19. Self-learning Linux: The logic of bash conditions and character testing of bash programming

pre-Class review : To find the sum of divisible by 3 divisible by 200

# !/bin/bash  -Isum= 0 for with {1..200};d o  if [$[$i%3]-eq 0]; then let Su M+="Thesum is: $sum"

Bash's knowledge points :

Conditional testing: Making a logical operation on a condition

　　　　　　　　　　　　with : Condition 1 && Condition 2

　　　　　　　　　　　　or : Condition 1 | | condition 2

　　　　　　　　　　　non- :! Conditions

　　　　　　example :#userName =root

#id $userName && echo "userName exist"

If the former fails, the latter will not execute because the final result of the operation is already available.

If the former succeeds, the latter still needs to be executed to determine whether the latter is true before the result can be obtained.

　　　　　　

Classic Conditional judgment :#id #userName &>/dev/null && echo "$userName exist" | | echo "$userName not exist"

parsing :

When condition 1 is satisfied, and because statement 2 is the output statement, it must be true, so the condition 1 and statement 2 are true, and then the statement 3 does or operations, condition 3 can not be executed.

When the condition 1 is not satisfied, because the condition 1 with the statement 2 is with the operation, so there is no need to do with the operation, the first two conditions are false, at this time, the 3rd statement must be executed.

The effect of this command is: if the condition 1 satisfies the execution statement 2, if the condition 1 is not satisfied, then executes the statement 3.

Bash Programming : character Testing

　　　　　　Binocular operator :

: Greater Than

<: Less than

= =: The left side is a string, the right is a pattern, determine the left side of the string, can be matched by the pattern on the right

Format: usually only used in [[]], the pattern can use the beginning of the line, the end of the row anchor characters, such as metacharacters, but the pattern can not be quoted

　　　　　　single-Mesh operators :

　　　　　　　　　　　　- N $stringVar: string is not empty, not empty is true, empty is false

　　　　　　　　　　　　- Z $stringVar: Whether the string is empty, empty is true, and not empty is false

　　　　　　　　　　　　　

Example :　　

#stringA ="root"#stringB="hello"  $stringA"$stringB"  ] #echo $?  "stringa" "$stringA" =~ OT]]: pattern matching

Exercise 1: Determine if a given user is a version of the shell

#userName =root#[[' grep ' ^ $userName \> "/etc/passwd  |  cut-d:-f7 ' =~ sh$]] #echo $?

　　

Exercise 2: Find the shell that can be landed and output

 for  in ' cut-d:-f1/etc/passwd ', do if"^ $userName \>" /etc/ passwd  |  cut-d:-f7 ' =~"login Shell: $userName"else"  nologin User: $userName"  fidone　　

　

19. Self-learning Linux: The logic of bash conditions and character testing of bash programming