shell test [ ]

http://man.linuxde.net/test
http://blog.chinaunix.net/uid-24780853-id-2578709.html

[root@server1 ~]# man test
TEST(1)                          User Commands                         TEST(1)

NAME
       test - check file types and compare values

SYNOPSIS
       test EXPRESSION
       test

       [ EXPRESSION ]
       [ ]
       [ OPTION

DESCRIPTION
       Exit with the status determined by EXPRESSION.

       --help display this help and exit

       --version
              output version information and exit

       An  omitted  EXPRESSION defaults to false.
       Otherwise, EXPRESSION is true or false and sets exit status.  It is one of:

       ( EXPRESSION )
              EXPRESSION is true

       ! EXPRESSION
              EXPRESSION is false

if 执行的选择与 $？返回值有关
test   ====  [  ]
------------------------------------------------------------------

[root@server1 ~]# test
[root@server1 ~]# echo $?
1

[root@server1 ~]# [ ]
[root@server1 ~]# echo $?
1

--------------------------------------------------------------------------

(ls) 与 $(ls) 区别：

(ls)在子shell中执行ls命令,屏幕显示结果

a=$(ls):把ls执行命令结果,即输出值放到变量a中,重定向输出到a(不包括错误输出）

[root@server1 kk]# (ls)
mm
[root@server1 kk]# echo $?
0

[root@server1 kk]# (ls tt)
ls: 无法访问tt: 没有那个文件或目录
[root@server1 kk]# echo $?
2

[root@server1 kk]# ! ls
mm
[root@server1 kk]# echo $?
1

说明"!":作用是$?取反

-----------------------------------------------------------------------------------------------------------------------------

echo $((2>1))    

1

echo $((2==1))  

0

echo $((2<1))  

0

a=$(( 2+1 ))
b=$(( 2-1 ))
c=$(( 2*1 ))
d=$(( 2/1 ))

-------------------------------------------------------------------------------------

[root@server1 kk]# true
[root@server1 kk]# echo $?
0
[root@server1 kk]# false
[root@server1 kk]# echo $?
1

--------------------------------------------------------------------------

-a: 
-o:
       EXPRESSION1 -a EXPRESSION2
              both EXPRESSION1 and EXPRESSION2 are true

       EXPRESSION1 -o EXPRESSION2
              either EXPRESSION1 or EXPRESSION2 is true
---------------------------------------------------------------------------------------
 字符串：
    
      -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
------------------------------------------------------------------------------------------------------------
整型：
       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
---------------------------------------------------------------------------------------------------------------------------------
文件：
       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
-------------------------------------------------------------------------------------------------------------------------------

       Except for -h and -L, all FILE-related tests dereference symbolic links.  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.

       NOTE: [ honors the --help and --version options, but test does not.  test treats each of those as it  treats
       any other nonempty STRING.

       NOTE:  your  shell may have its own version of test and/or [, which usually supersedes the version described
       here.  Please refer to your shell’s documentation for details about the options it supports.