Shellscripting

SHELL PROGRAMMING

Introduction to Shell
You communicate with a UNIX system through a command program known as a shell .
The shell interprets the commands that you type on the keyboard.
There are many different shells available for UNIX computers, and on some systems you can choose the shell in which you wish to work.
You can use shell commands to write simple programs (scripts) to automate many tasks

Introduction to Shell When you log in, you are in one of five shells. 1. Bourne Shll – /bin/sh – Steve Bourne (On AT & T UNIX) 2. Bourne Again SHell – /bin/bash – Improved Bourne shell (On Linux) 3. C Shell – /bin/csh – Bill Joy (On BSD UNIX) 4. TC Shell – /bin/tcsh (On Linux) 5. Korn Shell – /bin/ksh – David Korn (On AT & T UNIX) The system administrator determines which shell you start in.

Basics
Definition:
Shell is an agency that sits between the user and the UNIX system.
Description:
Understands all user directives and carries them out.
Processes the commands issued by the user.

Shell Scripts Example: script.sh #! /bin/sh # script.sh: Sample Shell Script echo “Welcome to Shell Programming” echo “Today’s date : `date`” echo “This months calendar:” echo “My Shell :$ SHELL”

Shell Variables Environmental Variables: Some global environment variables are, HOME Path to your home directory HOST The hostname of your system LOGNAME The name you login with PATH Paths to be searched for commands SHELL The login shell you’re using PWD Present working directory

Local Shell Variables A variable assignment is of the form variable=value , but its evaluation requires the $ as prefix to the variable name. count=5 echo $count total=$count You can assign value of one variable to another variable echo $total Note: There should not be any space around =. i.e. if we say x =5 then the shell interprets x as command running with the =5 as argument!

Shell scripts accept arguments from the command line.
Run non interactively
Arguments are assigned to special shell variables (positional parameters).
Represented by $1, $2, etc;
Position variables

Position variables

#! /bin/sh echo “Program Name : $0” echo “No of Arguments : $#” echo “Arguments are : $*” $ chmod +x 2.sh $ 2.sh A B C o/p  Program Name : 2.sh No of Arguments : 3 Arguments are : A B C Position variables

Bourne Shell Programming
Control structures
if … then
for … in
while
until
case
break and continue

if … then
Structure
if test-command
then
commands
fi
Example:
if test “$word1” = “$word2”
then
echo “Match”
fi

for: Looping with a List for is also a repetitive structure. Syntax: for variable in list do Commands done Note: list here comprises a series of character strings. Each string is assigned to variable specified.

A partial list of built-in
bg, fg, jobs job control
break, continue change the loop
cd, pwd working directory
echo, read display/read
eval scan and evaluate the command
exec execute a program
exit exit from current shell
export, unset export/ remove a val or fun
test compare arguments

kill sends a signal to a process or job
set sets flag or argument
shift promotes each command line argument
times displays total times for the current shell and
trap traps a signal
type show whether unix command, build-in, function
umask file creation mask
wait waits for a process to terminate.
ulimit print the value of one or more resource limits
A partial list of built-in

System Scripts
some system scripts
/etc/init.d/syslog
/etc/init.d/crond

Powerful scripts can be built using the excellent features offered in shell programming. A preliminary insight on shell programming has been offered here.
Scripts can make use of these as well as advanced features to help programmers/users of system to work with efficiency and ease.
conclusions:

Thank you

Shellscripting

by Narendra Sisodiya

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Shellscripting

by Narendra Sisodiya

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Shellscripting Presentation Transcript

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