4 MAILPATH This variable contains a list of filenames separated by colons. If set, the BourneShell will inform you when any of these files are modified (i.e. when new mail arrives). Normally, this variable is not set. 2.2.5 MAILCHECK This variable specifies how often, in seconds, the BourneShell will check for new mail. The default is 600 seconds. If set to 0, it will check for new mail each time before it gives you a prompt. 2.2.6 PATH This BourneShell variable will describe the directories that will be searched looking for the program that you want to execute. The BourneShell looks in several directories for a file that has the same name as the command that you entered. The PATH variable controls this search path. Normally, the first directory searched is the current working directory. If the program is not found, the search continues in the /bin and then the /usr/bin directory. Generally, these directories contain executable programs. If the program is not found in one of these directories, the BourneShell reports that the program can't be found (or executed). The PATH variable lists the pathnames in the order in which the search will proceed. The pathnames are separated by a colon (:). If nothing (null string) precedes the colon, that indicates to start the search at the current working directory. Example: ................................................................. . $PATH=:/user0/rharding/bin:/bin:/usr/bin . . $ . ................................................................. This PATH variable indicates to start the search for the program at the current working directory, then look in the directory /user0/rharding/bin, then /bin, and finally /usr/bin. If each user has a unique path specified, each user can execute a different program by giving the same command. The search for the program stops when it is satisfied; thus, you can use the same name for your own programs as the standard UNIX utilities. To do this, simply put your program in one of the first directories that the BourneShell searches. 2.2.7 PS1 This is the BourneShell prompt which lets you know that the shell is waiting for you to give it a command. The default BourneShell prompt is a dollar sign ($). The shell stores the prompt as a string variable in PS1. When you change the value of this variable, the appearance of the prompt will change. When you are working on several different machines, it might be useful to have the prompt be the name of the machine you are working on. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $pwd � � /user0/rharding � � $PS1='domax0: ' � � domax0: � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Notice that prompt is now domax0: 2.2.8 PS2 This variable is called the secondary prompt. If the command is not completed on one line and must be continued on the next line, the prompt for that continued line is PS2. The default is >. This prompt indicates that the BourneShell is expecting you to finish the previous command line. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $echo 'demonstration of prompt string � � >2' � � demonstration of prompt string � � 2 � � $PS2='Continue? ' � � $echo 'demonstration of � � Continue? prompt string 2' � � demonstration of � � prompt string 2 � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Notice how the secondary prompt was changed to "Continue? ". 2.3 Read-Only User Variables The contents of the user variables and the shell variables can be modified by the user. It is possible to assign a new value to them. The new value can be assigned from the dollar ($) prompt or from inside a BourneShell script. Read-only variables are different. The value of read-only variables can not be changed. The variable must be initialized to some value; and then, by entering the following command, it can be made read only. �袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴敲 � Command format: readonly variable_name � � � � variable_name = name of the variable to be made read only � 훤袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴� Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $person=Kathleen � � $readonly person � � $echo $person � � Kathleen � � $person=Richard � � person: is read only � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The readonly command given without any arguments will display a list of all the read-only variables. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $person=Kathleen � � $readonly person � � $example=Richard � � $readonly example � � $readonly � � readonly person � � readonly example � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 2.4 Read-Only Shell Variables The read-only shell variables are similar to the read-only user variables; except the value of these variables is assigned by the shell, and the user CANNOT modify them. 2.4.1 Name of the Calling Program The shell will store the name of the command you used to call a program in the variable named $0. It has the number zero because it appears before the first argument on the command line. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat name_ex � � echo 'The name of the command used' � � echo 'to execute this script was' $0 � � $name_ex � � The name of the command used � � to execute this script was name_ex � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 2.4.2 Arguments The BourneShell will store the first nine command line arguments in the variables named $1, $2, ..., $9. These variables appear in this section because you cannot change them using the equal sign. It is possible to modify them using the set command. