6. Conditional testing and comparison of shell scripts

(1) Common syntax for conditional expressions

1. Six ways to write conditional expressions (if, while)

Syntax 1:test<test expression>

Syntax 2: [<Test Expression>] #Both ends of brackets must have spaces

Syntax 3: [[[<Test Expression>] #Spaces are required at both ends

Syntax 4: ((Test Expression))#Spaces are not required at both ends

Syntax 5: (command expression)

Syntax 6: Command expression

Application Display

①[]Conditional expression
[root@centos6-kvm3 scripts]# [ -e /etc/hosts ] && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# [ -e /etc/host1 ] && echo 0 || echo 1
1
②test Conditional expression: test and[]Same function
[root@centos6-kvm3 scripts]# test -e /etc/host1  && echo 0 || echo 1
1
[root@centos6-kvm3 scripts]# test -e /etc/hosts  && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# # [] == test
③[[]]Double middle bracket conditional expression requiring spaces on both sides
[root@centos6-kvm3 scripts]# [[ -e /etc/host1 ]] && echo 0 || echo 1
1
[root@centos6-kvm3 scripts]# [[ -e /etc/hosts ]] && echo 0 || echo 1
0
④Double bracket conditional expression, no spaces on either side
[root@centos6-kvm3 scripts]# (( -e /etc/hosts )) && echo 0 || echo 1
-bash: ((: -e /etc/hosts : division by 0 (error token is "/hosts ")
1
⑤Double brackets are generally used to calculate:
[root@centos6-kvm3 scripts]# ((3>5)) && echo 0 || echo 1
1
[root@centos6-kvm3 scripts]# ((3<5)) && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# #(()) for calculation
⑥expr Expression: Use parentheses
[root@centos6-kvm3 scripts]# (expr 1 + 2 &>/dev/null) && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# (expr 1 + 2 &>/dev/null) && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# (expr 1 + a &>/dev/null) && echo 0 || echo 1
1
⑦expr Expression: Use quotation marks
[root@centos6-kvm3 scripts]# `expr 1 + a &>/dev/null` && echo 0 || echo 1
1
[root@centos6-kvm3 scripts]# `expr 1 + 2 &>/dev/null` && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# 

(2) The editing grammar of conditional expressions:

1, [<Test Expression>] &&Command 1 ||Command 2

If the previous expression succeeds, execute command 1, otherwise execute command 2

If [<test expression>]
then
   Command 1
else
   Command 2
fi

2. Multiple Command Conditions

When there are many commands, we can use curly brackets to enclose all the commands.The following:

[<Test Expression>] & & & {

Command 1

Command 2

}||{

Command 3

Command 4

}

3. Keep only those that execute successfully

[<Test Expression>] & &{

Command 1

Command 2

Command 3

}

4. Keep only those execution failures

[<Test Expression>] | {

Command 1

Command 2

Command 3

}

(3) File test expression

man test

-d:Files are directories and exist
[root@centos6-kvm3 scripts]# [ -d /etc/hosts ] && echo 0 || echo 1
1
[root@centos6-kvm3 scripts]# [ -d /etc ] && echo 0 || echo 1
0
-f: Judges as file and exists
[root@centos6-kvm3 scripts]# [ -f /etc/hosts ] && echo 0 || echo 1
0
-e: Determine existence, directory or file
[root@centos6-kvm3 scripts]# [ -e /etc/hosts ] && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# [ -e /etc ] && echo 0 || echo 1
0
-r: Determine that the file is readable:
[root@centos6-kvm3 scripts]# [ -r /etc/hosts ] && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# ll /etc/hosts
-rw-r--r--. 2 root root 352 Nov 19  2018 /etc/hosts
-x:Determine that the file is executable:
[root@centos6-kvm3 scripts]# [ -x /etc/hosts ] && echo 0 || echo 1
1
[root@centos6-kvm3 scripts]# chmod +x /etc/hosts
[root@centos6-kvm3 scripts]# [ -x /etc/hosts ] && echo 0 || echo 1
0
-s: Determine that the file size is not zero:
[root@centos6-kvm3 scripts]# [ -s /etc/hosts ] && echo 0 || echo 1
0
[root@centos6-kvm3 scripts]# touch oldboy.log
[root@centos6-kvm3 scripts]# [ -s oldboy.log ] && echo 0 || echo 1
1
//Example application: crond
[root@centos6-kvm3 scripts]# cat /etc/init.d/crond 

(4) Explanation of common functions of string test expressions

n:not zero ,[-n "Character string" ] String length is not zero, expression is true.
z:zero,[-z "Character string" ] The string length is 0 and the expression is true.
["String 1"=="String 2"] Two strings are true if they are identical.
["String 1"!="String 2"] Two strings that are not identical are true.
//Be careful:
1,The string is double quoted.
2,An equal sign can be one or two.
3,There must be spaces at both ends of the equal sign.

