Linux Shell Process Control

Linux Shell Process Control

Guide

Unlike the Java, PHP, and other languages, the Linux shell has its own process control statements, including conditional statements (IF), loop statements (for,while), and SELECT statements (case). I'll use the example below to illustrate how each statement is used.

One, Shell conditional statements (if usage)

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

If condition test statement thenaction[elif condition ACTIONELSEACTION]FI

Shell commands, which can be split by semicolons, or by line breaks. If you want to write multiple commands in one line, you can pass the ";" segmentation, such as:

[[email protected] ~]$ a=5;if [[A-GT 4]]; then echo ' OK '; fi;

Example: (test.sh)

#!/bin/shscores=40;if [[$scores-GT 90]]; Thenecho "Very good!"; elif [[$scores-GT 80]]; Thenecho "good!"; elif [[$scores-GT 60]]; Thenecho "pass!"; Elseecho "No pass!"; Fi

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

(1) For loop use Method (For/do/done)
1.for ... in statement--syntax structure

The for variable in seq string       # seq string is separated by a space character, and each time for...in reads, the                                                              value is read sequentially to the preceding variable. Doactiondone

Example (testfor.sh):

#!/bin/shfor i in $ (seq 10); Do     #seq 10 generate 1 2 3 ... 10 space-delimited string echo $i;d one;

2.for (assignment; condition; op-statement))

for (assignment; condition; Operation statement)) Doactiondone;

Example (testfor2.sh):

#!/bin/shfor ((i=1;i<=10;i++));d Oecho $i;d one;

(2) While loop use (While/do/done)

While conditional statement doactiondone;

Example 1:

#!/bin/shi=10;while [[$i-GT 5]];d oecho $i;((i--));d one;

Operation Result:

SH testwhile1.sh109876

Example 2: (Loop read file contents:)

#!/bin/shwhile Read Line;doecho $line;d one </etc/hosts;

Operation Result:

SH testwhile2.sh# don't remove the following line, or various programs# that require network functionality would fail.127. 0.0.1 centos5 localhost.localdomain localhost

(3) Until loop statement--Grammatical structure

Until condition             #直到满足条件, exit. Otherwise, the action is executed. Doactiondone

Example (testuntil.sh):

#!/bin/sha=10;until [[$a-lt 0]];d oecho $a;((a));d one;

Results:

SH testuntil.sh109876543210

Iii. Shell Selection statement (case, select usage)

(1) Case Selection statement use (CASE/ESAC)--syntax structure

Case $arg Inpattern | Sample) # arg in pattern or sample;; PATTERN1) # arg in PATTERN1;; *) #default;; Esac

Description: Pattern1 is a regular expression and can be used with the following characters:
* Any string? Any character
[ABC] A, B, or C three characters one of them
[A-n] Any character from A to n
| Multiple options

Instance:

#!/bin/shcase $ Instart | Begin) echo "Start something";; Stop | End) echo "Stop something";; *) echo "ignorant";; Esac

Operation Result:

Testcase.sh StartStart Something

(2) SELECT statement Use method (Generate menu selection)--syntax

Select variable name in seq variable doactiondone

Instance:

#!/bin/shselect ch in "Begin" "End" "Exit" Docase $ch "Begin") echo "Start Something";; " End ") echo" stop Something ";;" Exit ") echo" exit "break;;; *) echo "ignorant";; Esacdone;

Operation Result:

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Linux Shell Process Control