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat arg_ex � � echo 'The first five command line' � � echo 'arguments are' $1 $2 $3 $4 $5 � � $arg_ex Richard Kathleen Douglas � � The first five command line � � arguments are Richard Kathleen Douglas � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The script arg_ex will display the first five command-line arguments. The variables representing $4 and $5 have a null value. The BourneShell variable $* represents all of the command-line arguments as shown in the following example. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat display_all � � echo $* � � $display_all Richard Kathleen Douglas � � Richard Kathleen Douglas � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The BourneShell variable $# contains the number of arguments on the command line. This is a string variable that represents a decimal number. You can use the expr utility to perform calculations with that number and test to perform logical tests on it. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat num_args � � echo 'This script was called with' � � echo $# 'arguments' � � $num_args Richard Kathleen Douglas � � This script was called with � � 3 arguments � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 2.4.3 Shift The shift command promotes each of the command-line arguments. The second argument, represented by $2, is now the first argument, represented by $1. The third becomes the second and so on until the last argument becomes the next to last. You can access only the first nine command-line arguments (as $1 through $9). The shift command gives you access to the tenth, and the first becomes unavailable. There is no "unshift" command that will return the arguments that are no longer available. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat demo_shift � � echo 'arg1='$1 ' arg2='$2 ' arg3='$3 � � shift � � echo 'arg1='$1 ' arg2='$2 ' arg3='$3 � � shift � � echo 'arg1='$1 ' arg2='$2 ' arg3='$3 � � shift � � echo 'arg1='$1 ' arg2='$2 ' arg3='$3 � � shift � � $demo_shift Richard Kathleen Douglas � � arg1=Richard arg2=Kathleen arg3=Douglas � � arg1=Kathleen arg2=Douglas arg3= � � arg1=Douglas arg2= arg3= � � demo_shift: cannot shift � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The BourneShell will display an error message when the script executes a shift command after it has run out of variables. 2.4.4 Set The Set command will display a list of all the variables that are set when it has no arguments. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $set � � HOME=/user0/teacher � � IFS= � � � � LOGNAME=richard � � MAIL=/usr/mail/richard � � MAILCHECK=600 � � PATH=:/bin:/usr/bin � � PS1=$ � � PS2=> � � SHELL=/bin/sh � � TERM=vt100 � � TZ=MST7MDT � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� When set is called with arguments, it sets the value of the command-line arguments ($1-$n) to the arguments. The example sets the first three arguments. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat set_ex � � set who really cares � � echo $#: $* � � $set_ex � � 3: who really cares � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 2.4.5 expr The expr command will perform arithmetic in the BourneShell. �袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴敲 � Command format: expr expression � � � � See Appendix C for a complete list of expressions � 훤袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴袴� The arguments are taken as an expression. After the evaluation has taken place, the result is written to standard output. The terms of the expression must be separated by blanks. Special characters to the shell must be escaped. Strings containing blanks or other special characters must be quoted. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $expr 7 + 8 + 10 � � 25 � � $expr 10 - 8 � � 2 � � $expr 10 '*' 4 � � 40 � � $expr 135 / 5 � � 27 � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� expr will also work with user defined variables as in the following example: Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat data � � 8 � � 15 � � 25 � � $cat express � � count=0 � � tot=0 � � for a in `cat data` � � do � � tot=`expr $tot + $a` � � count=`expr $count + 1` � � done � � avg=`expr $tot / $count` � � echo "The average is $avg" � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Let's execute the script "express" with tracing on so we can follow the execution. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $sh -x express � � count=0 � � tot=0 � � + cat data � � + expr 0 + 8 � � tot=8 � � + expr 0 + 1 � � count=1 � � + expr 8 + 15 � � tot=23 � � + expr 1 + 1 � � count=2 � � + expr 23 + 25 � � tot=48 � � + expr 2 + 1 � � count=3 � � + expr 48 / 3 � � avg=16 � � + echo The average is 16 � � The average is 16 � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� NOTES 複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複� NOTES 複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複� Workshop 2 This workshop will reinforce your understanding of the ideas presented in Chapter 2. Login to the Multimax using the username and password given to you by the instructor. Each student is to complete the entire workshop. DESK EXERCISES 1. Any series of non-blank characters can be assigned to a user variable. True/False 2. How can you insert a space into a user variable? 