第四章 控制流结构

 

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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 and compare values
SYNOPSIS
       test EXPRESSION
       [ EXPRESSION ]
       [ OPTION
DESCRIPTION
       Exit with the status determined by EXPRESSION.
       –help display this help and exit
       –version
              output version information and exit
       EXPRESSION is true or false and sets exit status. It is one of:
       ( EXPRESSION )
              EXPRESSION is true
       ! EXPRESSION
              EXPRESSION is false
       EXPRESSION1 -a EXPRESSION2
              both EXPRESSION1 and EXPRESSION2 are true
       EXPRESSION1 -o EXPRESSION2
              either EXPRESSION1 or EXPRESSION2 is true
       [-n] STRING
              the length of STRING is nonzero
       -z STRING
              the length of STRING is zero
       STRING1 = STRING2
              the strings are equal
       STRING1 != STRING2
               the strings are not equal
       INTEGER1 -eq INTEGER2
              INTEGER1 is equal to INTEGER2
       INTEGER1 -ge INTEGER2
              INTEGER1 is greater than or equal to INTEGER2
       INTEGER1 -gt INTEGER2
              INTEGER1 is greater than INTEGER2
       INTEGER1 -le INTEGER2
              INTEGER1 is less than or equal to INTEGER2
       INTEGER1 -lt INTEGER2
              INTEGER1 is less than INTEGER2
       INTEGER1 -ne INTEGER2
              INTEGER1 is not equal to INTEGER2
       FILE1 -ef FILE2
              FILE1 and FILE2 have the same device and inode numbers
       FILE1 -nt FILE2
              FILE1 is newer (modification date) than FILE2
       FILE1 -ot FILE2
              FILE1 is older than FILE2
       -b FILE
              FILE exists and is block special
       -c FILE
              FILE exists and is character special
       -d FILE
              FILE exists and is a directory
       -e FILE
              FILE exists
       -f FILE
              FILE exists and is a regular file
       -g FILE
              FILE exists and is set-group-ID
       -h FILE
              FILE exists and is a symbolic link (same as -L)
       -G FILE
              FILE exists and is owned by the effective group ID
       -k FILE
              FILE exists and has its sticky bit set
       -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
              FILE exists and is a named pipe
       -r FILE
              FILE exists and is readable
       -s FILE
              FILE exists and has a size greater than zero
       -S FILE
              FILE exists and is a socket
       -t [FD]
              file descriptor FD (stdout by default) is opened on a terminal
       -u FILE
              FILE exists and its set-user-ID bit is set
       -w FILE
              FILE exists and is writable
       -x FILE
             FILE exists 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 greater than zero”
   else
    echo “$myfile exist but size is zero”
   fi
else
echo “file no exist”
fi
[test@szbirdora 1]$ 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 and 3”>&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

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