在64位Linux上安装MemCached
在一台64位Linux的机器上安装了MemCached，遇到一个小问题，特记录之。
MemCached使用了libevent，所以必须先安装libevent。安装libevent到/usr/lib
wget http://www.monkey.org/~provos/libevent-1.4.9-stable.tar.gz gzip -d libevent-1.4.9-stable.tar.gz tar xvf libevent-1.4.9-stable.tar cd libevent-1.4.9-stable ./configure --prefix=/usr make make install
安装MemCached的到/u01/memcached
wget http://www.danga.com/memcached/dist/memcached-1.2.6.tar.gz gzip -d memcached-1.2.6.tar.gz tar xvf memcached-1.2.6.tar cd memcached-1.2.6 ./configure --prefix=/u01/memcached --with-libevent=/usr make make install
但是执行memcached命令时出现错误：
#/u01/memcached/bin/memcached -h /u01/memcached/bin/memcached: error while loading shared libraries: libevent-1.4.so.2: cannot open shared object file: No such file or directory
一般对于这种依赖的库找不到的情况，在Linux中可以通过设置LD_DEBUG环境变量来获得更多的信息
#LD_DEBUG=help ls Valid options for the LD_DEBUG environment variable are: libs display library search paths reloc display relocation processing files display progress for input file symbols display symbol table processing bindings display information about symbol binding versions display version dependencies all all previous options combined statistics display relocation statistics unused determined unused DSOs help display this help message and exit To direct the debugging output into a file instead of standard output a filename can be specified using the LD_DEBUG_OUTPUT environment variable.
这里由于是库文件依赖有问题，则使用libs参数：
#LD_DEBUG=libs /u01/memcached/bin/memcached -h
30596: find library=libevent-1.4.so.2 [0]; searching
30596: search cache=/etc/ld.so.cache
30596: search path=/lib64/tls/x86_64:/lib64/tls:/lib64/x86_64:/lib64:/usr/lib64/tls/x86_64
/usr/lib64/tls:/usr/lib64/x86_64:/usr/lib64
(system search path)
30596: trying file=/lib64/tls/x86_64/libevent-1.4.so.2
30596: trying file=/lib64/tls/libevent-1.4.so.2
30596: trying file=/lib64/x86_64/libevent-1.4.so.2
30596: trying file=/lib64/libevent-1.4.so.2
30596: trying file=/usr/lib64/tls/x86_64/libevent-1.4.so.2
30596: trying file=/usr/lib64/tls/libevent-1.4.so.2
30596: trying file=/usr/lib64/x86_64/libevent-1.4.so.2
30596: trying file=/usr/lib64/libevent-1.4.so.2
30596:
/u01/memcached/bin/memcached: error while loading shared libraries: libevent-1.4.so.2:
cannot open shared object file: No such file or directory
可以看到是在加载/usr/lib64/libevent-1.4.so.2文件时出现了问题，系统中确实是没有该文件的，查找后发现libevent-1.4.so.2存在于/usr/lib目录，这可能是libevent在64位Linux系统上的一个bug吧，没有关系，复制一份或者建一个软链接即可解决问题。
