Linux&shell之结构化命令

写在前面:案例、常用、归类、解释说明。(By Jim)
使用if-then语句
如果命令的退出状态是0(成功执行命令),将执行then后面的所有命令。
如果命令的退出状态是0以外的其他值,那么then后面的命令将不会执行,bash shell会移动到脚本的下一条命令。

#!/bin/bash
# testing the if statement
if date
then
  echo "it worked"
fi

(date返回0,执行then语句it worked)

#!/bin/bash
#testing multiple commands in the then section
testuser=jiqing9006
if grep $testuser /etc/passwd
then
   echo The bash files for user $testuser are:
   ls -a /home/$testuser/.b*
fi

(The bash files for user jiqing9006 are:
/home/jiqing9006/.bash_logout  /home/jiqing9006/.bash_profile  /home/jiqing9006/.bashrc
if语句行使用grep命令搜索/etc/passwd文件,查看系统是否正在使用一个特定的用户名。
如果一个用户拥有该登录名,脚本会显示一些文本,然后列出用户HOME目录下的bash文件


if-then-else语句

#!/bin/bash
#testing multiple commands in the then section
testuser=jim
if grep $testuser /etc/passwd
then
   echo The bash files for user $testuser are:
   ls -a /home/$testuser/.b*
else
   echo "The user name $testuser does't exist on this system"
fi


(如果不存在,执行下面的语句)

嵌套if语句

#!/bin/bash
#testing multiple commands in the then section
user1=jim
user2=jiqing9006
if grep $user1 /etc/passwd
then
   echo The bash files for user $user1 are:
   ls -a /home/$user1/.b*
elif grep $user2 /etc/passwd
then
   echo The bash files for user $user2 are:
   ls -a /home/$user2/.b*
else
   echo "The user name $user1 and $user2 does't exist on this system"
fi

(不是elseif 而是elif ,注意与其它语言的区别)

test命令
if [condition]
then
  commands
 fi
 test命令能够评估三类条件
 数值
 字符串
 文件
 a.数值比较
 n1 -eq n2(是否等于)
 n1 -ge n2(是否大于等于)
 n1 -gt n2(是否大于)
 n1 -le n2(是否小于等于)
 n1 -lt n2(是否小于)
 n1 -ne n2(不等于)

#!/bin/bash
#using numeric test comparisons
val1=10
val2=11
if [ $val1 -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 not equal"
fi

(注意if与[之间有空格,[与$val1之间有空格)
test命令无法处理浮点数,bash shell只能处理整数数字

b.字符串比较
str1 = str2 (str1与str2是否相同)
str1 != str2 (是否不同)
str1 < str2(是否小于)
str1 > str2 (是否大于)
-n str1(长度是否大于0)
-z str1(长度是否为0)

#!/bin/bash
#testing string equality
testuser=root
if [ $USER = $testuser ]
then
  echo "Welcome $testuser"
fi

(比较相等)

#!/bin/bash
#testing string equality
testuser=baduser
if [ $USER != $testuser ]
then
  echo "This isn't $testuser"
else
  echo "Welcome $testuser"
fi

(不相等比较,所有标点符号和大小写都考虑在内)

使用大小于号,需要转义一下

#!/bin/bash
#mis-using string comparisons
val1=baseball
val2=hockey
val3=book
val4=Book
if [ $val1 \> $val2 ]
then
  echo "$val1 is greater than $val2"
else
  echo "$val1 is less than $val2"
fi
if [ $val1 \> $val3 ]
then
  echo "$val1 is greater than $val3"
else
  echo "$val1 is less than $val3"
fi
if [ $val1 \> $val2 ]
then
  echo "$val3 is greater than $val4"
else
  echo "$val3 is less than $val4"
fi

结果:
baseball is less than hockey
baseball is less than book
book is greater than Book
(test命令采用的是ascii码排序的,sort采用的是当前语言设置定义的排列顺序。由结果可知,比较第一个字母b小于h,如果第一个字符相同,比较第二个字符,a小于o。小写字符,大于大写字符a97,A65。)

ls -lt(按时间排序,最新的在最前面)
ls -l|sort -k1
ls -l|sort -k2
ls -l|sort -k3(按照第几列进行排序)
..

#!/bin/bash
#testing string length
str1="helloworld"
str2=""
if [ -n $str1 ]
then
  echo "str1 is not null"
else
  echo "str1 is null"
fi

if [ -z $str2 ]
then
  echo "str2 is null"
else
  echo "str2 is not null"
fi

(我自己写的代码,发现str中不能有空格,否则无法判断)

c.文件比较
test命令能够测试Linux文件系统上的文件状态和路径。
-d file 检查file是否存在并且是一个目录
-e file 检查file是否存在
-f file 检查file是否存在并且是一个文件
-r file 检查file是否存在并且可读
-s file 检查file是否存在并且不为空
-w file 检查file是否存在并且可写
-x file 检查file是否存在并且可执行
-O file 检查file是否存在并且被当前用户拥有
-G file 检查file是否存在并且被当前组拥有
file1 -nt file2 检查file1是否比file2新
file2 -ot file2 检查file1是否比file2旧
如果想将文件写到一个目录下,或试图改变到目录位置之前,最好检查一下-d

