Shell condition test, judgment statement and operator of shell system learning

Catalog

Conditions for testing

String test

Integer test

File test

Logical operators  

Conditional judgment sentence

if sentence

 if else sentence

if elif sentence

Multi conditional judgment statement case

Operator

Arithmetic operator

An operator

Self increasing / The subtraction operator

Perform arithmetic operations

Use $((expression))

Use $[expression]

Use expr command

Use let command

The base of a numeric constant  

  Summary

---

Conditions for testing

Conditional test statements are used to test whether the specified conditional expression is true or false . When the condition is true, it returns 0, Conversely, return non 0 value . This is contrary to general programming languages , Please pay attention .

There are two kinds of syntax for conditional testing : Namely test command and [ command .

test expression
[ expression ]

The parameter expression Is a conditional expression , Can be represented by a string 、 Numbers 、 file name , And various operators .

[ Command and test command , It also tests the following conditional expressions , But in order to increase the readability of the program ,Shell It is required to add a right square bracket to the right of the conditional expression ] Come with the front [ Command pairing ,

Be careful expression and [,] There is a space between ！ because " Left bracket " It's the same thing shell command , There must be a space between the command and the parameter .

String test

The judgment of string is mainly to judge whether it is empty , Whether it is equal or not . Users can use the 5 Operators

String operators

Operator	explain
string	Judge whether the specified string is empty , Not empty return 0
string1 = string2	Determine whether two strings are equal , Equal return 0
string1 != string2	Determine if two strings are not equal , Unequal return 0
-n string	Judge string Whether it is a non empty string , Not empty return 0
-z string	Judge string Whether it is an empty string , Empty string return 0

For the first operator in the above table , Only use test Order to test , rest 4 Kind of , Both test methods support .

In string comparison , It's a good habit to use quotation marks to define strings .

#!/bin/bash
a="abc"
b=""
test $a
# Not empty return 0
echo $?

test $b
# Null return non 0
echo $?

test -n $a
# Not empty return 0
echo $?

test -z $a
# Non empty returns non 0
echo $?

[ -n $a ]
# Not empty return 0
echo $?

[ -z $a ]
# Non empty returns non 0
echo $?

test "$a" = "$b"
# Inequality returns non 0
echo $?

[ "$a" != "$b" ]
# Unequal return 0
echo $?

give the result as follows ：

[email protected]:~/Documents/Shell$ ./4-1.sh 
0
1
0
1
0
1
1
0

  Be careful ： String comparisons are case sensitive .

there = and != All operators , In use , You need to add spaces before and after , Bear in mind .

Integer test

The two syntaxes of integer testing are similar to strings ：

test number1 op number2
[ number1 op number2]

number1 and number2 Represents the integer involved in the comparison , It can be constants and variables ,op Representation operator . The following table lists the commonly used integer operators ：

Commonly used integer operators

Operator	explain
num1 -eq num2	To determine if two Numbers are equal , Equal return 0
num1 -ne num2	Determine if two Numbers are not equal , Unequal return 0
num1 -gt num2	Judge num1 Is it greater than num2, Greater than return 0
num1 -lt num2	Judge num1 Is less than num2, Less than return 0
num1 -ge num2	Judge num1 Greater than or equal to num2, Greater than or equal to returns 0
num1 -le num2	Judge num1 Less than or equal to num2, Less than or equal to returns 0

example ：

#!/bin/bash
a=12
b=14

[ "$a" -eq "$b"]
# Inequality returns non 0
echo $?

test "$a" -ne "$b"
# Unequal return 0
echo $?

[ "$a" -eq 12 ]
# Equal return 0
echo $?

[ "$a" -gt "$b" ]
# Not greater than return non 0
echo $?

[ "$a" -lt "$b" ]
# Less than return 0
echo $?

