English | 简体中文 | 繁體中文 | Русский язык | Français | Español | Português | Deutsch | 日本語 | 한국어 | Italiano | بالعربية
Like other programming languages, Shell supports a variety of operators, including:
Arithmetic operators
" does not need to be escaped with a backslash "\".
Boolean operators
String operators
File test operators
Bash does not support simple mathematical operations natively, but can be implemented through other commands, such as awk and expr, with expr being the most commonly used.
expr is an expression evaluation tool that can be used to perform expression evaluation operations.
For example, adding two numbers (Note that the backquote ` is used instead of the single quote ') :
#!/bin/bash val=`expr 2 + 2` echo "The sum of the two numbers is : $val"
Execute the script, and the output is as follows:
The sum of the two numbers is : 4
Two points to note:
There must be spaces between expressions and operators, for example 2+2 is incorrect, and must be written as 2 + 2This is different from most programming languages we are familiar with.
The complete expression must be enclosed in ` ` (backticks), note that this character is not the commonly used single quote, located below the Esc key.
The following table lists commonly used arithmetic operators, assuming variable a is 10, variable b is 20:
| Operators | 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, and returns true if they are the same. | [ $a == $b ] returns false. |
| != | Not equal. Used to compare two numbers, and returns true if they are not the same. | [ $a != $b ] returns true. |
Note:Conditional expressions must be enclosed in square brackets and must have spaces, for example: [$a==$b] is incorrect, and must be written as [ $a == $b ].
The following are examples of arithmetic operators:
#!/bin/bash # author: Basic Tutorial # url:www.oldtoolbag.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 == $b ] then echo "a is equal to b" fi if [$a != $b] then echo "a is not equal to b" fi
Execute the script, and the output is as follows:
a + b : 30 a - b : -10 a * b : 200 b / a : 2 b % a : 0 a is not equal to b
Note:
Multiplication sign (*The asterisk (*) must be preceded by a backslash (\) to perform multiplication;
if...then...fi is a conditional statement, which will be explained later.
The expr syntax in shell on MAC is:$((表达式))$((expression)))*Here, the "
Relational operators
Relational operators only support numbers, not strings, unless the string value is a number. 10, variable b is 20:
| Operators | Description | Example |
|---|---|---|
| -eq | The following table lists commonly used relational operators, assuming variable a is | [ $a -Check if the two numbers are equal, if equal, return true. |
| -ne | eq $b ] returns false. | [ $a -Check if the two numbers are not equal, if not equal, return true. |
| -gt | ne $b ] returns true. | [ $a -Check if the number on the left is greater than the number on the right, if so, return true. |
| -lt | gt $b ] returns false. | [ $a -Check if the number on the left is less than the number on the right, if so, return true. |
| -ge | lt $b ] returns true. | [ $a -Check if the number on the left is greater than or equal to the number on the right, if so, return true. |
| -le | ge $b ] returns false. | [ $a -Check if the number on the left is less than or equal to the number on the right, if so, return true. |
le $b ] returns true.
#!/bin/bash # author:Basic Tutorial Website # url:www.oldtoolbag.com a=10 b=20 if [ $a -Examples of relational operators are as follows: then le $b: a is less than or equal to b" -eq $b ] else le $b: a is less than or equal to b" -eq $b : a is equal to b" fi if [ $a -eq $b: a is not equal to b" then le $b: a is less than or equal to b" -ne $b ] else le $b: a is less than or equal to b" -ne $b: a is not equal to b" fi if [ $a -ne $b : a is equal to b" then le $b: a is less than or equal to b" -gt $b ] else le $b: a is less than or equal to b" -gt $b: a is greater than b" fi if [ $a -gt $b: a is not greater than b" then le $b: a is less than or equal to b" -lt $b ] else le $b: a is less than or equal to b" -lt $b: a is less than b" fi if [ $a -lt $b: a is not less than b" then le $b: a is less than or equal to b" -ge $b ] else le $b: a is less than or equal to b" -ge $b: a is greater than or equal to b" fi if [ $a -ge $b: a is less than b" then le $b: a is less than or equal to b" -le $b ] else le $b: a is less than or equal to b" -echo "$a fi
Execute the script, and the output is as follows:
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 less than b 10 -le 20: a is less than or equal to b
The following table lists commonly used boolean operators, assuming variable a is 10, variable b is 20:
| Operators | Description | Example |
|---|---|---|
| ! | NOT operator, returns false if the expression is true, otherwise returns true. | [ ! false ] returns true. |
| -o | OR operator, returns true if at least one expression is true. | [ $a -lt 20 -o $b -gt 100 ] returns true. |
