1. Introduction to Unix (CA263) Passing ArgumentsBy
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. Example[2] $ who $ cat ison who | grep $1
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 $

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
phonebook: Susan Topple
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
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'
$ lu Tariq
Tariq Aziz

15. Phonebook $ cat phonebook Alice Chebba 596-2015 Bob Swingle 598-9257
Liz Stachiw
Susan Goldberg
Susan Topple
Tony Iannino
Tariq Aziz $

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'

17. Phonebook $ cat phonebook Alice Chebba 596-2015 Bob Swingle 598-9257
Liz Stachiw
Susan Goldberg
Susan Topple
Tony Iannino
Tariq Aziz
Billy Bach $

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

19. Remove from Phonebook Example[1]
$ rem 'Tariq Aziz'
$ cat phonebook
Alice Chebba
Bob Swingle
Liz Stachiw
Susan Goldberg
Susan Topple
Tony Iannino
Billy Bach $
$ rem 'Susan'
$ cat phonebook
Alice Chebba
Bob Swingle
Liz Stachiw
Tony Iannino
Billy Bach $
In next lecture you will learn how to alert user if more than one match found

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. The Shift Example $ cat tshift echo $# $* shift $ $ tshift a b c d e$

22. The Shift Example
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

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