[ "$a" -ge "12" ]
# Greater than or equal to returns 0
echo $?

result ：

[email protected]:~/Documents/Shell$ ./4-2.sh 
1
0
0
1
0
0

If [] There is no space between , Providing will report an error , For example, I ] No space before , The system prompts missing ] Symbol ：

 ./4-2.sh: line 5: [: missing `]'

File test

Shell Many file related operators are provided to detect files . The following table ：

Common file operators

The operator	explain
-a file	Judge whether the file exists , There is returned 0
-b file	Determine whether the file exists and the file type is block file , Is to return 0
-c file	Determine whether the file exists and the file type is character file , Is to return 0
-d file	Determine whether the file exists and the file type is directory , Is to return 0
-e file	Effect and -a identical
-f file	Determine whether the file exists and is a regular file , Is to return 0
-s file	Judge file Is it a non empty file , Is to return 0
-w file	Determine whether the file exists and can be written , Is to return 0
-r file	Determine whether the file exists and is readable , Is to return 0
-x file	Determine whether the file exists and is executable , Is to return 0
-L file	Determine whether the file exists and whether the file is a symbolic link , Is to return 0
-u file	Determine whether the file exists and whether it is set suid position , Is to return 0

Main detection 3 aspect ： Does the file exist ; Type of file ; Access to files .

First, create a directory and some files in a directory , The situation is as follows ：

[email protected]:~/Documents/Shell/4-3$ ll
total 16
drwxrwxr-x 3 eden_ubuntu eden_ubuntu 4096 Jul 26 00:26 ./
drwxrwxr-x 3 eden_ubuntu eden_ubuntu 4096 Jul 26 00:18 ../
-rw-rw-r-- 1 eden_ubuntu eden_ubuntu    0 Jul 26 00:26 file1
-rw-rw-r-- 1 eden_ubuntu eden_ubuntu    3 Jul 26 00:27 file2
lrwxrwxrwx 1 eden_ubuntu eden_ubuntu    9 Jul 26 00:19 shortcut.sh -> ../4-2.sh*
drwxrwxr-x 2 eden_ubuntu eden_ubuntu 4096 Jul 26 00:18 tmpDir/

  Execute the test case ：

#!/bin/bash
test -d tmpDir
echo $?

test -f file1
#file1 Exists and is a normal file 
echo $?

test -s file1
#file1 The size is 0, Return non 0
echo $?

test -s file2
#file1 Exist and file1 Size greater than 0, return 0
echo $?

test -L shortcut.sh
#shortcut.sh Exists and is a symbolic link , return 0
echo $?

test -b file1
#file1 Not a block file , Return non 0
echo $?

test -b /dev/sda
#/dev/sda Exist and /dev/sda It's a file , return 0
echo $?

test -c /dev/tty
# Control terminal /dev/tty Exist and /dev/tty Is a character file , return 0
echo $?

result :

[email protected]:~/Documents/Shell/4-3$ ./4-3.sh 
0
0
1
0
0
1
0
0

Logical operators  

stay Shell Programming , We often encounter the situation of judging multiple conditions , Combine multiple conditions into one condition for judgment , be used Not 、 or and And The logic of . Look at the table below

Common logical operators

The operator	explain
! expression	Logic is not
expression1 -a expression2	Logic and
expression1 -o expression2	Logic or

A simple example ：

#!/bin/bash
a=35
test "$a" -gt 30 -a "$a" -lt 40
echo $? 

give the result as follows ：

[email protected]:~/Documents/Shell$ ./4-4.sh 
0

Conditional judgment sentence

Conditional judgement sentences enable programs to execute different program branches according to different conditions , Mainly if sentence ,if else sentence ,if elif sentence , Examples of various statements are listed below .

if sentence

The grammar is ：

if expression
then
    statement1
    statement2
    ...
fi

 To make the code more compact , We can get if Clause and then Clause on one line , You need to be in expression Add a... After the expression ; Number 
if expression; then 
    statement1 statement2 ...
fi
 perhaps 
if expression; then statement1 statement2 ...; fi
    

Examples are as follows ： 

#!/bin/bash
if [ -f ./4-5.sh ]
    then 
    echo "./4-5.sh is a file"
fi

if test -f ./4-5.sh
    then echo "./4-5.sh is a file"
fi

if [ -f ./4-5.sh ]; then echo "./4-5.sh is a file"; fi

if :; then echo "always ture"; fi

You can see if It can be used in the back test command , It can also be used. [ command  

The result is ：

[email protected]:~/Documents/Shell$ ./4-5.sh 

./4-5.sh is a file
./4-5.sh is a file
./4-5.sh is a file
always ture

stay Shell There is also a special command in , be called An empty command , It is represented by a colon ：, Change the order and do nothing , But its exit state is always 0, Is always true , In the above example, we have shown .

