Shell Initial (2) Shell Operator

Shell basic operator

Shell, like other programming languages, supports a variety of operators, including:

• Arithmetic operator

• Relational operator

• Boolean operator

• String operator

Native bash does not support simple mathematical operations, but can be implemented by other commands, such as awk and expr, expr is the most commonly used.

Such as:

val=`expr 2 + 3`
echo $val

Output: 5

Note that there needs to be a space between 2 + 3

Arithmetic operator

a variable is 10, b variable is 20.

operator	Explain	Give an example
+	Plus	` expr $a + B `, result: 30
-	Minus sign	` expr $a - $b', result: - 10
*	Multiplication sign	` expr $a * $b `, result: 200
/	Division sign	` Expr $b / a `, result: 2
%	Remainder	` expr $b% $a `, result: 0
=	assignment	var=$a, assign variable a to var
==	Be equal to	[$a == $b], the result is 0
!=	Not equal to	[$a!= $b], the result is 1

eg:

[zhang@localhost home]$ a=10
[zhang@localhost home]$ b=20
[zhang@localhost home]$ echo $a + $b = `expr $a + $b`
10 + 20 = 30
[zhang@localhost home]$ echo $a - $b = `expr $a - $b`
10 - 20 = -10
[zhang@localhost home]$ echo $a \* $b = `expr $a \* $b`
10 * 20 = 200
[zhang@localhost home]$ echo $a / $b = `expr $a / $b`
10 / 20 = 0
[zhang@localhost home]$ echo $b / $a = `expr $b / $a`
20 / 10 = 2
[zhang@localhost home]$ echo $b % $a = `expr $b % $a`
20 % 10 = 0

[zhang@localhost home]$ var=$a
[zhang@localhost home]$ echo $var
10

[zhang@localhost home]$ echo $[$a == $b]
0
[zhang@localhost home]$ echo $[$a != $b]
1
[zhang@localhost home]$ echo $[$a == 10]
1

Note: $[$a == $b] can be replaced by $[a == b], such as:

[zhang@localhost ~]$ a=10
[zhang@localhost ~]$ b=20
[zhang@localhost ~]$ echo $[a + b]
30
[zhang@localhost ~]$ echo $[a - b]
-10
[zhang@localhost ~]$ echo $[a * b]
200
[zhang@localhost ~]$ echo $[a / b]
0
[zhang@localhost ~]$ echo $[a == b]
0
[zhang@localhost ~]$ echo $[a != b]
1
[zhang@localhost ~]$ echo $[a >= b]
0
[zhang@localhost ~]$ echo $[a <= b]
1
[zhang@localhost ~]$ echo $[a % b]
10

Relational operator

operator	Explain	Give an example
-eq	Check if the two numbers are equal and return true equally.	[$a-eq $b] returns false
-ne	Check whether the two numbers are not equal and return true equally.	[$a-ne $b] Returns true
-gt	Check if the number on the left is greater than that on the right, and if so, return true.	[$a-gt $b] Returns false
-lt	Check if the number on the left is less than the number on the right, and if so, return true.	[$a-lt $b] Returns true
-ge	Check if the number on the left is greater than or equal to the number on the right, and if so, return true.	[$a-ge $b] returns false
-le	Check if the number on the left is less than or equal to the number on the right, and if so, return true.	[$a-le $b] Returns true

eg:

#!/bin/bash

a=10
b=20
if [ $a -eq $b ]
then
    echo "$a -eq $b : a is equal to b"
else
    echo "$a -eq $b: a is not equal to b"
fi

if [ $a -ne $b ]
then
    echo "$a -ne $b : a is not equal to b"
else
    echo "$a -ne $b : a is equal to b"
fi

if [ $a -gt $b ]
then
    echo "$a -gt $b : a is greater than b"
else
    echo "$a -gt $b : a is not greater than b"
fi

if [ $a -lt $b ]
then
    echo "$a -lt $b : a is less than b"
else
    echo "$a -lt $b : a is not less than b"
fi

if [ $a -ge $b ]
then
    echo "$a -ge $b : a is greater or equal to b"
else
    echo "$a -ge $b : a is not greater or equal to b"
fi

if [ $a -le $b ]
then
    echo "$a -le $b : a is less or equal to b"
else
    echo "$a -le $b : a is not less or equal to b"
fi

Output results:

10 -eq 20: a is not equal to b
10 -ne 20 : a is not equal to b
10 -gt 20 : a is not greater than b
10 -lt 20 : a is less than b
10 -ge 20 : a is not greater or equal to b
10 -le 20 : a is less or equal to b

Boolean operator

operator	Explain	Give an example
!	If the expression is true, then false is returned, otherwise true is returned.	[! false] Returns true
-o	Or an operation that returns true if an expression is true	[$a-lt 20-o $b-gt 100], returns true
-a	With the operation, both expressions are true before returning true.	[$a-lt 20-a $b-gt 100], returns false

eg:

#!/bin/bash

a=10
b=20

if [ $a != $b ]
then
    echo "$a != $b"
else
    echo "$a == $b"
fi


if [ $a -lt 20 -o $b -gt 100 ]
then
    echo "$a less than 20 or $b greater than 100 : return true"
else
    echo "$a less than 20 or $b greater than 100 : return false"
fi


if [ $a -lt 20 -a $b -gt 100 ]
then
    echo "$a less than 20 and greater than 100 : return true"
else
    echo "$a less than 20 and greater than 100 : return false"
fi

Output results:

10 != 20
10 less than 20 or 20 greater than 100 : return true
10 less than 20 and greater than 100 : return false

Logical Operator

operator	Explain	Give an example
&&	Logical and	[[$a-lt 100 & & &$b-gt 100]] Returns false
||	Logical OR	[[[$a-lt 100 | | b-gt 100]] Returns true

eg:

#!/bin/bash

a=10
b=20

if [[ $a -lt 100 && $b -gt 100 ]]
then
    echo "return true"
else
    echo "return false"
fi


if [[ $a -lt 100 || $b -gt 100 ]]
then
    echo "return true"
else
    echo "return false"
fi

Output results:

return false
return true

String operator

Variable a is "abc" and variable b is "efg".

operator	Explain	Give an example
=	Check if two strings are equal and return true equally	[$a = $b], returns false
!=	Detecting whether two strings are equal and returning true unequally	[$a!= $b], return true
-z	Detecting whether the length of the string is 0, returning true if it is not 0	[-z $a] returns false
-n	Detecting whether the length of the string is 0, returning true if it is not 0	[-n $a] Returns true
str	Detect whether the string is empty and return true for none	[$a] Returns true

eg:

#!/bin/bash

a="abc"
b="efg"

if [ $a = $b ]
then
    echo "$a equal $b"
else
    echo "$a not equal $b"
fi


if [ $a != $b ]
then
    echo "$a != $b"
else
    echo "$a == $b"
fi


if [ -n $b ]
then
    echo "$b length ${#b}"
else
    echo "$b length 0"
fi

# eg: str
if [ $a ]
then
    echo "$a not null"
else
    echo "$a is null"
fi

Result:

abc not equal efg
abc != efg
abc length 3
efg length 3
abc not null