第四章 控制流结构

 1if 条件1
 2then
 3     命令1
 4elif 条件2
 5then
 6     命令2
 7else
 8     命令3
 9fi
10------------------
11if 条件
12then 命令
13fi
14eg：
15#!/bin/bash
16#if test
17#this is a comment line
18if [ "10" -lt "12" ];then
19#yes 10 is less than 12
20echo "yes,10 is less than 12"
21else
22echo "no"
23fi

  1man test
  2NAME
  3       test - check file types and compare values
  4SYNOPSIS
  5       test EXPRESSION
  6       [ EXPRESSION ]
  7       [ OPTION
  8DESCRIPTION
  9       Exit with the status determined by EXPRESSION.
 10       --help display this help and exit
 11       --version
 12              output version information and exit
 13       EXPRESSION is true or false and sets exit status. It is one of:
 14       ( EXPRESSION )
 15              EXPRESSION is true
 16       ! EXPRESSION
 17              EXPRESSION is false
 18       EXPRESSION1 -a EXPRESSION2
 19              both EXPRESSION1 and EXPRESSION2 are true
 20       EXPRESSION1 -o EXPRESSION2
 21              either EXPRESSION1 or EXPRESSION2 is true
 22       [-n] STRING
 23              the length of STRING is nonzero
 24       -z STRING
 25              the length of STRING is zero
 26       STRING1 = STRING2
 27              the strings are equal
 28       STRING1 != STRING2
 29               the strings are not equal
 30       INTEGER1 -eq INTEGER2
 31              INTEGER1 is equal to INTEGER2
 32       INTEGER1 -ge INTEGER2
 33              INTEGER1 is greater than or equal to INTEGER2
 34       INTEGER1 -gt INTEGER2
 35              INTEGER1 is greater than INTEGER2
 36       INTEGER1 -le INTEGER2
 37              INTEGER1 is less than or equal to INTEGER2
 38       INTEGER1 -lt INTEGER2
 39              INTEGER1 is less than INTEGER2
 40       INTEGER1 -ne INTEGER2
 41              INTEGER1 is not equal to INTEGER2
 42       FILE1 -ef FILE2
 43              FILE1 and FILE2 have the same device and inode numbers
 44       FILE1 -nt FILE2
 45              FILE1 is newer (modification date) than FILE2
 46       FILE1 -ot FILE2
 47              FILE1 is older than FILE2
 48       -b FILE
 49              FILE exists and is block special
 50       -c FILE
 51              FILE exists and is character special
 52       -d FILE
 53              FILE exists and is a directory
 54       -e FILE
 55              FILE exists
 56       -f FILE
 57              FILE exists and is a regular file
 58       -g FILE
 59              FILE exists and is set-group-ID
 60       -h FILE
 61              FILE exists and is a symbolic link (same as -L)
 62       -G FILE
 63              FILE exists and is owned by the effective group ID
 64       -k FILE
 65              FILE exists and has its sticky bit set
 66       -L FILE
 67              FILE exists and is a symbolic link (same as -h)
 68       -O FILE
 69              FILE exists and is owned by the effective user ID
 70       -p FILE
 71              FILE exists and is a named pipe
 72       -r FILE
 73              FILE exists and is readable
 74       -s FILE
 75              FILE exists and has a size greater than zero
 76       -S FILE
 77              FILE exists and is a socket
 78       -t [FD]
 79              file descriptor FD (stdout by default) is opened on a terminal
 80       -u FILE
 81              FILE exists and its set-user-ID bit is set
 82       -w FILE
 83              FILE exists and is writable
 84       -x FILE
 85             FILE exists and is executable
 86eg.
 87#!/bin/bash
 88#if test
 89#this is a comment line
 90echo "Enter your filename:"
 91read myfile
 92if [ -e $myfile ]
 93then
 94   if [ -s $myfile ];then
 95    echo "$myfile exist and size greater than zero"
 96   else
 97    echo "$myfile exist but size is zero"
 98   fi
 99else
100echo "file no exist"
101fi
102[test@szbirdora 1]$ sh iftest.sh
103Enter your filename:
10411
10511 exist but size is zero

 1case 值 in
 2模式1）
 3    命令1
 4    ;;
 5模式2)
 6    命令2
 7    ;;
 8esac
 9eg.
10#!/bin/bash
11#case select
12echo -n "enter a number from 1 to 3:"
13read ans
14case $ans in
151)
16echo "you select 1"
17;;
182)
19echo "you select 2"
20;;
213)
22echo "you select 3"
23;;
24*)
25echo "`basename $0`:this is not between 1 and 3">&2
26exit;
27;;
28esac

 1for 变量名 in 列表 (列表以空格作为分割)
 2do
 3   命令1
 4   命令2
 5done
 6eg：
 7#!/bin/bash
 8#forlist1
 9for loop in 1 2 3 4 5
10do
11echo $loop
12done

1until 条件
2do
3   命令1
4   命令2
5   ...
6done

1while 命令 （可以是一个命令也可以是多个，做条件测试）
2do
3      命令1
4      命令2
5      ...
6done