[root@centos6-kvm3 scripts]# [ -n "oldboy" ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ -z "oldboy" ] && echo 1 || echo 0
0
[root@centos6-kvm3 scripts]# char="oldboy"
[root@centos6-kvm3 scripts]# [ -n "$char" ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# unset char
[root@centos6-kvm3 scripts]# [ -n "$char" ] && echo 1 || echo 0
0
[root@centos6-kvm3 scripts]# [ "dd"=="ff" ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ "dd" == "ff" ] && echo 1 || echo 0
0
[root@centos6-kvm3 scripts]# [ "dd" == "dd" ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ "dd" != "ff" ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ "dd" != "dd" ] && echo 1 || echo 0
0
//Instance application:
cat /etc/init.d/crond 
cat /etc/init.d/network 

Example:

[root@centos6-kvm3 scripts]# cat select1.sh 
#!/bin/bash
cat << EOF
1.install lamp
2.install lnmp
3.exit
EOF
read -p "Please enter a serial number:" num
[ -z "$num" ] && exit 1 #Determine if the content is empty
expr 2 + $num &>/dev/null
if [ $? -ne 0 ]
then
   echo "usage:$0{1|2|3}"
   exit 1
fi

if [ $num -eq 1 ]
then
   echo "install lamp..."
elif [ $num -eq 2 ]
then
   echo "install lnmp ..."
elif [ $num -eq 3 ]
then
   echo "bye..."
   exit
else
   echo "usage:$0{1|2|3}"
   exit 1
fi

(5) Integer test expression

Example

[root@centos6-kvm3 ~]# [ 2 -eq 3 ] && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# [ 2 -gt 3 ] && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# [ 2 -lt 3 ] && echo 0 || echo 1
0
[root@centos6-kvm3 ~]# [ 2 > 3 ] && echo 0 || echo 1
0
[root@centos6-kvm3 ~]# [ 2 \> 3 ] && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# [[ 2 > 3 ]] && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# [[ 2 -gt 3 ]] && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# [[ 2 -lt 3 ]] && echo 0 || echo 1
0
[root@centos6-kvm3 ~]# (( 2 -lt 3 )) && echo 0 || echo 1
-bash: ((: 2 -lt 3 : syntax error in expression (error token is "3 ")
1
[root@centos6-kvm3 ~]# (( 2 > 3 )) && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# (( 2 < 3 )) && echo 0 || echo 1
0
[root@centos6-kvm3 ~]# test 2 -gt 3 && echo 0 || echo 1
1
[root@centos6-kvm3 ~]# test 2 -lt 3 && echo 0 || echo 1
0
//Summary:
1,Use alphabetic expressions in double brackets.
2,Double parentheses are not suitable for alphabetic expressions, only for symbolic expressions.
3,test Expressions only apply to symbolic expressions.

(6) Test questions: Use read's interaction to compare the size of two integers.

[root@centos6-kvm3 scripts]# cat test3.sh
#!/bin/bash
read -p "Please enter two integers:" a b
[ -z "$b" ] && {
   echo "Please enter two integers."
   exit 1
}
expr $a + $b + 1 &>/dev/null
[ $? -ne 0 ] && {
   echo "Please enter two integers."
   exit 2
}
[ $a -lt $b ] && {
   echo "$a less than $b."
   exit 0
}
[ $a -gt $b ] && {
   echo "$a greater than $b."
   exit 0
}

[ $a -eq $b ] && {
   echo "$a Be equal to $b."
   exit 0
}

================
//Use the if statement:
[root@centos6-kvm3 scripts]# cat test4.sh 
#!/bin/bash
read -p "Please enter two integers:" a b
[ -z "$b" ] && {
   echo "Please enter two integers."
   exit 1
}
expr $a + $b + 1 &>/dev/null
[ $? -ne 0 ] && {
   echo "Please enter two integers."
   exit 2
}
if [ $a -lt $b ]
then
   echo "$a less than $b."
elif [ $a -gt $b ]
then
   echo "$a greater than $b."
else 
   echo "$a Be equal to $b."
fi
[root@centos6-kvm3 scripts]# 

(7) Logical test expression

Example:

[root@centos6-kvm3 scripts]# [ 1 -eq 1 -a -f /etc/hosts ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ 1 -eq 2 -a -f /etc/hosts ] && echo 1 || echo 0
0
[root@centos6-kvm3 scripts]# [ 1 -eq 2 -o -f /etc/hosts ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ 1 -eq 2 ] -o [ -f /etc/hosts ] && echo 1 || echo 0
-bash: [: too many arguments
0
[root@centos6-kvm3 scripts]# [ 1 -eq 2 ] || [ -f /etc/hosts ] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [ 1 -eq 2 ] && [ -f /etc/hosts ] && echo 1 || echo 0
0
[root@centos6-kvm3 scripts]# [[ 1 -eq 2 || -f /etc/hosts ]] && echo 1 || echo 0
1
[root@centos6-kvm3 scripts]# [[ 1 -eq 2 && -f /etc/hosts ]] && echo 1 || echo 0
0