1. SHELL PROGRAMMING

2. SHELL PROGRAMMING
•A shell program is nothing but a series of commands
•We give the to-do list – a prg- that carries out an entire
procedure. Such programs are known as “shell scripts”
•We can write shell scripts for simple to full-fledged,
custom-made software applications
•We can exploit full power and versality of UNIX
•To execute ss we don’t need a separate compiler. Shell
itself interprets the commands and execute
•The commands like dfspace and shutdown are examples
for ss
•Atleast 280 ss come with UNIX OS

3. •The ss written for one shell may not work with
the other shell because different shell use
different mechanisms for executing the
commands and keywords may change also
•Ss written for bourne shell are likely to work with
korn shell reverse may not be true because k shell
supports arrays, command aliasing etc
•C shell programming language resemble the C-
language and it is quite different from Bourne
shell
•Basic ss will run under both

4. When to use shell scripts
Shell scripts can be used for a variety of tasks
•Customizing your work environment
•Every time login to see current date, welcome message etc
•Automating your daily tasks
•To take backup all your programs at the end of the day
•Automating repetitive tasks
•Producing sales report of every month etc
•Executing important system procedures
•Shutdown, formatting a disk, creating a file system on it,
mounting and un mounting the disk etc
•Performing same operation on many files
•Replacing printf with myprintf in all C programs present in a
dir etc

5. When not to use shell scripts
When the task :
• is too complex such as writing entire billing
system
•Require a high degree of efficiency
•Requires a variety of software tools

6. Shell script structure
# comments about script
Write commands in shell script
Execution
$sh ss1
•When we execute commands at $ they are executed in the shell
that was invoked when we logged in unless otherwise specified
•When we execute shell script the login shell creates a new shell, a
new command interpreter and waits ideally in background while
the new shell executes shell script
•When all the commands in ss have been completed the new shell
terminates and our login shell once again takes over the control

7. Interactive Shell script
•To read input - read
•To display output - echo
#ss2
#usage sh ss2
echo what is your branch?
read branch
echo hello $branch students
Output:
what is your branch?
Computer science
hello Computer science students

8. Shell variables
•Provide the ability to store and manipulate
information with in the shell script
•We create and destroy any number of variables
as needed
•Rules for building shell variables
•Any combination of alphabets, digits, underscore(_)
•No commas, blanks are allowed
•First character should be either alphabet or _
•No limit on number of characters
•Case sensitive
•Eg : avg average etc

9. Shell keywords
•These are words whose meaning has already
been explained to shell
•We cannot used as var names
•Also called as reserved words
echo if until trap
read else case wait
set fi esac eval
unset while break exec
shift do continue ulimit
export done exit umask
readonly for return

10. Assigning a value to Shell variable
•Use assignment operator (=)
•Eg name=lakshmi
avg=10
•While assigning values there should not be spaces either
side of the =
•If you leave space the shell will try to interpret value
being assigned as a command to be executed
•While assignment if the variable is not there it will create
otherwise it will over write the contents
•2 types
•System variables
•User defined variables

11. System variables
•Standard variables which are always accessible
•The shell provides values for these variables
•Govern the environment
•We can change the values as per our preference
and customize the system environment

12. variable meaning
PS1 system prompt1, default value”$”
PS2 system prompt2, default value”>”
PATH defines the path in which the shell must
search in order to execute the command or file
HOME default working dir of the user
LOGNAME login name of the user
MAIL define the file with path where the mail of the
user is stored
MAILCHECK period of check for mails of user
SHELL name of the default shell
TERM name of the terminal on which you are working
We can see all the system variables and their values using
$set

13. User defined variables
•All shell variables are string variables
•We can assign more than one word using “”
•Eg : c=“one two”
$name=“lakshmi”
$echo name is $name
•All the shell variables defined inside ss die at the
moment the execution of ss is over
•Defining null variables
1.d=“” 2. d=‘’ 3. d=
•On echoing null variables blank line appears on screen
•If we use null variables in commands shell ignores it
eg: $wc –l $var1 file1 //display number
of lines in file1 only

14. Unchanging variables
•To declare constants we use readonly key word
$a=20
$readonly a
•Shell does not allow us to change the value of a
$readonly // display all readonly vars

15. Wiping out variables
•Unset is used to wipe of shell variables
$unset a
•We cannot wipeout system variables
$unset PS1 // will not work

16. Positional parameters
•We can convey the information to a program using
command line arguments
•Shell uses positional parameters concept to know about
command line arguments
•They are 9 in number named $1 to $9
Eg: $sh ss this class is mtech(cs)
•Each word is automatically stored serially in the
positional parameters
$0 assigned ss
$1 assigned this
$2 assigned class
$3 assigned is
$4 assigned mtech(cs)

17. •Write a shell script to copy one file contents to other file
pass filenames as command line arguments
#ss
#usage sh ss <sf> <tf>
cp $1 $2
cat $2

18. Setting values of Positional parameters
•We can’t assign values to positional parameters directly
1)$set hello cs students
2)$set `cat f1` //f1 is a file
•If quoting meta characters are used the command given
with in the `` (reverse quotes) is replaced by the output
#ss
#usage sh ss <filename>
name=$1
set `who am i`
mv $name $name.$1

19. Using shift on Positional parameters
• Set command can set pp upto 9
• If we use more we can’t access after 1 directly
eg: $set a b c d e f g h I j k l
$echo $10 $11
a0 a1
• To access after 9 use shift
Eg: $shift 2
• First 2 words gone and lost forever
• $echo $* //will display all pps
• $echo $# //total number of pps