Introduction to Unix (CA263)

Passing Arguments

By

Tariq Ibn AzizDammam Community College

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

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.

Example[1]

$ cat run tbl $1 |nroff –mm –Tlp |lp$ chmod +x run

• Execute it with phonebook as the argument

$ run phonebookRequest id is laser1-15 (standard input)

Example[2]

$ whoroot tty02 Jul 7 08:37fred tty03 Jul 8 08:30tony tty04 Jul 8 08:17lulu tty05 Jul 8 08:27taziz tty06 Jul 8 08:57ahmed tty07 Jul 8 08:47

$ cat isonwho | grep $1

•Execute it with taziz as the argument

$ ison taziztaziz tty19 Jul 8 08:30

$ ison taziz$

The $# Variable

• The shell variable $# gives you the number of arguments that were typed on the command line.

$ cat argsecho $# arguments passedecho arg 1 =:$1: arg 2=:$2: arg 3=:$3:$ args a b c3 arguments passedarg 1 =:a: arg 2=:b: arg 3=:c:

$# Example[1]

$ args a b2 arguments passedarg 1 =:a: arg 2=:b: arg 3=::$ args0 arguments passedarg 1 =:: arg 2=:: arg 3=::$ args "a b c"1 arguments passedarg 1 =:a b c: arg 2=:: arg 3=::

$# Example[2]

• See what files start with x$ ls x*xact xtra$ args x*2 arguments passedarg 1 =:xact: arg 2=:xtra: arg 3=::$ my_bin=/usr/steve/bin$ args $my_bin1 arguments passedarg 1 =:/usr/steve/bin: arg 2=:: arg 3=::

$# Example[3]

• Pass the contents if names$ args `cat names`7 arguments passedarg 1 =:fil1: arg 2=:fil2: arg 3=:fil3:$

The $* Variable

• The special variable $* references all arguments passed to the program.

$ cat args2echo $# arguments passedecho they are :$*:$ args a b c3 arguments passedthey are :a b c:

The $* Examples [1]

$ args one two2 arguments passedthey are :one two:$ args0 arguments passedthey are ::$ args *7 arguments passedthey are :args args2 names nu phonebook stat xact xtra:

The $* Examples [2]

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

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

Add in Phonebook

$ cat add# # add someone to the phone book# echo "$1 $2" >> phonebook$$ add 'Tariq Aziz' 230-4958$ lu TariqTariq Aziz 230-4958$

Phonebook

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

$

Add in Phonebook Examples [1]

$ cat add# # add someone to the phone book version 2# echo "$1 $2" >> phonebookSort –o phonebook phonebook$$ add 'Billy Bach' 331-7618$

Phonebook

$ cat phonebookAlice Chebba 596-2015Bob Swingle 598-9257Liz Stachiw 775-2298Susan Goldberg 338-7776Susan Topple 243-4932Tony Iannino 386-1295Tariq Aziz230-4958Billy Bach331-7618$

Remove from Phonebook

$ cat rem# # remove someone to the phone book# grep –v "$1" phonebook >/tmp/phonebookmv /tmp/phonebook phonebook$

Remove from Phonebook Example[1]

$ rem 'Tariq Aziz'$ cat phonebookAlice Chebba 596-2015Bob Swingle 598-9257Liz Stachiw 775-2298Susan Goldberg 338-7776Susan Topple 243-4932Tony Iannino 386-1295Billy Bach 331-7618$

$ rem 'Susan'$ cat phonebookAlice Chebba 596-2015Bob Swingle 598-9257Liz Stachiw 775-2298Tony Iannino 386-1295Billy Bach 331-7618

$• In next lecture you will

learn how to alert user if more than one match found

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

The Shift Example

$ cat tshiftecho $# $*shiftecho $# $*shiftecho $# $*shiftecho $# $*shiftecho $# $*shiftecho $# $*$

$ tshift a b c d e5 a b c d e4 a b c d 3 a b c 2 a b 1 a0$

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

shiftshiftshift

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=$1shiftarg10=$9• Remember after executing shift command $1

contains the value of $2