Shell scripts, if judgments, case judgments

20.5 logical judgments in shell scripts

• If judgment

  #if If the 1:if condition of the Chinese language; fi[[email protected] ~]# a=5;if [$a-gt 3];then echo OK; fiok; The reference text format is as follows: # !/bin/basha=5if [$a-gt 3] #-gt means >= then echo OKFI; format 2:if condition; Then statement; else statement; Fi[[email protected] shell]# a=5;if [$a-gt 6];then echo ok;else echo no ok;fino OK; The reference text format is as follows: #!/bin/basha=5if [$a -GT 6]then Echo okelse echo no OKFI; format 3:if ...; Then ...; Elif ...; Then ...; else ...; Fi[[email protected] shell]# a=5;if [$a-gt];then echo 0k;elif [$a-gt 6];then echo $a \>6;else echo $a \< The 6;fi5<6; reference text format is as follows: [[email protected] shell]# sh-x if3.sh+ a=5+ ' [' 5-gt '] ' + ' [' 5-gt 6 '] ' + echo ' 5<6 ' 5&L    T;6[[email protected] shell]# cat if3.sh #!/bin/basha=5if [$a-gt]then echo ">10" elif [$a-gt 6]then echo $a ">6" else echo $a "<6" fi  

Logical Judgment Bar expression:
If [$a-gt $b];
If [$a-lt 5];
If [$b-eq 10] etc.
-GT (>); -lt (<); -ge (>=);
-le (<=); -eq (= =); -ne (! =)
Note that there are spaces everywhere.

#另一种写法：[[email protected] shell]# a=5;if (($a>10));then echo OK;else echo NO;fiNO[[email protected] shell]# a=5;if (($a>=5));then echo OK;else echo NO;fiOK[[email protected] shell]# a=5;if (($a>10));then echo OK;elif (($a>6));then echo $a">6";else echo $a"<6";fi5<6

• can use && | | Combine multiple conditions

If [$a-gt 5] && [$a-lt 10]; Then
If [$b-gt 5] | | [[$b-lt 3]; then

20.6 File Directory property judgment

• [-F file] Determines if it is a normal file, and there is

[[email protected] shell]# cat file.sh #!/bin/bashf="/tmp/taoyuan"if [ -f $f ]then    echo $f existelse    touch $ffi;没有文件，创建文件[[email protected] shell]# sh -x file.sh + f=/tmp/taoyuan+ ‘[‘ -f /tmp/taoyuan ‘]‘+ touch /tmp/taoyuan;文件已经存在[[email protected] shell]# sh -x file.sh + f=/tmp/taoyuan+ ‘[‘ -f /tmp/taoyuan ‘]‘+ echo /tmp/taoyuan exist/tmp/taoyuan exist

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

[[email protected] shell]# cat file1.sh #!/bin/bashf="/tmp/taoyuan"if [ -d $f ]then    echo $f existelse    mkdir $ffi[[email protected] shell]# sh -x file1.sh + f=/tmp/taoyuan+ ‘[‘ -d /tmp/taoyuan ‘]‘+ mkdir /tmp/taoyuan

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

[[email protected] shell]# cat file2.sh #!/bin/bashf="/tmp/taoyuan"if [ -e $f ]then    echo $f exist        else    mkdir $ffi[[email protected] shell]# sh -x file2.sh+ f=/tmp/taoyuan+ ‘[‘ -e /tmp/taoyuan ‘]‘+ echo /tmp/taoyuan exist/tmp/taoyuan exist

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

[[email protected] shell]# f="/tmp/taoyuan";if [ -r $f ];then echo $f read;else echo file reservation;fi/tmp/taoyuan read

• [-W file] Determines whether the file is writable

[[email protected] shell]# f="/tmp/taoyuan";if [ -w $f ];then echo $f write;else echo $f File unable to write;fi/tmp/taoyuan write

• [-X file] Determines whether the file is executable

[[email protected] shell]# f="/tmp/taoyuan";if [ -x $f ];then echo $f x file;else echo $f File no execution;fi/tmp/taoyuan x file

20.7 If special usage

• What happens if [-Z ' $a '] This indicates that the value of equivalent A is empty?

[[email protected] shell]# cat if.sh #!/bin/bashf="/tmp/lalal"if [ ! -f $f ]then    echo "$f not exist."    exitfin=`wc -l $f`if [ -z "$a" ] #变量需要双引号then    echo error    exitelif [ $n -gt 100 ]then    echo OKfi[[email protected] shell]# sh if.sh /tmp/lalal not exist.

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

[[email protected] shell]# if [ -n if.sh ]; then echo ok;fiok#可以判断一个文件是否为空[[email protected] shell]# if [ -n "$b" ]; then echo $b;else echo "b is null";fib is null

• If Grep-q ' 123 ' 1.txt;then indicates what happens if the 1.txt contains ' 123 ' Rows

[[email protected] shell]# if grep -w ‘user‘ /etc/passwd; then echo "user exist"; fiuser:x:1000:1000:user:/home/user:/bin/bashuser exist[[email protected] shell]# if grep -wq ‘user‘ /etc/passwd; then echo "user exist"; fiuser exist#grep -q 不显示过滤的结果

• if [!-e file];then indicates that the file does not exist
• Symbols such as <,>,==,!=,>=,<= cannot be used in []

20.8-20.9 Case Judgment

• Shellcase Judgment Script case

[[email protected] shell]# cat case.sh #!/bin/bashread -p "Please enter the score:" nif [ -z "$n" ]then    echo "For the input score."    exit 1fin1=`echo $n|sed ‘s/[0-9]//g‘`if [ ! -z "$n1" ]then    echo "Non-numeric input."    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=4elsetag=0ficase $tag in    1)       echo "C"    ;;    2)       echo "B"    ;;    3)       echo "A"    ;;    4)       echo "A+"    ;;    *)      echo "Please enter a scale of 0-100."    ;;esac

Shell scripts, if judgments, case judgments