| -a | AND operator, returns true only if both expressions are true. | [ $a -lt 20 -a $b -gt 100 ] returns false. |
Examples of boolean operators are as follows:
#!/bin/bash # author: Basic Tutorial # url:www.oldtoolbag.com a=10 b=20 if [$a != $b] then echo "$a != $b: a is not equal to b" else echo "$a == $b: a equals b" fi if [ $a -lt 100 -a $b -gt 15 ] then echo "$a less than 100 and $b greater than 15 : returns true else echo "$a less than 100 and $b greater than 15 : returns false fi if [ $a -lt 100 -o $b -gt 100] then echo "$a less than 100 or $b greater than 100: returns true else echo "$a less than 100 or $b greater than 100: returns false fi if [ $a -lt 5 -o $b -gt 100] then echo "$a less than 5 or $b greater than 100: returns true else echo "$a less than 5 or $b greater than 100: returns false fi
Execute the script, and the output is as follows:
10 != 20: a is not equal to b 10 less than 100: and 20: greater than 15 : returns true 10 less than 100: or 20: greater than 100: returns true 10 less than 5 or 20: greater than 100: returns false
The following introduces Shell's logical operators, assuming variable a is 10, variable b is 20:
| Operators | Description | Example |
|---|---|---|
| && | Logical AND | [[ $a -lt 100 && $b -gt 100 ]] returns false |
| || | Logical OR | [[ $a -lt 100 || $b -gt 100 ]] returns true |
Examples of logical operators are as follows:
#!/bin/bash # author:Basic Tutorial Website # url:www.oldtoolbag.com a=10 b=20 if [[ $a -lt 100 && $b -gt 100]] then echo "Returns true" else echo "Returns false" fi if [[ $a -lt 100 || $b -gt 100]] then echo "Returns true" else echo "Returns false" fi
Execute the script, and the output is as follows:
Returns false Returns true
The following table lists commonly used string operators, assuming variable a is "abc", variable b is "efg":
| Operators | Description | Example |
|---|---|---|
| = | Check if two strings are equal, return true if equal. | [ $a = $b ] returns false. |
| != | Check if two strings are not equal, return true if not equal. | [ $a != $b ] returns true. |
| -z | Check if the string length is 0, return true if 0. | [ -z $a ] returns false. |
| -n | Check if the string length is not 0, return true if not 0. | [ -n "$a" ] returns true. |
| $ | Check if the string is empty, return true if not empty. | [ $a ] returns true. |
Examples of string operators are as follows:
#!/bin/bash # author:Basic Tutorial Website # url:www.oldtoolbag.com a="abc" b="efg" if [$a = $b] then echo "$a = $b: a is equal to b" else echo "$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 != $b: a is equal to b" fi if [ -The end of the string length check then echo "}}-The string length is 0 else echo "}}-The string length is not 0 fi if [ -"$a" then echo "}}-The string length is not 0 else echo "}}-n $a : The string length is 0" fi if [ $a ] then echo "$a : The string is not empty" else echo "$a : The string is empty" fi
Execute the script, and the output is as follows:
abc = efg: a is not equal to b abc != efg : a is not equal to b -z abc : The string length is not 0 -n abc : The string length is not 0 abc : The string is not empty
File test operators are used to check various properties of Unix files.
The property detection description is as follows:
| Operator | Description | Example |
|---|---|---|
| -b file | Check if the file is a block device file, if it is, return true. | [ -b $file ] returns false. |
| -c file | Check if the file is a character device file, if it is, return true. | [ -c $file ] returns false. |
| -d file | Check if the file is a directory, if it is, return true. | [ -d $file ] returns false. |
| -f file | Check if the file is a regular file (neither a directory nor a device file), if it is, return true. | [ -f $file ] returns true. |
| -g file | Check if the file has the SGID bit set, if it does, return true. | [ -g $file ] returns false. |
| -k file | Check if the file has the Sticky Bit set, if it does, return true. | [ -k $file ] returns false. |
| -p file | Check if the file is a named pipe, if it is, return true. | [ -p $file ] returns false. |
| -u file | Check if the file has the SUID bit set, if it does, return true. | [ -u $file ] returns false. |
| -r file | Check if the file is readable, if it is, return true. | [ -r $file ] returns true. |
| -w file | Check if the file is writable, if it is, return true. | [ -w $file ] returns true. |
| -x file | Check if the file is executable, if it is, return true. | [ -x $file ] returns true. |
| -s file | Check if the file is empty (file size is greater than 0), if not empty, return true. | [ -s $file ] returns true. |
| -e file | Check if the file (including directories) exists, if it does, return true. | [ -e $file ] returns true. |
Other check characters:
-S: Determine if a file is a socket.
-L: Check if the file exists and is a symbolic link.
#!/bin/bash # author:Basic Tutorial Website # url:www.oldtoolbag.com file="/var/www/w3codebox/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, and 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