Shell Basic Operators
Like other programming languages, Shell supports a variety of operators, including:
- Arithmetic operators
- Relational operators
- Boolean operators
- String operators
- File test operators
Native bash does not support simple mathematical operations, but they can be implemented through other commands, such as awk and expr; expr is the most commonly used.
expr is an expression calculation tool that can be used to evaluate expressions.
For example, adding two numbers (Note the use of backticks`not single quotes')：
Execute the script, and the output result is as follows:
两数之和为 : 4
Two points to note:
- There must be spaces between expressions and operators. For example, 2+2 is incorrect; it must be written as 2 + 2. This is different from most programming languages we are familiar with.
- The complete expression must be enclosed by` ``, note that this character is not the commonly used single quote; it is below the Esc key.
Arithmetic Operators
The following table lists the commonly used arithmetic operators, assuming variable a is 10 and variable b is 20:
| Operator | Description | Example |
|---|---|---|
| + | Addition | `expr $a + $b` results in 30. |
| - | Subtraction | `expr $a - $b` results in -10. |
| * | Multiplication | `expr $a \* $b` results in 200. |
| / | Division | `expr $b / $a` results in 2. |
| % | Modulo | `expr $b % $a` results in 0. |
| = | Assignment | a=$b assigns the value of variable b to a. |
| == | Equal. Used to compare two numbers; returns true if they are the same. | [ $a == $b ] returns false. |
| != | Not equal. Used to compare two numbers; returns true if they are not the same. | [ $a != $b ] returns true. |
Note:Conditional expressions should be placed between square brackets with spaces, for example:[$a==$b]is wrong; it must be written as[ $a == $b ]。
Examples
An example of arithmetic operators is as follows:
Examples
# author:Example
# url:www.example.com
a=10
b=20
val=`expr $a + $b`
echo "a + b : $val"
val=`expr $a - $b`
echo "a - b : $val"
val=`expr $a \* $b`
echo "a * b : $val"
val=`expr $b / $a`
echo "b / a : $val"
val=`expr $b % $a`
echo "b % a : $val"
if [ $a == $b ]
then
echo a equals b
fi
if [ $a != $b ]
then
echo a does not equal b
fi
Execute the script, and the output result is as follows:
a + b : 30 a - b : -10 a * b : 200 b / a : 2 b % a : 0 a 不等于 b
Note:
- The multiplication sign (*) must be preceded by a backslash (\) to perform multiplication;
- if...then...fi is a conditional statement, which will be explained later.
- On MAC, the expr syntax in shell is:$((expression)), here the '*' in the expression does not need the escape character '\'.
Relational Operators
Relational operators only support numbers, not strings, unless the value of the string is a number.
The following table lists the commonly used relational operators, assuming variable a is 10 and variable b is 20:
| Operator | Description | Example |
|---|---|---|
| -eq | Checks whether two numbers are equal; returns true if equal. | [ $a -eq $b ] returns false. |
| -ne | Checks whether two numbers are not equal; returns true if not equal. | [ $a -ne $b ] returns true. |
| -gt | Checks whether the number on the left is greater than the one on the right; if so, returns true. | [ $a -gt $b ] returns false. |
| -lt | Checks whether the number on the left is less than the one on the right; if so, returns true. | [ $a -lt $b ] returns true. |
| -ge | Checks whether the number on the left is greater than or equal to the one on the right; if so, returns true. | [ $a -ge $b ] returns false. |
| -le | Checks whether the number on the left is less than or equal to the one on the right; if so, returns true. | [ $a -le $b ] returns true. |
Examples
An example of relational operators is as follows:
Examples
# author:Example
# url:www.example.com
a=10
b=20
if [ $a -eq $b ]
then
echo "$a -eq $b: a equals b
else
echo "$a -eq $b: a does not equal b
fi
