Shell Learning Notes VI (Test and judgment)

Test and judgment tests use the $ after command execution to determine if the command is performing properly. $?==0? Normal: Error test structure: 1, test expression 2, [expression] #注意表达式两侧的空格 mode 2 increases the readability of the code, and more easily with I        F,case,while the keywords associated with these conditions. File test: 1, Test file_operator file 2, [File_operator file] Example: Test-e/var/l                Og/message echo $?             #文件存在返回0, does not exist return non 0 value [-e/var/log/message] echo $?                File Comparator-B file exists and is a block file-C file exists and is a character device-e file or directory exists with a-D file exists and is a directory                -F file exists and is an ordinary file-X file exists and is an executable file-W file exists and is a writable file-R file exists and is a readable file -l file exists and is the connection file-p ...                Pipe file-S ... socket file-s size not 0 FILE1 nt FILE2 FILE1 than FILE2 new return True FILE1 ot FILE2 FILE1 than FILE2 old return True example: Test file read, write, execute properties #!/bin/bash read-P "input a filename which you want to test:" FileName if [!                    -E "$filename"];then echo "the file doesn ' t exist."                Exit 1 fi if [-W ' $filename "];then echo" the file is writeable. "                fi if [-r] $filename "];then echo" the file is readable. "                fi if [-X] $filename "];then echo" The file is executable "fi                               Note: There is a space string test between if and [] in if [expression]: string comparison (dictionary order) is mainly greater than, less than, equal to, not equal to, empty, not empty > < = =-z-n Example: str= "" Test-z        "$str" echo $?        #0 test-n "$str" echo $?        #1 test-n $str echo $? #0, preferably with double quotation marks ("") str= "Hello" [-Z "$STR "] echo $?        #1 [-N "$str"] echo $? #0 str1= "123" str2= "234" ["$str 1" = "$str 2"] #test $str 1 = $str 2, pay attention to spaces, or keep returning        Back to 0 echo $?                #1 ["$str 1"! = "$str 2"] echo $?                ["$str 1" \> "$str 2"] echo $?                ["$str 1" \< "$str 2"] echo $?        [["$str 1" > "$str 2"]] #不使用转义字符 echo $?            Integer comparison-eq =-lt <-gt >-le <=-ge >=-ne! =                1, Test num1 num_operator num2 2, [NUM1 num_operator num2] Example: num1=10                num2=10 num3=9 num4=11 #测试相等 [$num 1-eq $num 2]                echo $?            Test $num 3-eq $num 2 echo $? Note: Be sure to note the space, otherwiseThere is a possibility that the error logic and logical operators are logical with, logical, or logical non--a-o!     Non:!expression with: Expression1-a expression2 or: Expression1-o expression2 &&     ||               ！ [-e/var/log/message] && [-e/var/log/message01] =[-e/var/log/message-a-e/var/log/message01 ] [-e/var/log/message] | |        [-E/VAR/LOG/MESSAGE01] =[-e/var/log/message-o-E/VAR/LOG/MESSAGE01] Notice two different ways to judge            If statement if Expression;then Command1 Command2 ... fi                Example: Judging student achievement #!/bin/bash Echo-n "input a score of a student:" #-n means no line break                Read score if [$score-lt];then echo "D" fi If [$score-lt 80-a $score-ge];then echo "C" fi if [$score-lt 90-a $score-ge];then echo "B" fi if [$score-le 100                    -A $score-ge];then echo "a" fi if [$score-gt];then echo "The number is invalid" Fi has learned high-level language, is actually learning shell grammar and programming features, high-level language focus on large projects, and shell focus on the work            Make the work Automation If/else statement if expression; then command else command                Fi Example: Determine if a file exists #!/bin/bash echo-n "input a filename:"                Read filename If [-e $filename];then echo "file exists."                else echo "file doesn ' t exists." Fi if/elif/else Multiple If/else Statement Example: Modify student score script #!/bin/bash Echo-n "in Put a score of a student: "Read score if [$score-lt];then echo "D" Elif [$score-lt];then echo "C" elif                 [$score-lt];then echo "B" Elif [$score-le];then echo "A"             elif [$score-gt];then echo "The number is invalid" fi case                    Equivalent to Switch...case structure: Case VAR in var1) command;;                    VAR2) command;;                    VAR3) command;;            .. *) command;; Note: The var1, Var2, VAR3, and so on can only be constants or regular expressions, and can be regular, good example: detect the type of current system #!/bin/bash type= ' una                         Me-s ' #print The kernel name case $type in l*) echo "Linux"                    ;;                    SunOS) echo "SunOS";;    Darwin)                    echo "Darwin";;                *) echo "other";;                ESAC detects if the user input contains uppercase letters, lowercase letters, or numbers #!/bin/bash echo-n "input a string:" Read STR case $STR in *[[:lower:]]*) echo "the string                    Contains lower letters. ";;                        *[[:upper:]]*) echo "The string contains upper letters."                    ;;                        *[[:d igit:]]*) echo "The string contains digit."                    ;;                *) echo "other";; Esac

Shell Learning Note VI (Test and judgment)