History of Shell What is Shell Script What is Shell

1. Team
Md. Iftiar Uddin Ahmed [011 122 084 ]
Md.Tanvir Hossain [ 011 101 035]
Rafid asrar prottoy [ 011 132 124 ]
Amit ghosh [011 132 134]

2. About Shell | Shell Scripts

3. Overview
 A short history of Linux
 About Shell
 About Shell Script

4. Linux
Introduction
 In 80’s, Microsoft’s DOS was the dominated OS for PC
 Apple MAC was better, but expensive
 UNIX was much better, but much, much more
expensive. Only for minicomputer for commercial
applications
 People was looking for a UNIX based system, which is
cheaper and can run on PC
 Both DOS, MAC and UNIX were proprietary, i.e., the
source code of their kernel is protected
 No modification is possible without paying high license
fees
Before
Linux

5. Beginning of
Linux
 A famous professor Andrew Tanenbaum developed Minix,
a simplified version of UNIX that runs on PC
 Minix is for class teaching only. No intention for
commercial use
 In Sept 1991, Linus Torvalds, a second year student of
Computer Science at the University of Helsinki,
developed the preliminary kernel of Linux, known as
Linux version 0.0.1
Linux
Introduction

6. Linux
Introduction
 Linux has been used for many computing platforms
– PC, PDA, Supercomputer,…
 Not only character user interface but graphical user interface is
available
 Commercial vendors moved in Linux itself to provide freely
distributed code. They make their money by compiling up
various software and gathering them in a distributable format
– Red Hat, Slackware, etc
Linux
Today

7. Shell
Shell is an command language interpreter that executes
commands read from the standard input device (keyboard) or from a
file. Shell is not part of system kernel, but uses the system kernel to
execute programs, create files etc.
Simply put, the shell is a program that takes your commands from
the keyboard and gives them to the operating system to perform.
What is Shell

8. Shell
Commands
 Shell commands are interpreted directly by the
shell you specify.
 The commands are similar to the statement in
some programming languages, such as C.
 Popular shells include:
 Enhanced C-shell tchs (csh+)
 Bourne-Again Shell, bash (sh+)
 Korn Shell (ksh)

9. Shells’
Features
 In particular:
 Pass arguments to your script
 Set and reference variables
 Use of control flow
 Interact with the user (read user input)
 Comments…
 Info on commands a given shell offers can be found
in the man pages for that shell.
 There are many Linux/UNIX references that give
detailed information and tips.

10. Shell Scripts
 To automate certain common activities an user
performs routinely.
 They serve the same purpose as batch files in
DOS/Windows.
 Example:
 rename 1000 files from upper case to
lowercase
What are they for?

11. What are Shell
Scripts
 Just text/ASCII files with:
 a set of standard UNIX/Linux commands (ls,
mv, cp, less, cat, etc.) along with
 flow of control
 some conditional logic and branching
(if-then),
 loop structures (foreach, for, while), and
 I/O facilities (echo, print, set, ...).
 They allow use of variables.
 They are interpreted by a shell directly.
 Some of them (csh, tcsh) share some of C
syntax.
 DOS/Win equivalent - batch files (.bat)

12. Why not use
C/C++ for
that?
 C/C++ programming requires compilation and
linkage, maybe libraries, which may not be
available (production servers).
 For the typical tasks much faster in development,
debugging, and maintenance (because they are
interpreted and do not require compilation).

13. Shell Script
Invocation
 Specify the shell directly:
 % tcsh myshellscript
 % tcsh -v myshellscript
(-v = verbose, useful for debugging)
 Make the shell an executable first and then run is a
command (set up an execution permission):
 % chmod u+x myshellscript
 Then either this:
 % myshellscript
(if the path variable has ‘.’ in it; security issue!)
 Or:
 % ./myshellscript
(should always work)

14. Shell Script
Invocation
continue
 If you get an error:
“myshellscrip: command not found”
 The probably “.” is not in your path or there’s no
execution bit set.
 When writing scripts, choose unique names, that
preferably do not match system commands.
 Bad name would be test for example, since there
are many shells with this internal command.
 To disambiguate, always precede the shell with “./” or
absolute path in case you have to name your thing not
very creatively.

15. Start Writing a
Shell Script
 The very first line, often called 'shebang' (#!) should
precede any other line, to assure that the right shell is
invoked.
 Comments start with '#', with the exception of #!, $#,
which are a special character sequences.
 Everything on a line after # is ignored if # is not a part
of a quoted string or a special character sequence.
#!/bin/tcsh #!/bin/bash
# This is for tcsh # For Bourne-Again Shell
#!/bin/sh
# This is for Bourne Shell

16. Variables
 Variables start with a $ sign when they are used.
 $x, $val
 There's no $ when a variable is declared.
 set x = 3
 @ y = 1
 set input = "$<"
 There are some system, predefined variables:
 $0, $1, $3 .... - argument references (arguments
themselves)
 $* - all the arguments
 $< - user's input from STDIN
 $# - # of arguments passed to the script

17. Read
 To read user input from keyboard and store it into a
 variable use read var1,var2,.....varn
#!/bin/bash
echo ‐n "Enter your name:”
read name
echo ‐n "Enter your student no:”
read stdno
echo "Your Name:$name”
echo "Your Age:$stdno”

18. Shell
Arithmetic
 The expr command evaluates its arguments as an
 expression.
 It is commonly used for simple arithmetic operations.
#!/bin/bash expr 1 + 1 expr 1 ‐ 1 expr 1 * 1 expr
1 / 1
va r=`expr 1 + 1`
x=1
x=`expr $x + 1`

19. Shell
Arithmetic
Continue

20. if
if ( <expression> ) then
<statements>
else if ( <another-expression> ) then
<statements>
else
<statements>
endif

21. foreach
foreach var ( <list-of-values> )
<statements>
end

22. Switch
switch ( string )
case str1:
<statements>
breaksw
...
default:
<statements>
breaksw
endsw

23. while
while ( <expression> )
<statements>
end

24. File Inquiry
Operators:
-op file
r Read access
w Write access
xExecute access
eExistence
oOwnership
z Zero size
s Non-zero size
F Plain file
D Directory
L Symbolic link
B Block special file
C Character special file
P Named pipe (FIFO)
S Socket special file