Linux(四) 使用结构化命令

if command
then
　　commands
fi

# testing the if statement
if date
　　echo "it worked"
fi
echo "we‘re outside of the if statement"

# 输出: it worked ; we‘re outside of the if statement

#!/bin/bash
# testing multiple commands in the then section
testuser=root
if grep $testuser /etc/passwd
then
    echo The bash file for user $testuser are:
    ls -a /home/testuser/.b*
fi

if command
then
　　commands
else
　　commands
fi

#!/bin/bash
# testing the else section
testuser=badtest
if grep $testuser /etc/passwd
then 
　　echo The files for user $testuser are:
　　ls -a /home/$testuser/.b*
else
　　echo "The user name $testuser doesn‘t exxist on this system"
fi

输出: The user name badtest doesn‘t exist on this system

if command1
then
　　commed2
elif commed2
then 
　　more commands
elif commed3
then
　　more commands
fi

test condition

# condition是一系列test命令评估的参数和值. 在if-then语句中使用时, test命令如下所示
if test condition
then 
    commands
fi

# bash shell 提供一种在 if-then 语句中声明 test 命令的另一种方法:
if [ condition ]
then
    commands
fi
# 方括号定义在test命令中使用的条件. 
# 注意, 在前半个方括号的后面和后半个方括号的前面必须都有一个空格,否则会得到错误信息
# test命令能评估一下3类条件: 1.数值比较, 2.字符串比较, 3.文件比较

#!/bin/bash
# using numeric test comparisons
var1=10
var2=11

if [ $var -gt 5 ]
then
    echo "The test value $val1 is greater than 5"
fi

if [ $val1 -eq $val2 ]
then
    echo "The values are equal"
else
    echo "The values are different"
fi

# 第一个测试条件: if [ $val1 -ft 5 ]  测试变量 val1 的值是否大于5
# 第二个 test 条件: if [ $val1 -eq $val2 ] 测试变量val1的值是否等于变量val2的值
# 输出: The test value 10 is greater than 5 ; The values are different

# -----------------------------------------------------------------------
#!/bin/bash
# testing floating point numbers
val1=` echo "scale=4; 10 / 3 " | bc`
echo "The test value is $val1"
if [ $val1 -gt 3 ]
then
　　echo "The result is larger than 3"
fi

# 输出: The test value is 3.333 ; [: 3.333: integer expression expected

# 1.字符串相等 ( 比较字符串是否相等时, 测试比较将所有标点符号和大写都考虑在内 )
#!/bin/bash
# testing string equality
testuser=rich
if [ $USER = $testuser ]
then
    echo "Welome $testuser"
else
    echo "This isn‘t $testuser"
fi

# rich 登录 输出: Welcome rich
# baduser登录 输出: This isn‘t baduser

# ----------------------------------
# 2.字符串顺序
# 问题: 1.大于和小于符号一定要转义, 否则shell会将他们当作重定向符号,将字符串值看作文件名; 2.大于和小于顺序与在 sort 命令中的顺序不同
#!/bin/bash
# mis-using string comparisons

val1=baseball
val2=hockey
if [ $val1 > $val2 ]
then
    echo "$val1 is greater than $val2"
else
    echo "$val1 is less than $val2"
fi

# baseball is greater than hockey 
# 同时当前目录下生成了一个名为 hockey 的文件
# 脚本将大于号理解为输出重定向,所以创建了一个 hockey 文件. 由于重定向成功完成,所以测试命令返回突出状态代码 0 , if 语句认为事情已成功完成. 要解决这个问题,需要适当转义大于号
# 当字符串中存在大小写字母时, test命令使用标准的 ASCII 排序

#-------------------------------------------------------
# 3.字符串大小
#!/bin/bash
# testing string length
val1=testing
val2=‘‘

if [ -n $val1 ]
then
    echo "The string ‘$vall‘ is not empty"
else
    echo "The string ‘$val1‘ is empty"
fi

if [ -z $val2 ]
then
    echo "The string ‘$val2‘ is empty"
else
    echo "The string ‘val2‘ is not empty"
fi

if [ -z $val3 ]
then 
    echo "The string ‘$val3‘ is empty"
else
    echo "The string ‘$val3‘ is not empty"
fi

# 输出: The string ‘testing‘ is not empty ; The string ‘‘ is empty ; The string ‘‘ is empty
# if [ -n $val1 ] 确定变量val1的长度是否为非零值,如果是非零值,那么将处理then部分
# if [ -z $val2 ] 确定变量val2的长度是否为0,,如果是0, 那么将处理then部分;
# if [ -z $val3 ] 确定变量val3的长度是否为0,未定义,所以长度为零