Linux Structured Commands

内容概要: if-then, test condition ,case , for, while, until, nested loop ,looping on file data ,controlling loop…

1.if-then-fi:

The bash shell if statement is different from other languages. if the exit status of the if-command is 0, the commands listed under the then are executed.

if-then-elif-then-else-fi:
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if command1
then 
	commands
elif command2
then
	commands
elif command3
then
	commands
else
	commands
fi

Give a example:

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#!/bin/bash
if grep $1 /etc/passwd
then
	echo "1. The bash files for user $1 are: "
	ls -a /home/$2/.b*
elif grep $2 /etc/passwd
then
	echo "2. The bash files for user $2 are: "
	ls -a /home/$2/.b*
else
	echo "Invalid user!"
fi

2.The test command:

2.1 The test command provides a way to test different conditions in an if-then statement.
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#条件判断有两种可选的方式：
if test condtion
then
	commands
fi

#或者采用下面的,注意空格
if [ condition ]
then 
	commands
fi
2.2 There are three classes of conditions the test commands can evaluate:
  • Numeric comparisons
  • String comparisons
  • File comparisons
2.3 Numeric comparison:
Comparison Description
m -eq n m等于n
m -ne n m不等于n
m -ge n m大于或等于n
m -gt n m大于n
m -le n m小于或等于n
m -lt n m小于n
举例说明：
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#!/bin/bash
# 注意空格
m=10
n=20
if [ $m -eq 10  ]
then 
	echo "m==10"
fi

if test $m -ge $n
then 
	echo "m >= n"
elif [ $m -ne $n  ]
then
	echo "m != n"
else
	echo "m<n"
fi
2.4 String comparison:
comparison description
str1 = str2
str != str2
str1 < str2
str1 > str2
-n str1 str1长度大于0
-z str1 str1长度等于0
2.5 File comparison:
comparison description
-d file file存在并且是direcory
-e file file存在
-f file file存在并且是FILE
-r file file存在并且可读
-s file file存在并且不为空
-w file file存在并且可写
-x file file存在并且可执行
-O file file存在并且由当前用户拥有
-G file file存在并且默认group与当前用户的group相同
file1 -nt file2 file1 is newer than file2
file1 -ot file2 file1 is older than file2
举例说明:
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#!/bin/bash
if [ -e $HOME ]
then 
	echo "Your home directory exists!"
	if [ -f $HOME ]
	then 
		echo "yes,It is a file"
	else
		echo "NO, it is a directory!"
		if [ -e $HOME/.bash_history ]
		then
			#var=`cat $HOME/.bash_history`
			#echo $var
			if [ -w $HOME/.bash_history ]
			then
				echo ".bash_history is a file you can write!"
			else
				echo ".bash_history is not writable!"
			fi
		fi
	fi
else
	echo "sorry, your HOME file does not exists!"
fi
2.6 Compound Condition Testing:

[ condition1 ] && [ condition2 ]
[ condition1 ] || [ condition2 ]

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$ bash << EOF
> #!/bin/bash
> if [ -d $HOME  ] && [ -w $HOME/test  ];then
> echo The file exists and you can write to it!
> else
> echo I cannot write to the file
> fi
> EOF

3.Advanced if-then :

There are two advanced features that you can use in if-then statements

  • Double parentheses for mathematical expressions
  • Double square brackets for advanced string handling functions
3.1 Double parentheses: (( expression ))

The double parentheses command allows you to incorporate advanced mathematical formulas in your comparisons.

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#!/bin/bash
val1=10

if (( $val1 ** 2 > 90 ))
then
	(( val2 = $val1 **2 ))
	echo The square of $val1 is $val2
fi

For循环

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#!/bin/bash

for((x=0;x<10;x++))
do
	echo $x
done

The double parentheses command symbols:
Symbol Description
val++
val–
++val
–val
! logical negation
~ bitwise negation
** exponentiation
<< left bitwise shift
>> right bitwise shift
& bitwise Boolean AND
“ “ bitwise Boolean OR
&& logical AND
“ “ logical OR
3.2 Double brackets: [[ expression ]]

The double brackets command provides advanced features for string comparisons.

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#!/bin/bash

if [[ $USER == r* ]]
then
	echo "Hello $USER"
else
	echo "Sorry, I don't know you"
fi

4.Case command:

The case command checks multiple values of a single variable in a list-oriented format,

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case variable in
	pattern1) commands1;;
	pattern2) commands2;;
	pattern3 | pattern4) commands;;
	*) default commands;;
esac

举例说明：

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#!/bin/bash

case $1 in
	jay)
		echo "Hello jay!";;
	root)
		echo "This is root user!";;
	zhang | bryant)
		echo "zhang or bryant!";;
	*)
		echo "This is default commands!";;
esac

5.for command:

A loop that iterates through a series of values.
for

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for var in list
do 
	commands
done

5.1 Reading values in a list && from a variable:
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#!/bin/bash
list="Alabama Alaska Arizona Arkansas California Colorado"
list=$list" Washington"
for var in $list
do 
	echo "The next state is $var"
done
echo "The last state you visit is $var"

i=0
for var in Alabama Alaska Arizona Arkansas California Colorado "New Mexico"
do
	((i=$i+1))
	echo "state $i is $var"
done
5.2 Reading values from a command:

Another way to generate values for use in the list is to use the output of a command.

