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Source: Cheng Mo
www.cnblogs.com/chengmo/archive/2010/10/14/1851434.html
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The Linux Shell has its own set of control flow statements, including conditional statements (if), loop statements (for, while), and selection statements (case). Below, I will introduce the usage of each statement through examples.
1. Shell Conditional Statements (Usage of if)
The structure of the if statement is [if/then/elif/else/fi]
if condition test statement
then
action
[elif condition
action
else
action
]
fi
If you are not very clear about the condition test statement, you can refer to: Linux shell logical operators and logical expressions detailed explanation.
Shell commands can be separated by semicolons or newline characters. If you want to write multiple commands in one line, you can separate them with ‘;’.
For example:
[chengmo@centos5 ~]$ a=5;if [[ a -gt 4 ]] ;then echo ‘ok’;fi;
ok
Example: (test.sh)
#!/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;
Condition tests can be: [[]], [], test, these types, note: [[]] must be separated from the variable by a space.
2. Loop Statements (Usage of for, while, until):
-
Usage of for loop (for/do/done)
Syntax structure:
1. for … in statement
for variable in seq string
do
action
done
Note: The seq string is separated by space characters, and each time for…in reads, it will sequentially assign the read value to the preceding variable.
Example (testfor.sh):
#!/bin/sh
for i in $(seq 10); do
echo $i;
done;
seq 10 generates a space-separated string of 1 2 3 … 10.
2. for((assignment; condition; operation statement))
for((assignment; condition; operation statement))
do
action
done;
Example (testfor2.sh):
#!/bin/sh
for((i=1;i<=10;i++));do
echo $i;
done;
-
Usage of while loop (while/do/done)
Structure of while statement
while condition statement
do
action
done;
Example 1:
#!/bin/sh
i=10;
while [[ $i -gt 5 ]];do
echo $i;
((i–));
done;
Running result:========================
sh testwhile1.sh109876
Example 2: (Loop reading file content:)
#!/bin/sh
while read line;do
echo $line;
done < /etc/hosts;
Running result:===================
sh testwhile2.sh# Do not remove the following line, or various programs# that require network functionality will fail.127.0.0.1 centos5 localhost.localdomain localhost
-
Until loop statement
Syntax structure:
until condition
do
action
done
This means: until the condition is met, exit. Otherwise, execute action.
Example (testuntil.sh):
#!/bin/sh
a=10;
until [[ $a -lt 0 ]];do
echo $a;
((a–));
done;
Result:
sh testuntil.sh
109876543210
3. Shell Selection Statements (Usage of case, select)
-
Usage of case selection statement (case/esac)
Syntax structure
case $arg in
pattern | sample) # arg in pattern or sample
;;
pattern1) # arg in pattern1
;;
*) #default
;;
Note: pattern1 is a regular expression, and can use the following characters:
* any string? any character[abc] any one of a, b, or c[a-n] any character from a to n| multiple choices
Example:
#!/bin/sh
case $1 in
start | begin)
echo “start something”
;;
stop | end)
echo “stop something”
;;
*)
echo “Ignorant”
;;
esac
Running result:======================
testcase.sh startstart something
-
Usage of select statement (producing menu selection)
Syntax:
select variable name in seq variable
do
action
done
Example:
#!/bin/sh
select ch in “begin” “end” “exit”
do
case $ch in
“begin”)
echo “start something”
;;
“end”)
echo “stop something”
;;
“exit”)
echo “exit”
break;
;;
*)
echo “Ignorant”
;;
esac
done;
Running result:
Note: select is a loop selection, generally used with case statements.
The above are the control flow statements of the shell: condition, loop, and selection. Welcome to discuss and exchange!
This series:
-
Detailed Explanation of Linux Shell String Operations (Length/Searching/Replacing)
-
Examples of Linux Shell Control Flow Statements
-
Introduction to Custom Functions in Linux Shell (Definition, Return Values, Variable Scope)
-
Linux Shell Array Creation and Usage Tips
-
Detailed Explanation of Linux Shell Data Redirection
-
Linux Shell Time Calculations and Methods for Calculating Time Differences
-
Simple Methods for Performing Arithmetic Operations (Integers and Floating Points) in Linux Shell
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