Linux Learning Note--bash script programming the second, condition judgment

Bash Script Programming second, conditional judgment

Condition Judgment:

If the user does not exist

Add user, give password and display add success;

Otherwise

Show that the user is already present, not added

How do I make conditional judgments in bash?

Integer test

Character Test

File test

Expressions for Conditional tests:

[Expression]

[[Expression]]

Test expression

Integer comparison:

-eq: Tests whether two integers are equal, such as $A-eq $B

-ne: Test whether the two integers are unequal, unequal, true, equal, false;

-QT: Tests whether one number is greater than the other, greater than true;

-LT: Tests if one number is less than the other; less than true; otherwise, false;

-ge: greater than or equal to

-le: Less than or equal to

The logical relationship between commands:

Logic and:&&

when the first condition is false, the second condition is no longer judged and the final result is already there;

When the second condition is true, the second condition must be judged;

logical OR: | |

If the user user2 does not exist, the user is added User2

！ ID user2 && useradd user2

ID User2 | | Useradd User2

variable name:${varname}, parentheses can sometimes be omitted

1, can only contain sub-mother, number and underscore, and can not start with a number;

2, should not be in the system with the existing system variables duplicate name;

3, it is best to see the name of the meaning;

If the user User1 exists, it shows that the user already exists; otherwise, it is added;

ID user1 && echo "user1 exists." | | Useradd user1

If the user User1 does not exist, add it, otherwise, show that it already exists;

! ID user1 && Useradd user1 | | echo "User1 exists."

If user User1 does not exist, add and give password; otherwise, show that it already exists;

! ID user1 && useradd user1 && echo "user1" | passwd--stdin User1 | | echo "User1 exists."

conditional judgment, control structure :

Single Branch if statement

if judgment condition; Then

Statement1

Statement2

...

Fi

Dual-Branch If statement:

if judgment condition; Then

Statement1

Statement2

Else

Statement3

Statement4

Fi

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Linux Learning Note--bash script programming the second, condition judgment