Introduction to Unix (CA263) Passing Arguments

1. Introduction to Unix (CA263)Passing Arguments By Tariq Ibn Aziz Dammam Community College

2. Objectives • In this lecture you will learn • Shell variables • Writing shell program • Pass an arguments • Add, remove and lookup into a file • The shift command

3. Special Shell Variables • Whenever you execute a shell program, the shell automatically stores the first argument in the special shell variable 1, the second argument in the variable 2, and so on. • The special variables as known as positional parameters.

4. Example[1] $ cat run tbl $1 |nroff –mm –Tlp |lp $ chmod +x run • Execute it with phonebook as the argument $ run phonebook Request id is laser1-15 (standard input)

5. $ who root tty02 Jul 7 08:37 fred tty03 Jul 8 08:30 tony tty04 Jul 8 08:17 lulu tty05 Jul 8 08:27 taziz tty06 Jul 8 08:57 ahmed tty07 Jul 8 08:47 $ cat ison who | grep $1 Execute it with taziz as the argument $ ison taziz taziz tty19 Jul 8 08:30 $ ison taziz $ Example[2]

6. The $# Variable • The shell variable $# gives you the number of arguments that were typed on the command line. $ cat args echo $# arguments passed echo arg 1 =:$1: arg 2=:$2: arg 3=:$3: $ args a b c 3 arguments passed arg 1 =:a: arg 2=:b: arg 3=:c:

7. $# Example[1] $ args a b 2 arguments passed arg 1 =:a: arg 2=:b: arg 3=:: $ args 0 arguments passed arg 1 =:: arg 2=:: arg 3=:: $ args "a b c" 1 arguments passed arg 1 =:a b c: arg 2=:: arg 3=::

8. $# Example[2] • See what files start with x $ ls x* xact xtra $ args x* 2 arguments passed arg 1 =:xact: arg 2=:xtra: arg 3=:: $ my_bin=/usr/steve/bin $ args $my_bin 1 arguments passed arg 1 =:/usr/steve/bin: arg 2=:: arg 3=::

9. $# Example[3] • Pass the contents if names $ args `cat names` 7 arguments passed arg 1 =:fil1: arg 2=:fil2: arg 3=:fil3: $

10. The $* Variable • The special variable $* references all arguments passed to the program. $ cat args2 echo $# arguments passed echo they are :$*: $ args a b c 3 arguments passed they are :a b c:

11. The $* Examples [1] $ args one two 2 arguments passed they are :one two: $ args 0 arguments passed they are :: $ args * 7 arguments passed they are :args args2 names nu phonebook stat xact xtra:

12. The $* Examples [2] $ cat lu # # Look someone up in the phone book # grep $1 phonebook $ $ lu "Susan T" grep: can’t open T phonebook: Susan Goldberg 338-7776 phonebook: Susan Topple 243-4932 $ Here, it passed 2 arguments not 1 argument

13. The $* Examples [3] $ cat lu # # Look someone up in the phone book # grep "$1" phonebook $ $ lu "Susan T" Susan Topple 243-4932 $ Here, it passed "Susan T" as 1 argument

14. Add in Phonebook $ cat add # # add someone to the phone book # echo "$1 $2" >> phonebook $ $ add 'Tariq Aziz' 230-4958 $ lu Tariq Tariq Aziz 230-4958 $

15. Phonebook $ cat phonebook Alice Chebba 596-2015 Bob Swingle 598-9257 Liz Stachiw 775-2298 Susan Goldberg 338-7776 Susan Topple 243-4932 Tony Iannino 386-1295 Tariq Aziz 230-4958 $

16. Add in Phonebook Examples [1] $ cat add # # add someone to the phone book version 2 # echo "$1 $2" >> phonebook Sort –o phonebook phonebook $ $ add 'Billy Bach' 331-7618 $

17. Phonebook $ cat phonebook Alice Chebba 596-2015 Bob Swingle 598-9257 Liz Stachiw 775-2298 Susan Goldberg 338-7776 Susan Topple 243-4932 Tony Iannino 386-1295 Tariq Aziz 230-4958 Billy Bach 331-7618 $

18. Remove from Phonebook $ cat rem # # remove someone to the phone book # grep –v "$1" phonebook >/tmp/phonebook mv /tmp/phonebook phonebook $

19. $ rem 'Tariq Aziz' $ cat phonebook Alice Chebba 596-2015 Bob Swingle 598-9257 Liz Stachiw 775-2298 Susan Goldberg 338-7776 Susan Topple 243-4932 Tony Iannino 386-1295 Billy Bach 331-7618 $ $ rem 'Susan' $ cat phonebook Alice Chebba 596-2015 Bob Swingle 598-9257 Liz Stachiw 775-2298 Tony Iannino 386-1295 Billy Bach 331-7618 $ In next lecture you will learn how to alert user if more than one match found Remove from Phonebook Example[1]

20. The Shift Command • If you supply more than 9 arguments, there is no way to reference the argument 10 and up, because shell only accepts a single digit following the $ sign. • $10 shell will actually substitute $1 followed by a 0. • The shift command allow you to effectively left shift your positional parameters. • If you execute the command shift then whatever stored in $2 will be assigned to $1, similarly $3 to $2 and so on. But value of $1 will be lost. • When this command is executed, $# is also automatically decremented by one

21. $ cat tshift echo $# $* shift echo $# $* shift echo $# $* shift echo $# $* shift echo $# $* shift echo $# $* $ $ tshift a b c d e 5 a b c d e 4 a b c d 3 a b c 2 a b 1 a 0 $ The Shift Example

22. If you try to shift when there are no variables to shift, then you will get the following error message. prog: cannot shift prog is the name of the program that executed the shift. shift 2 This above command has the same effect as performing 3 separate shift shift shift shift The Shift Example

23. The Shift Example • If you really need to access the 10th argument, the easiest way is to execute the shift command, and then access the value as $9. You should save the value of $1 if you need later in the program. arg1=$1 shift arg10=$9 • Remember after executing shift command $1 contains the value of $2