test命令在shell脚本中的应用

   
     接触shell脚本后，发现test命令在脚本中的应用非常多。学习shell脚本test命令是必须要掌握的，而且他很容易被掌握。
    test命令可以处理shell脚本的各类工作。它产生的不是一般的输出（文本内容等），而是可使用的退出状态（true和false）。test接受各种不同的参数，可控制它要执行哪一种测试。
   
    test命令有两种模式：
    第一种：可以通过man test查看：test [-OPTION] EXPRESSION   
    不带参数： 

if test "$str1 = $str2"
then
    ......
fi

    带参数：

if test -x "$file"
then
    ......
fi

    第二种：[‘空’[-OPTION] EXPRESSION‘空’] 注意左右方括号和测试表达式之间必须存在一个空格.
    不带参数：

if [ "$str1 = $str2" ]
then
    ......
fi

    带参数：

if [ -z "$str1" -a -z "$str2" ]
then
    ......
fi

  这两种只是格式不同，使用时情况因人而异。表达式的逻辑如下：
 ( EXPRESSION )
              EXPRESSION is true


       ! EXPRESSION
              EXPRESSION is false

表达式组合：
       EXPRESSION1 -a EXPRESSION2
              both EXPRESSION1 and EXPRESSION2 are true


       EXPRESSION1 -o EXPRESSION2
              either EXPRESSION1 or EXPRESSION2 is true

[-OPTION]参数如下（栽man test）：

       -n STRING
              the length of STRING is nonzero


       STRING equivalent to -n STRING


       -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


       -G FILE
              FILE exists and is owned by the effective group ID


       -h FILE
              FILE exists and is a symbolic link (same as -L)


       -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 read permission is granted


       -s FILE
              FILE exists and has a size greater than zero


       -S FILE
              FILE exists and is a socket


       -t FD  file descriptor FD is opened on a terminal
    
     -u FILE
              FILE exists and its set-user-ID bit is set


       -w FILE
              FILE exists and write permission is granted


       -x FILE
              FILE exists and execute (or search) permission is granted