if [ $a -ne $b ]
then
echo "$a -ne $b: a does not equal b
else
echo "$a -ne $b: a equals 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 $b: a is greater than or equal to b
else
echo "$a -ge $b: a is less than b
fi
if [ $a -le $b ]
then
echo "$a -le $b: a is less than or equal to b
else
echo "$a -le $b: a is greater than b
fi
Execute the script, and the output result is as follows:
10 -eq 20: a 不等于 b 10 -ne 20: a 不等于 b 10 -gt 20: a 不大于 b 10 -lt 20: a 小于 b 10 -ge 20: a 小于 b 10 -le 20: a 小于或等于 b
Boolean Operators
The following table lists the commonly used Boolean operators, assuming variable a is 10 and variable b is 20:
| Operator | Description | Example |
|---|---|---|
| ! | Negation operation; returns false if the expression is true, otherwise returns true. | [ ! false ] returns true. |
| -o | OR operation; returns true if one expression is true. | [ $a -lt 20 -o $b -gt 100 ] returns true. |
| -a | AND operation; returns true only if both expressions are true. | [ $a -lt 20 -a $b -gt 100 ] returns false. |
Examples
An example of Boolean operators is as follows:
Examples
# author:Example
# url:www.example.com
a=10
b=20
if [ $a != $b ]
then
echo "$a != $b: a does not equal b
else
echo "$a == $b: a equals b
fi
if [ $a -lt 100 -a $b -gt 15 ]
then
echo "$ais less than 100 and$bgreater than 15 : returns true
else
echo "$ais less than 100 and$bgreater than 15 : returns false
fi
if [ $a -lt 100 -o $b -gt 100 ]
then
echo "$ais less than 100 or$bgreater than 100 : returns true
else
echo "$ais less than 100 or$bgreater than 100 : returns false
fi
if [ $a -lt 5 -o $b -gt 100 ]
then
echo "$ais less than 5 or$bgreater than 100 : returns true
else
echo "$ais less than 5 or$bgreater than 100 : returns false
fi
Execute the script, and the output result is as follows:
10 != 20 : a 不等于 b 10 小于 100 且 20 大于 15 : 返回 true 10 小于 100 或 20 大于 100 : 返回 true 10 小于 5 或 20 大于 100 : 返回 false
Logical Operators
The following introduces Shell's logical operators, assuming variable a is 10 and variable b is 20:
| Operator | Description | Example |
|---|---|---|
| && | Logical AND | [[ $a -lt 100 && $b -gt 100 ]] returns false |
| || | Logical OR | [[ $a -lt 100 || $b -gt 100 ]] returns true |
Examples
An example of logical operators is as follows:
Examples
# author:Example
# url:www.example.com
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
Execute the script, the output is as follows:
返回 false 返回 true
String Operators
The following table lists commonly used string operators, assuming variable a is "abc" and variable b is "efg":
| Operator | Description | Example |
|---|---|---|
| = | Check whether two strings are equal; return true if they are equal. | [ $a = $b ] returns false. |
| != | Check whether two strings are not equal; return true if they are not equal. | [ $a != $b ] returns true. |
| -z | Check whether the string length is 0; return true if it is 0. | [ -z $a ] returns false. |
| -n | Check whether the string length is not 0; return true if it is not 0. | [ -n "$a" ] returns true. |
| $ | Check whether the string is not empty; return true if it is not empty. | [ $a ] returns true. |
Examples
An example of string operators is as follows:
Examples
# author: Example
# url:www.example.com
a="abc"
b="efg"
if [ $a = $b ]
then
echo "$a = $b: a equals b"
else
echo "$a = $b: a is not equal to b"
fi
if [ $a != $b ]
then
echo "$a != $b: a is not equal to b"
else
echo "$a != $b: a equals b"
fi
if [ -z $a ]
then
echo "-z $a: string length is 0"
else
echo "-z $a: string length is not 0"
fi
if [ -n "$a" ]
then
echo "-n $a: string length is not 0"
else
echo "-n $a: string length is 0"
fi
if [ $a ]
then
echo "$a: string is not empty"
else
echo "$a: string is empty"
fi
Execute the script, the output is as follows:
abc = efg: a 不等于 b abc != efg : a 不等于 b -z abc : 字符串长度不为 0 -n abc : 字符串长度不为 0 abc : 字符串不为空
File Test Operators
File test operators are used to check various attributes of Unix files.