  Others like to use && Operator instead of if sentence , Like the following example ：

#!/bin/bash
test -f './4-5.sh' && echo './4-5.sh is a file'
! test -f './4-5-.sh' && echo './4-5.sh is not a file'; exit 1

result ：

[email protected]:~/Documents/Shell$ ./4-6.sh 
./4-5.sh is a file
./4-5.sh is not a file

 if else sentence

The grammar is ：

if expression
then 
    statement1
    statement2
    ...
else
    statement3
    statement4
    ...
fi

if elif sentence

The grammar is ：

if expression1
then
    statement1
    statement2
    ...
elif expression2
then
    statement3
    statement4
    ...
...
else
    statementn
fi

I won't talk more about the above two sentences , Similar to other languages , know if Usage of , The two are similar .

Multi conditional judgment statement case

Be similar to C++,java Yes. switch sentence , The grammar is as follows ：

case variable in
value1)
    statement1
    ...
    statementn
    ;;
value1)
    statement1
    ...
    statementn
    ;;
value1)
    statement1
    ...
    statementn
    ;;
*)
    statement1
    ...
    statementn
    ;;
esac

among *） Namely default Options , When there are no other branches, it will enter this branch .

case The sentence structure is in esac ending , and if Statement to fi Same structure , They all end with the command name in reverse order .

Operator

In addition to the operators listed above , There are also some operators related to numbers , Like arithmetic operators 、 An operator 、 Self increasing / Subtraction operator, etc . First, let's look at arithmetic operators , Similar to other programming languages

Arithmetic operator

Common arithmetic operators

Operator	explain
+	Find the sum of two numbers
-	seek 2 The difference between numbers
*	seek 2 The product of numbers
/	seek 2 Quotient of numbers
%	Seeking remainder / modulus
**	Power operation , This is special , give an example ：3**4 Namely 3 Of 4 The next power

Let's look at the compound arithmetic operator , This and C The language is the same , Don't explain too much

Common compound arithmetic operators

Operator	explain
+=	After summing up , Then assign and to the variable on the left
-=	After calculating the difference , Then assign and to the variable on the left
*=	After finding the product , Then assign and to the variable on the left
/=	After seeking business , Then assign and to the variable on the left
%/	After taking the rest , Then assign and to the variable on the left

An operator

Shell Programming also supports bitwise operators , Usage and C The language is the same , But it may be used less , Now let's look at the watch

Common bitwise operators

Operator
<<	Move left
>>	Move right
&	Bitwise AND
|	Press bit or
~	Bitwise non
^	Bitwise XOR

Bitwise operators also have compound operators

Compound bit operators

Operator
>>=	After moving left, assign the value to the variable on the left
<<=	After moving to the right, assign the value to the variable on the left
&=	Assign the value to the variable on the left by bitwise and post
|=	Assign the value to the variable on the left by bit or after
^=	Assign the value to the variable on the left after bitwise XOR

Self increasing / The subtraction operator

Usage and C The language is the same

Self increasing / The subtraction operator

Operator	explain
++variable	First the variable The value of the add 1, And then assign it to variable
--variable	First the variable The value of the reduction 1, And then assign it to variable
variable++	First use variable Value , And then variable The value of the add 1
variable--	First use variable Value , And then variable The value of the reduction 1

Perform arithmetic operations

stay Shell There is 4 There are three ways to perform arithmetic operations , They are as follows ：

Use $((expression))

Using this form for arithmetic operations is relatively free , There is no need to escape operators and parentheses , Expressions can be written in a loose or compact format . give an example ：