3. What utility can be used to display the contents of a user variable to standard output? 4. The backslash (\) character is used to remove the special meaning of some characters. True/False 5. What other character can be used to prevent the shell from doing the substitution? 6. Double quote marks will prevent the shell from making the substitution. True/False Continue on the next page 7. What do the following shell variables do? HOME IFS MAIL MAILPATH MAILCHECK PATH PS1 PS2 Continue on the next page 8. What is the command to create a read-only user variable? 9. What is the read-only shell variable that represents the calling program? 10. What do $1,$2,...,$9 represent? 11. What BourneShell variable represents all of the command line arguments? 12. What does the shift command do? 13. What is displayed when you enter set with no arguments? Continue on the next page COMPUTER EXERCISES 14. Login to the Multimax (domax1) using the username and password given to you by the instructor. 15. Create a subdirectory called sub_dir. 16. Modify your .profile to include the following: a) Change the home directory to sub_dir b) Set the internal-field separator to a comma c) Have mail messages saved into mail1. d) Set the PATH to look for programs in the following directories: $HOME/bin /bin /usr/bin e) Change the prompt to reflect the name of the system f) Change the secondary prompt to 'More?' 17. Execute the .profile Enter $. .profile 18. Verify that the changes are correct. If you have extra time go to the next page. Extra Mile on the next page Extra Mile Change the .profile file so the date/time and a list of all users that are currently logged in will be displayed on your monitor screen automatically when you login. NOTES 複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複複� 3. POSITIONAL PARAMETERS A BourneShell script can also read in command-line arguments. The first argument is referred to as $1, the second is $2, and so on. Command-line arguments are referred to as positional parameters. Let's look at an example BourneShell script to see how these are used. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat neat_shell � � echo $1 $2 $3 � � echo $0 is the name of the shell script � � echo "There were $# arguments." � � echo $* � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Insure that the BourneShell script is executable by issuing this command: Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $chmod a+x neat_shell � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Now, if we type the name of the BourneShell script with no arguments, we get the following results. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $neat_shell � � � � neat_shell is the name of the shell script � � There were 0 arguments. � � � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� In this sample session, there were no arguments given so none were printed. $0 is the positional parameter that refers to the name of the script. Since there were no arguments given with this invocation of neat_shell, there were zero arguments listed. 3.1 Reading Input Into a Shell Variable The BourneShell script can read user input from standard input. The read command will read one line from standard input and assign the line to one or more variables. The following example shows how this works. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat read_script � � echo "Please enter a string of your choice" � � read a � � echo $a � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� This simple script will read one line from standard input (keyboard) and assign it to the variable a. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $read_script � � Please enter a string of your choice � � Here it is � � Here it is � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The line read from standard input can also be assigned to several variables as shown in the following example. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat reads � � echo "Please enter three strings" � � read a b c � � echo $a $b $c � � echo $c � � echo $b � � echo $a � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� This time, we will turn on the trace mechanism and follow the execution of this BourneShell script. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $sh -x reads � � + echo Please enter three strings � � Please enter three strings � � + read a b c � � this is more than three strings � � + echo this is more than three strings � � this is more than three strings � � + echo more than three strings � � more than three strings � � + echo is � � is � � + echo this � � this � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� It is interesting to note that the spaces separate the values for the variables a,b, and c. For example, the variable a was assigned the string this, the variable b was assigned the string is, and the remainder of the line was assigned to c (including the spaces). Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat read_ex � � echo 'Enter line: \c' � � read line � � echo "The line was: $line" � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� In this example, the \c option will suppress the carriage return. The single quote marks protect the backslash from being interpreted by the shell. Also notice that the double quote marks have no effect on the substitution of the variable line. