控制流结构



1.if语句

if条件1

then 

     命令1

elif条件2

then 

     命令2

else

     命令3

fi

------------------

if条件

then命令

fi

eg：

#!/bin/bash

#if test

#this is a comment line

if [ "10" -lt "12" ];then

#yes 10 is less than 12

echo "yes,10 is less than 12"

else

echo "no"

fi

注意：if语句必须以fi终止

   "10"前一个空格，“12”后也有一个空格。这个条件都是通过test命令来指定。条件表达为test expression或者[expression]

 

条件表达式中的比较函数

man test

NAME

       test - check file types andcompare values

SYNOPSIS

       test EXPRESSION

       [ EXPRESSION ]

       [ OPTION

DESCRIPTION

       Exit with the statusdetermined by EXPRESSION.

       --help display this help andexit

       --version

              outputversion information and exit

       EXPRESSION is true or falseand sets exit status. It is one of:

       ( EXPRESSION )

             EXPRESSION is true

       ! EXPRESSION

             EXPRESSION is false

       EXPRESSION1 -a EXPRESSION2

              bothEXPRESSION1 and EXPRESSION2 are true

       EXPRESSION1 -o EXPRESSION2

              eitherEXPRESSION1 or EXPRESSION2 is true

       [-n] STRING

              thelength of STRING is nonzero

       -z STRING

              thelength of STRING is zero

       STRING1 = STRING2

              thestrings are equal

       STRING1 != STRING2

              the strings are not equal

       INTEGER1 -eq INTEGER2

              INTEGER1is equal to INTEGER2

       INTEGER1 -ge INTEGER2

              INTEGER1is greater than or equal to INTEGER2

       INTEGER1 -gt INTEGER2

              INTEGER1is greater than INTEGER2

       INTEGER1 -le INTEGER2

              INTEGER1is less than or equal to INTEGER2

       INTEGER1 -lt INTEGER2

              INTEGER1is less than INTEGER2

       INTEGER1 -ne INTEGER2

              INTEGER1is not equal to INTEGER2

       FILE1 -ef FILE2

              FILE1and FILE2 have the same device and inode numbers

       FILE1 -nt FILE2

              FILE1 isnewer (modification date) than FILE2

       FILE1 -ot FILE2

              FILE1 isolder than FILE2

       -b FILE

              FILEexists and is block special

       -c FILE

              FILEexists and is character special

       -d FILE

              FILEexists and is a directory

       -e FILE

              FILEexists

       -f FILE

              FILEexists and is a regular file

       -g FILE

              FILEexists and is set-group-ID

       -h FILE

              FILEexists and is a symbolic link (same as -L)

       -G FILE

              FILEexists and is owned by the effective group ID

       -k FILE

              FILEexists and has its sticky bit set

       -L FILE

              FILEexists and is a symbolic link (same as -h)

       -O FILE

              FILEexists and is owned by the effective user ID

       -p FILE

              FILEexists and is a named pipe

       -r FILE

              FILEexists and is readable

       -s FILE

              FILEexists and has a size greater than zero

       -S FILE

              FILEexists and is a socket

       -t [FD]

              filedescriptor FD (stdout by default) is opened on a terminal

       -u FILE

              FILEexists and its set-user-ID bit is set

       -w FILE

              FILEexists and is writable

       -x FILE

             FILEexists and is executable

eg.

#!/bin/bash

#if test

#this is a comment line

echo "Enter your filename:"

read myfile

if [ -e $myfile ]

then

   if [ -s $myfile ];then

    echo "$myfile exist and size greaterthan zero"

   else

    echo "$myfile exist but size iszero"

   fi

else

echo "file no exist"

fi

[test@szbirdora 1]$ sh iftest.sh 

Enter your filename:

11

11 exist but size is zero

2.case语句

case语句为多选择语句。

case值 in

模式1）

   命令1

    ;;

模式2)

   命令2

    ;;

esac

eg.

#!/bin/bash

#case select

echo -n "enter a number from 1 to 3:"

read ans

case $ans in

1)

echo "you select 1"

;;

2)

echo "you select 2"

;;

3)

echo "you select 3"

;;

*)

echo "`basename $0`:this is not between 1 and3">2

exit;

;;

esac

3.for循环

for循环一般格式：

for变量名 in 列表 (列表以空格作为分割)

do

   命令1

   命令2

done

eg：

#!/bin/bash

#forlist1

for loop in 1 2 3 4 5

do

echo $loop

done

4.until循环

until条件

do

   命令1

   命令2

   ...

done

条件测试发生在循环末尾，所以循环至少可以执行一次。

5.

while循环

while命令 （可以是一个命令也可以是多个，做条件测试）

do

     命令1

     命令2

      ...

done

注意：如果从文件中读入变量<filename要放到done后

6.break和continue控制

break跳出，continue跳过