shell学习之条件测试（参考shell脚本编程诀窍）

rgar12

1关于test测试，查看man文档知

表达式的判断
( EXPRESSION )                    #EXPRESSION is true
! EXPRESSION                      #EXPRESSION is false
EXPRESSION1 -a EXPRESSION2        #both are true,-o means or

字符串是否为空，相等
-n STRING  #the length of STRING is nonzero,-n can be removed
-z STRING  #the length of STRING is zero（nonexists,or null)
#when comes to string, we use =

STRING1 = STRING2       #the strings are equal
STRING1 != STRING2      #the strings are not equal

数字的比较
#when comes to number,we use different eq ne and so on
INTEGER1 -eq INTEGER2   #INTEGER1 is equal to INTEGER2
        -ge  greater or equal
        -gt  greater than
    -le   -lt   -ne   I
比较文件是否为链接文件     
FILE1 -ef FILE2         #FILE1 and FILE2 have the same device and inode numbers
                eg

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[iyunv@www shell]# test  txt  -ef txthardlink                             [iyunv@www shell]# echo $?                 0                 [iyunv@www shell]# test  txt  -ef txtlink(soft)                 [iyunv@www shell]# echo $?                 0

                所以这个是测试是否是文件链接（包括软硬），大家可以自己试试

比较文件的修改时间
FILE1 -nt FILE2                        #FILE1 is newer/older (modification date) than FILE2，timestamp
FILE1 -ot FILE2                    文件有修改时间modify time （强调内容改变） 改变时间change time（本质即相关属性改变）  接触时间access time（被访问）

文件类型
-b FILE     #type:block special
-c FILE     #type:character special
-d FILE     # a directory
-e FILE     #FILE exists是否存在
-s FILE     #FILE exists and has a size greater than zero是否为空
-f FILE     #a regular file
-u FILE     #set user-ID   SUID
-g FILE     #FILE exists and is set-group-ID使用时暂时拥有文件所属组的权限SGID
-G FILE     #FILE exists and is owned by the effective group ID真的Group
-h FILE     #a symbolic link (same as -L)
-k FILE     #FILE exists and has its sticky bit set   sticky位其他用户只能添加不能删除
-L FILE     #FILE exists and is a symbolic link (same as -h)
-O FILE     #FILE exists and is owned by the effective user ID
-p FILE     #a named pipe
-S FILE     #FILE exists and is a socket
-r FILE     #FILE exists and read permission is granted
                        you can  use -w  ,write  ,-x  execute

-t FD       #file descriptor FD is opened on a terminal
很多，但有规律，常用的并不多

2 上面的测试还可用于类似[ -e $num ]结构中

3 if/then条件测试结构（还有case，while，等结构可能后面会涉及，不懂请自行baidu）
①if  [ condition ]
then
COMMAND
fi
        or
if  [ condition ];then
COMMAND
fi
②if  [ condition ];then
COMMAND1
else
COMMAND2
fi
③if  [ condition ];then
COMMAND1
elif  [ condition ];then
COMMAND2
else
COMMAND3
fi

big eg.

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#if1.sh #!/bin/bash # read -p "guss who am i?"  HELLO test -z $HELLO&&echo "wrong argv please reinput!"&&exit 1 if [ "$HELLO" = "Jack" ];then echo "oh,you are jack!" elif [ "$HELLO" = "Kitty" ];then echo "oh,you are Kitty!" elif [ "$HELLO" = "Mike" ];then echo "oh,you are Mike!" else echo "sorry i don't remember." echo "ok,my name is $HELLO" fi

通常if then可以改为[[ conditon ]]&&条件ok||条件不ok
3 [[ ]]与[]的区别
通常表现在以下几个方面
[ ]使用-a，-o连接多个逻辑表达式，而[[ ]]使用&&，||连接多个表达式
[ ]：通配符不起作用，[[ ]]会展开通配符
[ ]不能使用比较运算符，如>，<，而[[ ]]可以4 (())
测试数字表达式
结果为0返回1，结果为其他返回0

[iyunv@www ~]# ((1-1))
[iyunv@www ~]# echo $?
1
[iyunv@www ~]# ((1+1))
[iyunv@www ~]# echo $?
0
5 条件判断与测试的小程序

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#-N可以表示文件是否被修改 #所以有下面的一段监控文件是否被修改的shell script #watchfile.sh #!/bin/bash TT=5 #5 seconds NFILE=$1 #the watched file #origin length of the file len=`wc -l $NFILE |awk '{print $1 }'` echo " $NFILE has  $len lines " while : do   if [ -N $NFILE ]   then   echo "`date`:new entry in $NFILE"   newlen=`wc -l $NFILE |awk '{print $1 }'`   newlines=`expr $newlen - $len`   tail -$newlines $NFILE   len=$newlen   fi   sleep $TT done

6 正则表达式的测试程序

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#RE.sh #!/bin/bash for deb in /root/shell/* do pkgname=`basename $deb` if [[ $pkgname =~ .*\.deb ]];then         #提醒 if紧跟着的[有空格，$前面有一个空格，deb后面有个空格   echo "FILE $pkgname is a debian package" else   echo "File $pkgname is not a debian package" fi done

另一个添加捕获的

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#如何捕获括号中的字符串 #保存在BASH_REMATCH[]数组中 #RE1.sh #!/bin/bash # manage to know your name read -p "please input your fullname：" NAME if [[ $NAME =~  (.*)[[:space:]](.*) ]];then echo "你姓 ${BASH_REMATCH[2]}" echo "你叫 ${BASH_REMATCH[1]}" echo "全称 ${BASH_REMATCH[0]}" fi 执行结果 [iyunv@www shell]# ./RE1.sh please input your fullname:liancao liu 你姓 liu 你叫 liancao 全称 liancao liu

7 一个猜数字的脚本

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#一个猜数字的sh #numguess.sh #!/bin/bash #you may want to guess between 0-50 WORDS=51 GUESS=-1 TIME=0 BELOW=0 TOP=`expr $WORDS - 1 ` let ANSWER=($RANDOM % $WORDS) let ANSWER+=-1 while [ "$GUESS" -ne "$ANSWER" ] do     echo "the number is between $BELOW and $TOP"         read -p "your guess:" GUESS     let TIME+=1        if [ "$GUESS" -lt "$ANSWER" ] ;then             echo "please input larger"         BELOW=`expr $GUESS + 1`     elif [ "$GUESS" -gt "$ANSWER" ];then         echo "please input smaller"         TOP=`expr $GUESS - 1 `     else             echo "right! you guessed $TIME times"         fi done

国安信仰

不错，学习了！