Linux Operating System Basics (ix)

operator and process Control for shell scripts 1. operator

1.1 Arithmetic operators

+

-

*

/

%

[[email protected] ~]# echo $[3+1]
4

1.2 Relationship Operations

With (())

<

>

<=

>=

==

!=

&&

||

Test command related, [] can achieve the same effect
[Email protected] ~]# x=1
[Email protected] ~]# [$x-gt 1]
[[email protected] ~]# echo $?
0

1.3 Assignment operators

=

+=

*=

/=

%=

[Email protected] ~]# x=10
[Email protected] ~]# ((x%3))
[Email protected] ~]# echo $x
10
[Email protected] ~]#
[Email protected] ~]# ((x%=3))
[Email protected] ~]# echo $x
1

1.4 All the calculators in the shell
$[] (()) $ (()) expr BC Bc-l

Floating point arithmetic: Yum install bc-y

[Email protected] ~]# echo ' SCALE=2;1/3 ' |bc-l
.33

1.5 Test actions

When the command executes, it returns to a system variable of $?
A value of 0 indicates that the command executed successfully or failed.


Testing command test [] [[]] (())
Open man test to describe each parameter individually
1.6. test file status
-D Directory
-S file length > 0, non-empty
-F Regular File
-W Writable

-R Readable

-X Executable

-L Symbolic connection

-u file has suid bit settings

1.7. String test
= Two strings equal
! = two strings are not equal
-Z Empty string
-N Non-empty string

[[email protected] ~]# var1= ' abc '
[[email protected] ~]# var2= ' 123 '
[[Email protected] ~]# [$var 1 = = $var 2]
[[email protected] ~]# echo $?
1

1.8. Test value
-eq equals
-ne Not equal to
-GT Greater than
-lt less than
-ge greater than or equal to
-le less than or equal to

[Email protected] ~]# [10000-GT] #不要使用大于号小于号等于号等, to use the man test specified, see the next section 4 expand
[[email protected] ~]# echo $?
0

1.9. Expand the test symbol [[]] (())
Digital Test Symbols
# [< 2] # syntax error
-bash:2: No file or directory
#

# [[2 > 10]] # result error
# echo $?
1
# [[> 10]] # Right
# echo $?
0
# ((< 20))
# echo $?
0

Character test
# ["AA" = "AA"]
# echo $?
0
# [["AA" = "AA"]]
# echo $?
0
# (("AA" = "AA")) #结果错误
# echo $?
1
Hybrid testing
# [A = A-a < 20]
-BASH:20: No file or directory
[[Email protected] ~]# [[a = A-a < 20]]
-bash:syntax Error in conditional expression
-bash:syntax error near '-a '
[[Email protected] ~]# [[a = a && < 20]]
[[email protected] ~]# echo $?
0
[[Email protected] ~]# [[a = a | | < 20]]
[[email protected] ~]# echo $?
0
[[Email protected] ~]# ((a = a | | < 20))
[[email protected] ~]# echo $?
0
[[Email protected] ~]# ((a = a && < 20))
[[email protected] ~]# echo $?
0

Two. Process Control

Use of 1.if

If is usually

If [Judging condition];then #如果条件满足那么

echo ' Output content ' #输出内容

Else

echo ' Output content ' #否则输出

Fi #fi来关闭

You can also add elif to make multiple judgments.

2. Cycle control

while (condition)

Do action

Done

If the while is followed by: then this control goes into an infinite loop.

For loop

For i in [1..100]

Do

Done

Syntax structure for A for loop

In the Loop

Use continue to cycle through a layer of exit operations.

Break is a piece of the loop that exits the operation.

Linux Operating System Basics (ix)