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#!/bin/bash
file="/home/jay/.bash_history"

count=0
for var in `cat $file`
do
	((count++))
	echo "Command NO.$count : $var"
done


# Reading a directory using wildcards
for file in /home/jay/* /etc/*
do
	if [ -d $file ]
	then
		echo "$file is a directory!"
	elif [ -f $file ]
	then
		echo "$file is a file"
	fi
done

5.3 The C-style for command:
for(( varibale assignment; condition; iteration process ))
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#!/bin/bash
sum=0
for((i=1;i<=100;i++ ))
do	
	((sum=sum+i))
done
echo "sum of 1+...+100 = $sum"

6. while command:

When the test command returns a non-zero exit status, the while command stops executing the set of commands.
Basic format of the while command is

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while test command
do 	
	other commands
done

举例说明:

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#!/bin/bash

var=10
while [ $var -gt 0 ]
do
	echo $var
	var=$[$var-1]
done

# using multuple test commands
var=10
while echo $var
 [ $var -gt 0 ]
do
	echo "Inside the loop!"
	var=$[$var-1]
done

7. until command:

The util command works exactly the opposite way from the while command. Once the test command returns a zero exit status, the loop stops.
The format of the until command is:

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until test commands
do
	other commands
done

举例说明:

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var=100
until [ $var -eq 0 ]
do 	
	echo $var
	var=$[ $var-10 ]
done

8. Nesting Loops:

Some examples for nesting loops:

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#!/bin/bash

var=5
while [ $var -gt 0 ]
do 
	echo "Outer loop: $var"
	for ((var2=1;$var2<3;var2++))
	do
		var3=$[$var*$var2]
		echo "	  Inner loop: $var * $var2 = $var3"
	done
	var=$[$var-1]
done

9. Looping on File Data:

Sometimes, you must iterate through items stored inside a file. This require combining two of the techniques covered:

  • using nested loops
  • changing the IFS environment variable
    By changing the IFS environment variable, you can force the for command to handle each line in the file as a separate item for processing, even if the data contains spaces.
    Here is an example of doing that:
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    #!/bin/bash

    IFS_OLD=$IFS
    echo "Initial IFS=$IFS"

    #change IFS value to parse the individual lines 
    IFS=$'\n'
    echo "Changed IFS=$IFS"

    count=0
    for entry in `cat /etc/passwd`
    do 
    	count=$[ $count+1 ]
    	echo "Rows NO.$count >>>  " $entry
    	#change IFS value to the colon, which allows you to parse the individual values within each line.
    	IFS=:
    	for value in $entry
    	do 
    		echo "	$value"
    	done
    done

10. Controlling the loop:

  • break command
  • continue command
    10.1 break command
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    #!/bin/bash

    #for loop
    for var in 1 2 3 4 5 6 7 8 9
    do 
    	if [ $var -eq 5 ]
    	then 
    		break
    	fi
    	echo "for Iteration NO.$var"
    done

    #while loop
    var=1
    while [ $var -lt 10 ]
    do
    	if [ $var -eq 5 ]
    	then 
    		break
    	fi
    	echo "while Iteraion NO.$var"
    	var=$[$var+1]
    done
10.2 break command
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#!/bin/bash

#continue in for loop
for (( var=1;var<15;var++ ))
do
	if [ $var -gt 5 ] && [ $var -lt 10 ]
	then 
		continue
	fi
	echo "for iteratin NO.$var"
done

#continue in while loop
var=0
while echo "while iteraion NO.$var"
	[ $var -lt 15 ]
do
	if [ $var -gt 5 ] && [ $var -lt 10 ]
	then
		var=$[$var+2]
		continue
	fi
	echo " Inside iteration NO.$var"
	var=$[$var+1]
done

11. 数组

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#!/bin/bash
#1.创建一个空数组
arrs=()

#2.初始化数组
arrs=("helo" "beijing" "china" 12 58 "well")

#3.数组的第二个元素
echo ${arr[1]}

#4.取得数组的所有元素
${arrs[@]}

#5.数组长度
${#arrs[@]}

#6.修改第一个元素
arrs[0]=2

#7.数组尾部添加一个元素
arrs+="jay"

#8.把ls的输出保存到字符串
str=$(ls)

#9.把ls的输出保存到数组
arrs=($(ls))

#10.取得从索引m开始的n个元素
${arr[@]:m:n}


#举例: 数组迭代
for i in ${arrs[@]}
do
	echo $i
done

#数组迭代with index
for i in ${!arrs[@]}
do
	echo "arrs["$i"]="${arrs[$i]}
done