Logical judgments in shell scripts, file directory attribute judgments, if special usages, case judgments

I. Logical judgments in shell scripts

Grammar
1. Format 1
if condition; Then statement; Fi
Example: If a is greater than 3, print OK

[[email protected] ~]# vi 2.sh[[email protected] ~]# bash -v 2.sh #!/bin/basha=5if [ $a -gt 3 ]then      

2. Format 2
if condition; Then statement; else statement; Fi
Example: If a is less than 3, print no OK

[[email protected] ~]# bash -v 3.sh #!/bin/basha=2if [ $a -gt 3 ]then      echo okelse     

3. Format 3
If ...; Then ...; Elif ...; Then ...; else ...; Fi
Cases:

 [[email protected] ~]# sh -x 4.sh+ a=3+ ‘[‘ 3 -gt 4 ‘]‘+ ‘[‘ 3 -lt 6 ‘]‘+ echo ‘<6 && >1‘<6 && >1[[email protected] ~]# sh -v 4.sh#!/bin/basha=3if [ $a -gt 4 ]then      echo ">1"elif [ $a -lt 6 ]then      echo "<6 && >1"else     

Logical Judgment Expression
If [$a-gt $b]; If [$a-lt 5]; If [$b-eq 10] etc.
-GT (>); Greater than
-lt (<); Less than
-ge (>=); Greater than or equal
-le (<=); Less than or equal to
-eq (= =) equals
; -ne (! =)
Note that there are spaces everywhere.
can use && | | Combine multiple conditions
If [$a-gt 5] && [$a-lt 10]; Then
If [$b-gt 5] | | [$b-lt 3]; Then

Second, the file directory attribute judgment

1. If judgment file, directory attribute
[-F file] Determines if it is a normal file, and there is
Example: If the riven file does not exist, create

 #!/bin/bashc="/tmp/riven"if [ -f $c ]then    echo $c existelse    touch $cfi

[-D file] Determines if it is a directory and exists

 #!/bin/bashc="/tmp/2018"if [ -d $c ]then    echo $c existelse    touch $cfi

[-E file] to determine whether files or directories exist

 #!/bin/bashc="/tmp/2018"if [ -e $c ]then    echo $c existelse    touch $cfi

[-R File] to determine if the document is readable

 #!/bin/bashc="/tmp/2018"if [ -r $c ]then    echo $c readableelse    touch $cfi

[-W file] Determines whether the file is writable

 #!/bin/bashc="/tmp/2018"if [ -w $c ]then    echo $c writeableelse    touch $cfi

[-X file] Determines whether the file is executable

 #!/bin/bashc="/tmp/2018"if [ -x $c ]then    echo $c exeableelse    touch $cfi

Because the file is not executable, all have no output.
2. Script abbreviation

#!/bin/bashc="/tmp/2018"[ -f $f ] && rm -f $f#上面的命令等同于下面的命令if [-f $f ]then    rm -f $ffi

#!/bin/bashc="/tmp/2018"[ -f $f ] || touch $f   #如果这个文件不存在就执行后面的文件#上面的命令等同于下面的命令if [ !-f $f ]    #叹号表示取反then    touch $ffi

Third, if special usage

1,if [-Z "$a"]?? This indicates that an error occurs when the value of variable A is empty

#!/bin/bashif [ ! -f /tmp/a1 ]then       echo "/tmp/a1 not exist."             exit n=`wc -l /tmp/a1`if [ -z "$n" ]then     echo error    exitelse    if [ $n -gt 100 ]then    echo okfi

If [-n ' $a '] means that the value of variable A is not empty

If Grep-q ' 123 ' 1.txt; Then?? What happens if the 1.txt contains a ' 123 ' row?

 [[email protected] ~]# grep -w ‘root‘ /etc/passwdroot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/root:/sbin/nologin[[email protected] ~]# if grep -w ‘root‘ /etc/passwd; then echo "root exist"; firoot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/root:/sbin/nologinroot exist[[email protected] ~]# if grep -wq ‘root‘ /etc/passwd; then echo "root exist"; fi     #-q不显示过滤内容root exist[[email protected] ~]# if ! grep -w ‘user1‘ /etc/passwd;then useradd user1;fi   # ！取反

if [!-e file]; Then what happens when the file doesn't exist?

if (($a <1)); Then ... Equivalent to if [$a-lt 1]; Then ...
Symbols such as <,>,==,!=,>=,<= cannot be used in []

Iv. Case Judgment

Case judgment in the 1?shell
Format:

case??量名 in          value1)         command            ;;                    value2)                    command                    ;;                    *)                    commond                    ;;                    esac

In a case program, you can use the |, meaning, or mean of a condition such as
2|3)
Command
;;
2. Script case

#!/bin/bashread -p "Please input a number: " n      #read命令让用户输入字符串，n表示要捕获的变量。if [ -z "$n" ]       then    echo "Please input a number."    exit 1                              fin1=`echo $n|sed ‘s/[0-9]//g‘`    #if [ -n "$n1" ]then echo "Please input a number." exit 1fiif [ $n -lt 60 ] && [ $n -ge 0 ]then    tag=1elif [ $n -ge 60 ] && [ $n -lt 80 ]then    tag=2elif [ $n -ge 80 ]  && [ $n -lt 90 ]then    tag=3elif [ $n -ge 90 ] && [ $n -le 100 ]then    tag=4else     tag=0ficase $tag in    1)    echo "not ok"        ;;    2)        echo "ok"        ;;    3)        echo "ook"        ;;    4)        echo "oook"        ;;    *)        echo "The number range is 0-100."        ;; esac

Logical judgments in shell scripts, file directory attribute judgments, if special usages, case judgments