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $read_ex � � Enter line: All's well that ends well � � The line was: All's well that ends well � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 3.2 Command Substitution You can execute a command by enclosing it within two grave accent marks [these are sometimes called backquotes (`)]. The BourneShell will replace the command and the grave marks with the output from the command. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat dir � � dir=`pwd` � � echo 'You are using the' $dir 'directory' � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� NOTE: The grave marks lean to the left, and the apostrophes lean to the right. The grave marks enclose the pwd command. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $dir � � You are using the /user0/rharding directory � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The important thing to notice here is that the pwd command was executed; and the output, /user0/rharding, was then assigned to the variable dir. It is not necessary to assign the output of a command to a variable as shown in the previous example. The command substitution can occur directly as shown in the next example. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat dir2 � � echo 'You are using the' `pwd` 'directory' � � $dir2 � � You are using the /user0/rharding directory � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� One final example will show a practical use of command substitution. This BourneShell script will use the date command to provide the date in a useful format. The normal output from the date command looks like the following. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $date � � Wed Sep 12 18:02:05 MDT 1990 � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� Here's a BourneShell script that rearranges the output into a more useable format. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat dateset � � set `date` � � echo $* � � echo � � echo 'Argument 1:' $1 � � echo 'Argument 2:' $2 � � echo 'Argument 3:' $3 � � echo 'Argument 4:' $4 � � echo � � echo $2 $3, $6 � � $dateset � � Wed Sep 12 18:02:05 MDT 1990 � � � � Argument 1: Wed � � Argument 2: Sep � � Argument 3: 12 � � Argument 4: 18:02:05 � � � � Sep 12, 1990 � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� The first command in the BourneShell script dateset uses the grave accent marks to set the command-line argument variables to the output of the date command. The next commands show the first four of these argument variables. The final command displays the arguments in a different order that could be useful in a report or a letter. 3.3 Comments in BourneShell Scripts Comments can be inserted into the BourneShell script by beginning each comment line with the pound symbol (#) or a colon (:). All characters after the comment character will be ignored by the shell. The only exception to this rule is that the first character of the first line must not be a pound symbol; if the first character is a pound sign, the BourneShell tries to execute the script as if it was written in CShell syntax. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat com_sub � � # The first line sets your present working directory � � # to the variable 'directory' � � directory=`pwd` � � # The second line sets the date to the variable 'when' � � when=`date` � � : The third line will echo on the screen � � echo "You are in $directory on $when" � � : You could have said echo : � � : "You are in `pwd` on `date`" � � : to have a one line program � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 3.4 BourneShell Environment - Exporting Variables Within a process, you can declare, initialize, read, and modify variables. The variable is local to that process. When a process forks a child process, the parent process does not automatically pass the value of the variable to the child process. Here is an example of the variables not being exported. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat no_export � � car=mercedes # set the variable � � echo $0 $car $$ # $0 = name of file executed � � # $car =value of variable car � � # $$ = PID number (process id) � � inner # execute another BourneShell script � � echo $0 $car $$ # display same as above � � $cat inner � � echo $0 $car $$ # display variables for this process � � $chmod a+x no_export � � $chmod a+x inner � � $no_export � � no_export mercedes 4790 � � inner 4792 � � no_export mercedes 4790 � � $ � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� When no_export was executed, it, of course, assigned a value of mercedes to the variable car and printed it out. The call to inner created a child process. Its PID is 4792, while the parent PID is 4790. Notice, when inner tried to print the value of car, it printed nothing. The reason is because the value of car was not passed by the parent. Can the value be passed from parent to child process? Yes, by using the export command. Let's look at an example. Sample Session: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� � $cat export_it � � car=mercedes � � export car � � echo $0 $car $$ � � inner1 � � echo $0 $car $$ � � $cat inner1 � � echo $0 $car $$ � � car=chevy � � echo $0 $car $$