Introduction to Unix (CA263) Command File

1. Introduction to Unix (CA263)Command File By Tariq Ibn Aziz

2. Objectives • In this lecture you will learn • How to write your own commands and how to use shell variables.

3. Command File • A shell program can be typed directly at the terminal • $ who | wc –l • Or it can be type into a file and then file can be executed by the shell • $ cat > nu Who | wc -l • $

4. Execute the Command • Type nu at the command name to the shell • $ nu sh: nu: cannot execute • $ • You need to add execute permission to this nu script file

5. chmod Command • Ls –l nu • rw-rw-rw- 1 taziz taziz 12 jul 10 11:42 nu • $ chmod +x nu • rwxrwxrw- 1 taziz taziz 12 jul 10 11:42 nu • $ • Now try $ nu 8 $ nu > tally $ cat tally 8

6. Another Example[1] • Suppose you are working on a proposal called sys.caps and followig command sequence is needed every time you want to generate a new copy of a proposal. tbl sys.caps |nroff –mm –Tlp |lp • You can save it in a file and give it executable permission and execute script when you need.

7. Another Example[2] $ cat run tbl sys.caps |nroff –mm –Tlp |lp $ chmod +x run $ run Request id is laser1-15 (standard input)

8. Another Example[3] $ cat stats date Who | wc –l pwd $ chmod +x stats $ stats Wed Mar 23 11:55:50 ETD 2008 13 /home/aziz/documents/proposals

9. Another Example[4] • You can add some echo command to stats to make the output more informative. $ cat stats echo The current date and time is: date echo echo The number of users on the system is: who | wc –l echo echo Your current working directory is: pwd

10. Another Example[5] $ stats The current date and time is: Wed Mar 23 11:55:50 ETD 2008 The number of users on the system is: 13 Your current working directory is: /home/aziz/documents/proposals $

11. Comments[1] • A shell programming cannot be complete without comment statement. • Whenever the shell encounters the special character # at the state of a word, it takes as a comment and ignore the line. # Here is an entire commentary line Who | wc –l # count the number of users # # Test to see if the correct arguments were # supplied $

12. Comments[2] $ cat stats # # stats – prints: date, number of users # logged on and current directory # echo The current date and time is: date echo echo The number of users on the system is: who | wc –l echo echo Your current working directory is: pwd

13. Variables[1] • Like all programming languages, the shell allow you to store values into variables. • A shell variable begins with an alphabetic or underscore (_) character, and followed by zero or more alphanumeric or underscore characters. variable=value

14. Variables[2] • To assign the value 1 to the shell variable count count=1 • To assign the value /home/aziz/bin to the shell variable my_bin my_bin=/home/aziz/bin

15. Displaying the value of Variables • The echo command is used to display the value that is stored inside a shell variable echo $variable • The $ is a special character to the shell. If a valid variable name follows the $, then shell substitute the value stored inside the variable. $ echo $count 1 $

16. Displaying the value of Variables • You can have the value of more than one variable substituted at a time. $ echo $my_bin /home/aziz/bin $ echo $my_bin $count /home/aziz/bin 1 $

17. Use of Variables value[1] • The value of variables can be used anywhere on the command line. $ ls $my_bin mon nu textx $ pwd /home/aziz/documents/memos $ cd $my_bin $ pwd /home/aziz/bin

18. Use of Variables value [2] $ number=99 $ echo There are $number pens There are 99 pens

19. $ cat names Ziad Amir Salem Khalid $ command=sort $ $command names Amir Khalid Salem Ziad $ command=wc $ option=-l $ file=names $ command $option $file 7 names Variables Example[1]

20. $ value1=10 $ value2=value1 $ echo $value2 value1 $ value1=10 $ value2=$value1 $ echo $value2 10 Variables Example[2]

21. Display the value of a variable that was never assigned $ echo $nosuch $ You don’t get an error message $ echo :$nosuch: :: $ A variable that contains no value is said to contain the null value The Null Value

22. Here is a puzzle for you $ x=* $ Will the shell store character * into variable x, or will it store the names of all files in your current directory $ x=* $ echo $x Addresses into names nu numbers stat phonebook $ File Name Substitution and Variable[1]

23. File Name Substitution and Variable[2] • Was the list of files stored into the variable x when $ echo $x was executed?

24. File Name Substitution and Variable[3] • Shell does not perform file name substitution when assigning values to variables. Therefore, x=* • Assigns the single character * to x. This means shell did the file substitution when executing the echo command

25. File Name Substitution and Variable[4] $ echo $x was executed as follows: • The shell scanned the line, substituting * as the value of x • The shell then rescanned the line, encountered * and then substituted the name of the files in the current directory. • The shell then initiated execution of echo passing it the file name as arguments

26. The ${Variable} Construct • Suppose name of file is store in a variable filename. If you want to rename a file so that the new name was same as old, except with an X added to the end $ mv $filename $filenameX • The shell thinks that filenameX is the full name of variable because it is a valid variable name. To avoid this problem use curly braces{} $ mv $filename ${filename}X