Learning note -5.3 Shell Programming 2

5.3 Shell Programming 2

Condition Judgment:

If the user does not exist

Add user, give password and show add success

Otherwise

Displays if it already exists, does not add

How do I make conditional judgments in bash?

Condition Test Type:

Integer test

Character test

File test

Expressions for Conditional tests:

[EXPRESSOPM] must have a space before and after

[[EXPRESSOPM]]

Test expression

Integer comparison:

-eq: Test two integers for equality, 0 for example $A-eq $B

-ne: Test whether two integers are unequal; 0, equal, false

-GT: Tests whether a number is greater than another number: greater Than, true

-LT: Test whether a number is less than another number

-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:

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

Logical OR: | |

ID user1 &>/dev/null && echo "hello,student."

If the user User6 does not exist, the user is added User6

! ID USER6 && useradd user6

ID User6 | | Useradd User6

Variable name: can only contain letters, numbers and underscores, and cannot start with a number, should not be followed

The names of the existing environment variables in the system

It's better to see the name and righteousness

Nano second.sh

#!/bin/bash

LINES = ' Wc-l/etc/inittab '

#echo $LINES

Finlines = ' echo $LINES | Cut-d '-f1 '

#echo $FINLINES

[$FINLINES-GT] && echo "/etc/inittab is a big file."

|| echo "/etc/inittab is a small file."

If the user exists, the user is already present, otherwise the user is added:

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

If the user does not exist, add: Otherwise, it shows that it already exists

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

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

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

echo "User1 exists."

Practice, write a script to complete the following requirements:

1 Add 3 User User1,user2,user3, but first determine if a user exists,

does not exist and then adds

2 After the add is complete, the display adds a total of several users: Of course, it cannot be included because prior

exists without adding the

3 finally shows how many users are on the current system

Nano adduser.sh

#!/bin/bash

! ID user1 &>/dev/null && useradd user1 && echo "user1" | passwd

--stdin user1 &>/dev/null | | echo "User1 exists."

! ID user2 &>/dev/null && useradd user2 && echo "User2" | passwd

--stdin user2 &>/dev/null | | echo "User2 exists."

! ID user3 &>/dev/null && useradd user3 && echo "User3" | passwd

--stdin user3 &>/dev/null | | echo "User3 exists."

USERS = ' Wc-l/etc/passwd | cut-d:-f1 '

echo "$USERS USERS."

If the UID is 0: Then

Show as Administrator

Otherwise

Show as normal user

NAME = User16

USERID = ' Id-u $NAME ' (~ down quote) command Reference indicates the execution result of the command

if[$USERID-eq 0];then

echo "Admin"

Else

echo "Common user."

Fi

An anti-quote reference indicates the execution result of the command

No anti-quotation marks indicate the execution status of the command (succeeded)

If ID $NAME;

Practice writing a script to complete the following requirements:

Given a user:

1 if its UID is 0, this is displayed as an administrator

2 Otherwise, it will be displayed as a normal user

Nano third.sh

#!/bin/bash

#

NAME = User1

USERID = ' Id-u $NAME '

[$USERID-eq 0] && echo "Admin" | | echo "Common user."

Conditional judgment, control structure:

Single Branch if statement

if judgment condition; then (then write next line, semicolon can be omitted)

Statement1

Statement2

...

Fi

Two-branch if statement:

if judgment condition; then

Statement1

Statement2

...

Else

Statement3

Statement4

...

Fi

Nano tt.sh

#!/bin/bash

#

NAME = User1

If ID $NAME &>/dev/null;then

echo "$NAME EXISTS."

Else

Useradd $NAME

echo $NAME | passwd--stdin $NAME &>/dev/null

echo "Add $NAME finished."

Fi

Exercise: Write a script to complete the following tasks

1 use a variable to save a user name

2 Delete the user in this variable and delete its home directory

3 Display the "User Delete complete" class information

Learning note -5.3 Shell Programming 2