Shell Programming Shells a shell can be used in one of two ways

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Shell Programming

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
Enter name and how many girlfriends:
BillG 63
BillG has 63 girlfriends!
$ test3
Enter name and how many girlfriends:
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

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
$

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Shell Programming Shells a shell can be used in one of two ways

Shell Programming

Shells

A shell can be used in one of two ways:

Shell Scripts

A shell script is just a file containing shell commands, but with a few extras:

#!/bin/sh

chmod u+rx scriptname

Shell Script Example

Here is a “hello world” shell script:

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:

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 “$”:

User Input

Use the read command to get and store input from the user.

$ cat test2

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

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

$

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

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

Backquotes: Command Substitution

A command or pipeline surrounded by backquotes causes the shell to:

You can use backquotes anywhere:

$ whoami

gates

$ cat test7

#!/bin/sh

Control Flow

The shell allows several control flow statements:

if

The if statement works mostly as expected:

$ whoami

clinton

$ cat test7

#!/bin/sh

However, the spaces before and after the square brackets [ ] are required.

if then else

The if then else statement is similar: