Linux man test

        先来看看Linux下对test的解释（使用man test或者info test）

TEST(1)                               FSF                              TEST(1)

NAME
       test - check file types and compare values

SYNOPSIS
       test EXPRESSION
       [ EXPRESSION ]
       test 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

       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
       Beware  that  parentheses need to be escaped (e.g., by backslashes) for
       shells.  INTEGER may also be -l STRING, which evaluates to  the  length
       of STRING.

AUTHOR
       Written by FIXME: ksb and mjb.

REPORTING BUGS
       Report bugs to <bug-coreutils@gnu.org>.

COPYRIGHT
       Copyright (C) 2002 Free Software Foundation, Inc.
       This is free software; see the source for copying conditions.  There is
       NO warranty; not even for MERCHANTABILITY or FITNESS FOR  A  PARTICULAR
       PURPOSE.

SEE ALSO
       The  full documentation for test is maintained as a Texinfo manual.  If
       the info and test programs are properly installed  at  your  site,  the
       command

              info test

       should give you access to the complete manual.

GNU coreutils 4.5.3              February 2003                         TEST(1)

        再看两个简单的例子：

#示例一
#!/bin/bash  
#iftest2  
echo -n "Enter your name:"  
read NAME  
#did the user just hit return  
if [ "$NAME" == "" ];  
then  
        echo "You did not enter any infomation"  
else  
        echo "Your name is ${NAME}"  

#示例二
#!/bin/bash  
#ifelif  
echo -n "Enter your name:"  
read NAME  
if [ -z $NAME ] || [ "$NAME" = "" ];  
then  
        echo "You did not enter a name."  
elif [ "$NAME" = "root" ];  
then  
        echo "Hello root"  
elif [ "$NAME" = "chinaitlab" ];  
then  
        echo "Hello chinaitlab"  
else  
        echo "You are not root or wgb,but hi,$NAME"  
fi  

        小技巧：

        1.Linux对Shell的要求很严格，不要把代码洁癖（比如定义变量的空格、if、while等等中为了美观留下的空格）带到Linux Shell编程中，稍不注意就会报错；

        2.多用man或者info，会收到意想不到的收获。

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