Linux with Operating System Concepts

Arithmetic Operators
Meaning
+
, 
−
, *, /
Addition, subtraction, multiplication, division
**
Exponent (e.g., x**2 is x
2
)
%
Modulo (division retaining the remainder only)
~
Negation of 1’s complement value
<
, 
>
, 
< =
, 
> =
, 
= =
, 
< >
, ! 
=
Relational operators, the last two are available for not equal
a
<<
, 
>>
, &, 
∧
, |
Bit operations (left/right shift, AND, XOR, OR)
&&, ||, !
Logical AND, OR, NOT
=
Assignment
+ =
, 
− =
, * 
=
, / 
=
, % 
=
Reassignment (e.g., x 
=
x 
+
y can be written as x
+
=
y)
& 
=
, 
∧ =
, | 
=
, 
<< =
, 
>> =
Reassignment using &, 
∧
, |, 
<<
, 
>>
a 
Note that we do not use 
<
, 
<=
, and so on in conditional statements. We cover this in Section 7.4.

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• 
let N+=1
• Reassign N to be 1 greater, N is now 3, note that without the “let” statement, N
+
=
1 
is interpreted instead as string concatenation
• 
let Y=$N+1
• Y is 4, the $ in front of N is not needed so this could be let 
Y = N + 1
• 
let Z=Y%N
• Z is 1 (4/3 has a remainder of 1)
• 
Y=$(($Y+1))
• Y is now 5, we can omit the $ before Y
• 
let Y=$((Y << 2))
• Y is now 20. 
<<
is a left shift which results in a number being doubled. The num-
ber 2 in this instruction indicates that the left shift should be done twice. As Y was 
5, the left shift doubles Y to 10 and then 20.
• 
((Y++))
• Y is now 21
• 
((Q––))
• As Q had no value, it is now 
−
1
• N
=
$N
+
1
• N has the value of 3 
+
1 (literally a 3, a plus, and a 1) because $N
+
1 is treated as a 
string concatenation operation instead of an arithmetic operation
You must specify the arithmetic operations correctly. For instance, a statement like 
X 
=
$((Y 
+
Z*))
will yield an error because the operation is lacking an operand. The spe-
cific error will look like: 
Y 
+
Z*: syntax error: operand expected (error 
token is “*”).
You can specify multiple assignment statements on the same line. This is accomplished 
by separating each assignment with a space. For instance, rather than assigning three vari-
ables on separate lines as in
X 
=
1
Y 
=
2
Z 
=
3
we could do this in one instruction as 
X 
=
1 Y 
=
2 Z 
=
3
.
The Bash interpreter also has a command called 
expr
which performs and returns the 
result of a string or arithmetic operation. This is not part of an assignment statement; instead 

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256
◾
Linux with Operating System Concepts
the result is printed to the terminal window. You might use this if you need to see the result 
of a quick computation. Here are some examples. Assume N is 3.
• 
expr $((N+1))
• This returns 4 without changing N
• 
expr $N+1
• This is treated as string concatenation and returns the string 3 
+
1
• 
X=‘expr $((N+1))‘
• The expr command computes and returns 4, which is then stored in X, so X stores 
4 when this instruction concludes
7.3.5 String Operations Using 
expr
The 
expr
command is commonly used with string operations. Three string operations 
are 
substr
, 
index,
and 
length
. The format of the expr statement using a string oper-
ation is
expr