简单判断式

利用 test 命令的测试功能

[root@www ~]# test -e /dmtsai && echo "exist" || echo "Not exist"
Not exist  <==结果显示不存在啊！

利用判断符号 [ ]

中括号的两端需要有空白字节来分隔

[root@www scripts]# vi sh06.sh
#!/bin/bash
# Program:
#     This program shows the user's choice
# History:
# 2005/08/25    VBird    First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH

read -p "Please input (Y/N): " yn
[ "$yn" == "Y" -o "$yn" == "y" ] && echo "OK, continue" && exit 0
[ "$yn" == "N" -o "$yn" == "n" ] && echo "Oh, interrupt!" && exit 0
echo "I don't know what your choice is" && exit 0

Shell script 的默认变量($0, $1...)： shift

• $# ：代表后接的参数『个数』，以上表为例这里显示为『 4 』；
• $@ ：代表『 "$1" "$2" "$3" "$4" 』之意，每个变量是独立的(用双引号括起来)；
• $* ：代表『 "$1c$2c$3c$4" 』，其中 c 为分隔字节，默认为空白键， 所以本例中代表『 "$1 $2 $3 $4" 』之意。

[root@www scripts]# vi sh07.sh
#!/bin/bash
# Program:
#    Program shows the script name, parameters...
# History:
# 2009/02/17    VBird    First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH

echo "The script name is        ==> $0"      //程序的档名为何？
echo "Total parameter number is ==> $#"       //共有几个参数？
[ "$#" -lt 2 ] && echo "The number of parameter is less than 2.  Stop here." \
    && exit 0                               //若参数的个数小於 2 则告知使用者参数数量太少
echo "Your whole parameter is   ==> '$@'"      //全部的参数内容为何？
echo "The 1st parameter         ==> $1"          //第一个参数为何？
echo "The 2nd parameter         ==> $2"         //第二个参数为何

运行结果

[root@www scripts]# sh sh07.sh theone haha quot 
The script name is        ==> sh07.sh            <==档名
Total parameter number is ==> 3                  <==果然有三个参数
Your whole parameter is   ==> 'theone haha quot' <==参数的内容全部
The 1st parameter         ==> theone             <==第一个参数
The 2nd parameter         ==> haha               <==第二个参数

• shift：造成参数变量号码偏移

[root@www scripts]# vi sh08.sh
#!/bin/bash
# Program:
#    Program shows the effect of shift function.
# History:
# 2009/02/17    VBird    First release
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:~/bin
export PATH

echo "Total parameter number is ==> $#"
echo "Your whole parameter is   ==> '$@'"
shift   # 进行第一次『一个变量的 shift 』
echo "Total parameter number is ==> $#"
echo "Your whole parameter is   ==> '$@'"
shift 3 # 进行第二次『三个变量的 shift 』
echo "Total parameter number is ==> $#"
echo "Your whole parameter is   ==> '$@'"

运行结果

[root@www scripts]# sh sh08.sh one two three four five six <==给予六个参数
Total parameter number is ==> 6   <==最原始的参数变量情况
Your whole parameter is   ==> 'one two three four five six'
Total parameter number is ==> 5   <==第一次偏移，看底下发现第一个 one 不见了
Your whole parameter is   ==> 'two three four five six'
Total parameter number is ==> 2   <==第二次偏移掉三个，two three four 不见了
Your whole parameter is   ==> 'five six'