The attribute detection descriptions are as follows:
| Operator | Description | Example |
|---|---|---|
| -b file | Check whether the file is a block device file; if so, return true. | [ -b $file ] returns false. |
| -c file | Check whether the file is a character device file; if so, return true. | [ -c $file ] returns false. |
| -d file | Check whether the file is a directory; if so, return true. | [ -d $file ] returns false. |
| -f file | Check whether the file is a regular file (neither a directory nor a device file); if so, return true. | [ -f $file ] returns true. |
| -g file | Check whether the SGID bit is set on the file; if so, return true. | [ -g $file ] returns false. |
| -k file | Check whether the sticky bit is set on the file; if so, return true. | [ -k $file ] returns false. |
| -p file | Check whether the file is a named pipe; if so, return true. | [ -p $file ] returns false. |
| -u file | Check whether the SUID bit is set on the file; if so, return true. | [ -u $file ] returns false. |
| -r file | Check whether the file is readable; if so, return true. | [ -r $file ] returns true. |
| -w file | Check whether the file is writable; if so, return true. | [ -w $file ] returns true. |
| -x file | Check whether the file is executable; if so, return true. | [ -x $file ] returns true. |
| -s file | Check whether the file is empty (whether the file size is greater than 0); return true if it is not empty. | [ -s $file ] returns true. |
| -e file | Check whether the file (including directories) exists; if so, return true. | [ -e $file ] returns true. |
Other check operators:
- -S: Check whether a file is a socket.
- -L: Check whether the file exists and is a symbolic link.
Examples
The variable file represents a file/var/www/example/test.sh, it has a size of 100 bytes and hasrwxpermissions. The following code will check various attributes of this file:
Examples
# author: Example
# url:www.example.com
file="/var/www/example/test.sh"
if [ -r $file ]
then
echo "File is readable"
else
echo "File is not readable"
fi
if [ -w $file ]
then
echo "File is writable"
else
echo "File is not writable"
fi
if [ -x $file ]
then
echo "File is executable"
else
echo "File is not executable"
fi
if [ -f $file ]
then
echo "File is a regular file"
else
echo "File is a special file"
fi
if [ -d $file ]
then
echo "File is a directory"
else
echo "File is not a directory"
fi
if [ -s $file ]
then
echo "File is not empty"
else
echo "File is empty"
fi
if [ -e $file ]
then
echo "File exists"
else
echo "File does not exist"
fi
Execute the script, the output is as follows:
File is readable File is writable File is executable File is a regular file File is not a directory File is not empty File exists
Increment and Decrement Operators
Although Shell itself does not have ++ and -- operators like C, C++, or Java, the same functionality can be implemented in other ways. The following are some common methods:
Using the let command
letThe command allows arithmetic operations on integers.
Examples
# Initialize variables
num=5
# Increment
let num++
# Decrement
let num--
echo $num
Using $(( )) for arithmetic operations
$(( ))The syntax is also a way to perform arithmetic operations.
Examples
# Initialize variables
num=5
# Increment
num=$((num + 1))
# Decrement
num=$((num - 1))
echo $num
Using the expr command
exprThe command can be used for arithmetic operations, but in modern scripts it is not asletand$(( ))commonly used.
Examples
# Initialize variables
num=5
# Increment
num=$(expr $num + 1)
# Decrement
num=$(expr $num - 1)
echo $num
Using (( )) for arithmetic operations
Similar to $(( )), the (( )) syntax can also be used for arithmetic operations.
Examples
# Initialize variables
num=5
# Increment
((num++))
# Decrement
((num--))
echo $num
Examples
The following is a complete example script demonstrating the use of increment and decrement operations:
Examples
# Initialize variables
num=5
echo "Initial value:$num"
# Increment
let num++
echo "After increment:$num"
# Decrement
let num--
echo "After decrement:$num"
# Using $(( ))
num=$((num + 1))
echo "Using$(( )) after increment:$num"
num=$((num - 1))
echo "Using$(( )) after decrement:$num"
# Using expr
num=$(expr $num + 1)
echo "Using expr after increment:$num"
num=$(expr $num - 1)
echo "Using expr after decrement:$num"
# Using (( ))
((num++))
echo "Using (( )) after increment:$num"
((num--))
echo "Using (( )) after decrement:$num"
Running this script will output the following:
初始值: 5 自增后: 6 自减后: 5 使用 $(( )) 自增后: 6 使用 $(( )) 自减后: 5 使用 expr 自增后: 6 使用 expr 自减后: 5 使用 (( )) 自增后: 6 使用 (( )) 自减后: 5Other extensions