#!/bin/bash
# In compact format 
result=$((3+7))
echo "$result"

# In a loose format 
result=$(( 3 - 7 ))
echo "$result"

result=$((3*7))
echo "$result"

result=$((8/3))
echo "$result"

result=$((8%3))
echo "$result"

result=$((2**3))
echo "$result"

# Compound operation 
result=$(((3+7)*2))
echo "$result"

result ：

[email protected]:~/Documents/Shell$ ./4-7.sh 
10
-4
21
2
2

8
20

Use $[expression]

This method of use is similar to $((expression)) Basically the same , give an example ：

#!/bin/bash
# In compact format 
result=$[3+7]
echo "$result"

# In a loose format 
result=$[ 3 - 7 ]
echo "$result"

result=$[3*7]
echo "$result"

result=$[8/3]
echo "$result"

result=$[8%3]
echo "$result"

result=$[2**3]
echo "$result"

# Compound operation 
result=$[(3+7)*2]
echo "$result"

The result is consistent with the above .

[email protected]:~/Documents/Shell$ ./4-8.sh 
10
-4
21
2
2
20

[] And (())  The difference is [] Assignment operation is not allowed inside , and (()) It supports assignment , This difference can be found later We can see in the example of the base of numerical constants , If you are interested, you can also try .

Use expr command

expr It's a Shell command , Use the syntax ：

expr expression

It has more attention to use , It's not easy to use . When using operators , Operators need to be escaped , If there are parentheses , You also need to escape the parentheses . Cannot use compact format , Use loose formatting , give an example ：

#!/bin/bash
result=`expr 2 + 100`
echo "$result"

result=`expr 2 - 100`
echo "$result"

result=`expr 2 \* 100`
echo "$result"

result=`expr 100 / 2`
echo "$result"

result=`expr 100 % 3`
echo "$result"

result=`expr \( 1 + 3 \) \* 5`
echo "$result"

# Wrong grammar 
result=`expr 2+5`
echo "$result"

# Wrong grammar 
result=`expr 2+5*10`
echo "$result"

# Wrong grammar 
result=`expr 2*(2+5)`
echo "$result"

result ：

[email protected]:~/Documents/Shell$ ./4-9.sh 
102
-98
200
50
1
20
2+5
2+5*10
./4-9.sh: command substitution: line 29: syntax error near unexpected token `('
./4-9.sh: command substitution: line 29: `expr 2*(2+5)'

You can see the front 6 Items are normal , To the first 7 Xiang and di 8 The result did not meet expectations , Not an integer value, but an expression  , The first 9 Item reporting error . So pay attention to avoid the above mistakes when using .

Use let command

Use let The command can execute one or more arithmetic expressions , There is no need to use... For variable names $ Symbol . If the expression contains spaces or other special characters , Need to quote . give an example ：

#!/bin/bash
n=10
let n=n+1
echo "$n"

let n-=1
echo "$n"

let n=n**2
echo "$n"

let n--
echo "$n"

let n**3
echo "$n"

let The expression of must have an assignment at the end to be effective . let n**3 It will not be modified n The value of the .

give the result as follows ：

[email protected]:~/Documents/Shell$ ./4-10.sh 
11
10
100
99
99

The base of a numeric constant  

By default ,Shell Numbers are always represented in decimal . But users can also Shell Use other base numbers in to represent numbers , Like binary , octal , Hexadecimal . stay Shell in , Users can use two syntaxes to represent different base numbers

1. Add prefix ：0 The first number indicates 0 Base number ,0x The first number indicates hexadecimal

2. Use # Number ：2#1001 For binary ,8#20 For octal  

Examples are as follows ：

#!/bin/bash
a=$((2#1000))
((b=8#20))
((c=16#10))

d=$[0x10]
e=$[020]
f=20
((g=0x10))

((h=(1+3)*5))
echo "$a $b $c $d $e $f $g $h"

  give the result as follows ：

[email protected]:~/Documents/Shell$ ./4-11.sh 
8 16 16 16 16 20 16 20

  Summary

This article mainly discusses conditional testing , Judgment statement , Operator and how to use it