#ln -s /usr/lib/libevent-1.4.so.2 /usr/lib64/libevent-1.4.so.2 #/u01/memcached/bin/memcached -h memcached 1.2.6 -pTCP port number to listen on (default: 11211) -U UDP port number to listen on (default: 0, off) -s unix socket path to listen on (disables network support) -a access mask for unix socket, in octal (default 0700) -l interface to listen on, default is INDRR_ANY -d run as a daemon -r maximize core file limit -u assume identity of (only when run as root) -m max memory to use for items in megabytes, default is 64 MB -M return error on memory exhausted (rather than removing items) -c max simultaneous connections, default is 1024 -k lock down all paged memory. Note that there is a limit on how much memory you may lock. Trying to allocate more than that would fail, so be sure you set the limit correctly for the user you started the daemon with (not for -u user; under sh this is done with 'ulimit -S -l NUM_KB'). -v verbose (print errors/warnings while in event loop) -vv very verbose (also print client commands/reponses) -h print this help and exit -i print memcached and libevent license -b run a managed instanced (mnemonic: buckets) -P save PID in , only used with -d option -f chunk size growth factor, default 1.25 -n minimum space allocated for key+value+flags, default 48
启动MemCached，-m表示分配的内存
#/u01/memcached/bin/memcached -d -m 1024 -u admin -l 127.0.0.1 -p 11211
Linux中如何让进程在后台运行
在Linux中，如果要让进程在后台运行，一般情况下，我们在命令后面加上&即可，实际上，这样是将命令放入到一个作业队列中了：
$ ./test.sh & [1] 17208 $ jobs -l [1]+ 17208 Running ./test.sh &
对于已经在前台执行的命令，也可以重新放到后台执行，首先按ctrl+z暂停已经运行的进程，然后使用bg命令将停止的作业放到后台运行：
$ ./test.sh [1]+ Stopped ./test.sh $ bg %1 [1]+ ./test.sh & $ jobs -l [1]+ 22794 Running ./test.sh &
Linux中如何将文件dump成16进制值
在linux中有多种方式可以将文件dump成16进制显示，也可以将16进制值再反向成文件。
0000000 524f 2d41 3030 3036 0a30 524f 2d41 3030
0000010 3630 0a30
0000014
$ od -x test.txt
0000000 524f 2d41 3030 3036 0a30 524f 2d41 3030
0000020 3630 0a30
0000024
$ xxd test.txt
0000000: 4f52 412d 3030 3630 300a 4f52 412d 3030 ORA-00600.ORA-00
0000010: 3036 300a 060.
注意到hexdump和od出来的结果都是按实际存储的字节序，因为基于x86的linux是little-endian的，也就是高低字节是颠倒了的。但是xxd的结果是将字节序调整过了的。而IBM的Power CPU是big-endian的，所以在AIX上od的结果如下：
0000000 4f52 412d 3030 3630 300a 4f52 412d 3030
0000020 3036 300a
0000024
xxd还可以实现从16进制反向生成文件，只需要加上-r选现即可。
ORA-00600
ORA-00060
$ echo 0000000: 4f52 412d 3030 3630 300a 4f52 412d 3030 | xxd -r
ORA-00600
ORA-00
但是要注意xxd接受的是big-endian格式的16进制值，如果输入的是little-endian的，则生成的文件字符(包括换行符)是两两颠倒的。
RO-A0006
0RO-A0060
0
在vi编辑器中可以使用:%!xxd 调用xxd来将文件转换成16机制编辑模式，编辑完成后再调用:%!xxd -r转换文件模式，从而使得vi具有16进制编辑的功能，:%!其实就是调用外部shell命令，需要注意的是xxd的字节序是big-endian的，不要搞错了。
如果你的Linux系统中找不到xxd命令，那么检查下是否有安装vim-common包
vim-enhanced-6.3.046-0.40E.7
vim-X11-6.3.046-0.40E.7
vim-minimal-6.3.046-0.40E.7
vim-common-6.3.046-0.40E.7
Redhat Linux如何设置用户默认属性
和Oracle的profile类似，在Redhat Linux新建一个用户或组，有很多默认继承的属性，比如密码过期时间，密码最短长度，UID/GID的范围等。这些属性是通过文件/etc/login.defs获得的，所以可以通过修改该文件改变后续所有新建用户的默认设置。
一些主要的属性包括：
PASS_MAX_DAYS 99999
PASS_MIN_DAYS 0
PASS_MIN_LEN 5
PASS_WARN_AGE 7
UID_MIN 500
UID_MAX 60000
GID_MIN 500
GID_MAX 60000
CREATE_HOME yes
更多的设置及其详细说明，参考man login.defs。