#!/bin/bash
#look before you leap
if [ -d $HOME ]
then
  echo "Your HOME directory exists"
  cd $HOME
  ls -l
else
  echo "There is a problem with your HOME directory"
fi

(检查一个目录是否存在,存在就做一些事情,不存在就提示错误。)
在脚本中使用文件或目录之前,-e比较能够检查它们是否存在

#!/bin/bash
# check if a directory exists
if [ -e $HOME ]
then
  echo "Your home directory exists"
  #check if file exists in the directory
  if [ -e $HOME/testing ]
  then
    echo "$HOME/tesing exist in the directory"
    date >>$HOME/testing
  else
    echo "Create a new file"
    date >$HOME/testing
  fi
else
  echo "Your home directory doesn't exists"
fi

(这里稍微复杂了一点,用了嵌套,并且注释很清晰,提示很到位)
-f检测文件

#!/bin/bash
#testing if a file
if [ -e $HOME ]
then
  echo "The obj exist,if it is a file?"
  if [ -f $HOME ]
  then
    echo "Yes,it is a file!"
  else
    echo "No,it is not a file!"
    if [ -f $HOME/testing ]
    then
      echo "But $HOME/testing is a file!"
    else
      echo "$HOME/testing is not a file too!"
    fi
  fi
else
  echo "The obj doesn't exist"
fi

(这个好无聊,不过展示了一个流程,逐级向下进行查询)
是否能读
在尝试从文件中读取数据之前,通常首先检查一下是否能读取文件。-r

#!/bin/bash
#testing if can read a file
pwfile=/etc/shadow
#first,check if it is a file
if [ -f $pwfile ]
then
  #now test if you can read it
  if [ -r $pwfile ]
  then
    tail $pwfile
  else
    echo "sorry,the file can be read."
  fi
else
  echo "sorry,it is not a file."
fi

(注意变量使用是一定要加上$符,不然就会出错)

检查空文件
应该用-s比较来检查文件是否为空,尤其是想删除文件时。注意,-s比较成功时,它表明文件包含数据

#!/bin/bash
#testing a file is empty
file=testfile
touch $file

if [ -s $file ]
then  
  echo "The $file exists and has data in it"
else
  echo "The $file doesn't exist or is empty"
fi
 
date >$file
if [ -s $file ]
then
  echo "The $file exists and has data in it"
else
  echo "The $file doesn't exist or is empty"
fi

(真表示有数据,假表示无数据)

检查是否能够写入数据-w

#!/bin/bash
#checking if a file if writeable
logfile=$HOME/logtest
touch $logfile
chmod u-w $logfile
now=`date +%Y%m%d-%H%M`

if [ -w $logfile ]
then
  echo "The program ran at:$now">>$logfile
  echo "The first attempt succeeded"
else
  echo "The first attempt failed"
fi

chmod u+w $logfile
if [ -w $logfile ]
then
  echo "The program ran at:$now">>$logfile
  echo "The second attempt succeeded"
else
  echo "The second attempt failed"
fi

(这里的判断有点问题,需要再研究)

..
检查文件日期

#!/bin/bash
#testing file dates
if [ ./test1 -nt ./test10 ]
then
  echo "The test1 file is newer than test10"
else
  echo "The test10 file is newer than test1"
fi

if [ ./test1 -ot ./test10 ]
then
  echo "The test1 file is older than test10"
else
  echo "The test10 file is older than test1"
fi

(比较两个文件的新旧)

复合条件检查
&&
||

#!/bin/bash
#testing compound comparisons
if [ -d $HOME ] && [ -x $HOME/testing ]
then
  echo "The file exists and you can execute it"
else
  echo "You can't execute the file"
fi

(并列执行)

if-then的高级特征
(())双圆括号,可以进行复杂的算术操作
val++ 后增量
val-- 后减量
++val 前增量
--val 前减量
!     逻辑否定
~     取反
**    取幂
<<    左移
>>    右移
&
|
&&    逻辑与
||    逻辑或

#!/bin/bash
#using double parenthesis
val1=10
if (($val1**2>90))
then
  ((val2 =$val1**2))
  echo "The square of $val1 is $val2"
fi

(双圆括号里面的内容更加智能,会识别很多东西,不用总是空格空格的编写代码了)

[[]]双方括号
可以进行字符串比较,更加智能
模式匹配,也就是正则表达式可以更好的使用

#!/bin/bash
# using pattern matching
if [[ $USER==r* ]]
then
  echo "Hello $USER"
else
  echo "Sorry,I don't know you"
fi

(规范编写)

case使用

#!/bin/bash
#using the case command
case $USER in
rich | barbara)
  echo "Welcome,$USER"
  ehco "Please enjoy your visit";;
testing)
  echo "Special testing account";;
jessica)
  echo "Don't forget to log off when you're done";;
root)
  echo "Welcome,Manager";;
*)
  echo "Sorry,you're not allowed here";;
esac

 

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