Linux shell Process Control Statement instance explanation (if, for, while, Case statement instance) _linux Shell

A, Shell conditional statement (if usage)

If statement structure [IF/THEN/ELIF/ELSE/FI]

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If condition test statement

Then

Action

[Elif condition

Action

Else

Action

]

Fi

If you are not very clear about: conditional test statements, you can refer to: Linux shell logical operators, logical expression of the detailed
Shell commands, which can be separated by semicolons or by line breaks. If you want to write multiple commands on one line, you can pass "';" Segmentation.
Such as:

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[Chengmo@centos5 ~]$ a=5;if [[A-GT 4]]; then echo ' OK '; fi;
Ok

Example: (test.sh)

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#!/bin/sh

scores=40;
if [[$scores-GT 90]]; Then
echo "Very good!";
elif [[$scores-GT 80]]; Then
echo "good!";
elif [[$scores-GT 60]]; Then
echo "pass!";
Else
echo "No pass!";
Fi

The conditional tests are: [[]],[],test], note: [[]] separated from the variable by a space.

Second, the circular statement (For,while,until usage):

1.for Recycling Method (For/do/done)

Syntax structure:

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1.for.. in statement

For variable in SEQ string

Todo

Action

Done

Description: Seq string as long as the space character segmentation, each time for...in read, will be read in order to the value, to the previous variable.

Instance (testfor.sh):

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#!/bin/sh

For I in $ (seq 10); Todo
echo $i;
Done

Seq 10 produces 1 2 3 .... 10 space-delimited string.

2.for (assignment; condition; Operation statement)

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for ((assignment; condition; Operation statement)

Todo

Action

Done

Instance (testfor2.sh):

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#!/bin/sh

For ((i=1;i<=10;i++));d o
echo $i;
Done

3.while Recycling (While/do/done)

While statement structure

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While conditional statement

Todo

Action

Done

Example 1:

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#!/bin/sh
i=10;
while [[$i-GT 5]];d o
echo $i;
((i--));
Done

Run Result: ========================

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SH testwhile1.sh
10
9
8
7
6

Instance 2: (Looping through the contents of the file:)

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#!/bin/sh

While Read Line;do
Echo $line;
Done </etc/hosts;

Run Result: ===================

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SH testwhile2.sh

# do don't remove the following line, or various programs
# that require network functionality would fail.
127.0.0.1 centos5 localhost.localdomain localhost

4.until Loop statement

Syntax structure:

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Until conditions

Todo

Action

Done

Until the condition is met, exit. Otherwise, the action is executed.

Instance (testuntil.sh):

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#!/bin/sh

a=10;

until [[$a-lt 0]];d o

echo $a;

((a));

Done

Run Result:

SH testuntil.sh

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10
9
8
7
6
5
4
3
2
1
0

III. Shell Selection Statement (case, select usage)

1.case SELECT statement Use (CASE/ESAC)

Grammatical structure

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Case $arg in
Pattern | Sample) # arg in-pattern or sample

PATTERN1) # arg in PATTERN1

*) #default

Esac

Note: Pattern1 is a regular expression, you can use the following characters:

* Any string
? Any character
[ABC] A, B, or C-one of the three characters
[A-n] Any character from A to n
| Multiple selection

Instance:

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#!/bin/sh

Case is in
Start | Begin
echo "Start Something"

Stop | End
echo "Stop Something"

*)
echo "Ignorant"

Esac

Run Result: ======================

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testcase.sh start
Start something

2.select Statement Use Method (Generate menu selection)

Grammar:

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Select variable name in SEQ variable

Todo

Action

Done

Instance:

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#!/bin/sh

Select ch in "Begin" "End" "Exit"
Todo
Case $ch in
"Begin")
echo "Start Something"

"End")
echo "Stop Something"

"Exit")
echo "Exit"
Break

*)
echo "Ignorant"

Esac
Done

Run Result:



Note: Select is a circular selection, typically used with case statements.