ShellProgramming and Script in operating system

Shell
Programming
Software Tools

Shells
 A shell can be used in one of two
ways:
A command interpreter, used interactively
A programming language, to write shell
scripts (your own custom commands)

Shell Scripts
 A shell script is just a file containing shell
commands, but with a few extras:
 The first line of a shell script should be a comment of the
following form:
#!/bin/sh
for a Bourne shell script. Bourne shell scripts are the most
common, since C Shell scripts have buggy features.
 A shell script must be readable and executable.
chmod u+rx scriptname
 As with any command, a shell script has to be “in your
path” to be executed.
– If “.” is not in your PATH, you must specify “./scriptname”
instead of just “scriptname”

Shell Script Example
 Here is a “hello world” shell script:
$ ls -l
-rwxr-xr-x 1 horner 48 Feb 19 11:50 hello*
$ cat hello
#!/bin/sh
# comment lines start with the # character
echo "Hello world"
$ hello
Hello world
$
 The echo command functions like a print command in
shell scripts.

Shell Variables
 The user variable name can be any sequence of
letters, digits, and the underscore character, but
the first character must be a letter.
 To assign a value to a variable:
number=25
name="Bill Gates"
 There cannot be any space before or after the “=“
 Internally, all values are stored as strings.

Shell Variables
 To use a variable,
precede the name
with a “$”:
$ cat test1
#!/bin/sh
number=25
name="Bill Gates"
echo "$number $name"
$ test1
25 Bill Gates
$

User Input
 Use the read command to get and store input from
the user.
$ cat test2
#!/bin/sh
echo "Enter name: "
read name
echo "How many girlfriends do you have? "
read number
echo "$name has $number girlfriends!"
$ test2
Enter name:
Bill Gates
How many girlfriends do you have?
too many
Bill Gates has too many girlfriends!

User Input
 read reads one line of input from the keyboard and assigns
it to one or more user-supplied variables.
$ cat test3
#!/bin/sh
echo "Enter name and how many girlfriends:"
read name number
echo "$name has $number girlfriends!"
$ test3
Enter name and how many girlfriends:
Bill Gates 63
Bill has Gates 63 girlfriends!
$ test3
BillG 63
BillG has 63 girlfriends!
$ test3
Bill
Bill has girlfriends!
 Leftover input words are all assigned to the last variable.

$
 Use a backslash before $ if you really want to
print the dollar sign:
$ cat test4
#!/bin/sh
echo "Enter amount: "
read cost
echo "The total is: $$cost"
$ test4
Enter amount:
18.50
The total is $18.50

$
 You can also use single quotes
for printing dollar signs.
 Single quotes turn off the special meaning of all
enclosed dollar signs:
$ cat test5
#!/bin/sh
echo "Enter amount: "
read cost
echo ‘The total is: $’ "$cost"
$ test5
Enter amount:
18.50
The total is $ 18.50

expr
 Shell programming is not good at numerical
computation, it is good at text processing.
 However, the expr command allows simple integer
calculations.
 Here is an interactive Bourne shell example:
$ i=1
$ expr $i + 1
2
 To assign the result of an expr command to
another shell variable, surround it with backquotes:
$ i=1
$ i=`expr $i + 1`
$ echo "$i"
2

expr
 The * character normally means “all the files in
the current directory”, so you need a “” to use it
for multiplication:
$ i=2
$ i=`expr $i * 3`
$ echo $i
6
 expr also allows you to group expressions, but
the “(“ and “)” characters also need to be
preceded by backslashes:
$ i=2
$ echo `expr 5 + ( $i * 3 )`
11

expr Example
$ cat test6
#!/bin/sh
echo "Enter height of rectangle: "
read height
echo "Enter width of rectangle: "
read width
area=`expr $height * $width`
echo "The area of the rectangle is $area"
$ test6
Enter height of rectangle:
10
Enter width of rectangle:
5
The area of the ractangle is 50
$ test6
Enter height of rectangle:
10.1
Enter width of rectangle:
5.1
expr: non-numeric argument
Does not work for floats!

Backquotes:
Command Substitution
 A command or pipeline surrounded by
backquotes causes the shell to:
 Run the command/pipeline
 Substitute the output of the command/pipeline for
everything inside the quotes
 You can use backquotes anywhere:
$ whoami
gates
$ cat test7
#!/bin/sh
user=`whoami`
numusers=`who | wc -l`
echo "Hi $user! There are $numusers users logged on."
$ test7
Hi gates! There are 6 users logged on.

Control Flow
 The shell allows several control flow
statements:
if
while
for

if
 The if statement works mostly as expected:
$ whoami
clinton
$ cat test7
#!/bin/sh
user=`whoami`
if [ $user = "clinton" ]
then
echo "Hi Bill!"
fi
$ test7
Hi Bill!
 However, the spaces before and after the square
brackets [ ] are required.

if then else
 The if then else statement is similar:
$ cat test7
#!/bin/sh
user=`whoami`
if [ $user = "clinton" ]
then
echo "Hi Bill!"
else
echo "Hi $user!"
fi
$ test7
Hi horner!

if elif else
 You can also handle a list of cases:
$ cat test8
#!/bin/sh
users=`who | wc -l`
if [ $users -ge 4 ]
then
echo "Heavy load"
elif [ $users -gt 1 ]
then
echo "Medium load"
else
echo "Just me!"
fi
$ test8
Heavy load!

Boolean Expressions
 Relational operators:
-eq, -ne, -gt, -ge, -lt, -le
 File operators:
-f file True if file exists and is not a directory
-d file True if file exists and is a directory
-s file True if file exists and has a size > 0
 String operators:
-z string True if the length of string is zero
-n string True if the length of string is nonzero
s1 = s2 True if s1 and s2 are the same
s1 != s2 True if s1 and s2 are different
s1 True if s1 is not the null string

File Operator Example
$ cat test9
#!/bin/sh
if [ -f letter1 ]
then
echo "We have found the evidence!"
cat letter1
else
echo "Keep looking!"
fi
$ test9
We have found the evidence!
How much would it cost to buy Apple Computer?
Best,
Bill

And, Or, Not
 You can combine and negate expressions with:
-a And
-o Or
! Not
$ cat test10
#!/bin/sh
if [ `who | grep gates | wc -l` -ge 1 -a `whoami` != “gates" ]
then
echo "Bill is loading down the machine!"
else
echo "All is well!"
fi
$ test10
Bill is loading down the machine!

while
 The while statement loops indefinitely, while
the condition is true, such as a user-controlled
condition:
$ cat test11
#!/bin/sh
resp="no"
while [ $resp != "yes" ]
do
echo "Wakeup [yes/no]?"
read resp
done
$ test11
Wakeup [yes/no]?
no
Wakeup [yes/no]?
y
Wakeup [yes/no]?
yes
$

while
 while can also do normal incrementing loops:
$ cat fac
#!/bin/sh
echo "Enter number: "
read n
fac=1
i=1
while [ $i -le $n ]
do
fac=`expr $fac * $i`
i=`expr $i + 1`
done
echo "The factorial of $n is $fac"
$ fac
Enter number:
5
The factorial of 5 is 120

break
 The break command works like in C++,
breaking out of the innermost loop :
$ cat test12
#!/bin/sh
while [ 1 ]
do
echo "Wakeup [yes/no]?"
read resp
if [ $resp = "yes" ]
then
break
fi
done
$ test12
Wakeup [yes/no]?
no
Wakeup [yes/no]?
y
Wakeup [yes/no]?
yes
$

ShellProgramming and Script in operating system

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