1. program：instruction+data
? ? ?? data structures + algorathms ??
2. IO redirection：change the position of standard input and output
?? ?2.1 Output redirection : COMMAND > NEW_POS,COMMAND >> NEW_POS
?? ??? ?1) >：overwrite redirection : the original contents of the object file will be cleaned up
?? ??? ??? ?[[email?protected] ~]# ls -l > /tmp/io.txt
? ? ? ? ? ? overwrite redirection will overwrite the object file which is a dangerous operation,we can set it not to overwrite when the file is existed.
?? ??? ??? ??? ?[[email?protected] ~]# set -C? //forbid overwrite an existed file
?? ??? ??? ??? ?[[email?protected] ~]# set +C? //adverse to above operation
? ? ? ? ? ? forced overwrite redirection ：>|
?? ??? ??? ??? ?[[email?protected] ~]# ls -l >| /tmp/io.txt?
?? ??? ?2) >>: append redirection，new contents will append to the end of the object file；
?? ??? ?3) 2>: overwrite redirection of error output data stream : when an error occurred during command execution,redirect error message output.
?? ??? ?4) 2>>: append direction of error output data stream
?? ??? ?5) standard output and error output are redirected to different locations.
?? ??? ??? ?COMMAND > /path/to/file.out 2> /path/to/error.out
?? ??? ?6) standard output and error output are redirected to a same location：
?? ??? ??? ?&>：overwrite redirection ? ?Is equivalent to ： COMMAND > /path/to/file.out 2> &1
?? ??? ??? ?&>>：append redirection ? is equivalent to ： COMMAND >> /path/to/file.out 2>> &1
?? ?2.2 Input redirection < ? change the position of standard input.
?? ??? ?比如 tr 的输入方式是标准输入，我们可以把它的输入改编成文件
?? ??? ??? ?[[email?protected] ~]# tr ‘a-z‘ ‘A-Z‘ < /tmp/io.txt
?? ?2.3 << : HERE Documentation 此处文档
?? ??? ?# cat << EOF
?? ??? ??? ?[[email?protected] ~]# cat << EOF?? //遇到给定的字符结束
?? ??? ??? ?> how are you
?? ??? ??? ?> how old are you
?? ??? ??? ?> EOF? //这里结束之后打印到屏幕上,也就是标准输出
?? ??? ??? ?how are you
?? ??? ??? ?how old are you
?? ??? ?# cat > /path/to/somefile << EOF
?? ??? ??? ?[[email?protected] ~]# cat > /tmp/io.txt << EOF
?? ??? ??? ?> how are you
?? ??? ??? ?> how old are you
?? ??? ??? ?> EOF?? //这里结束后没有打印到屏幕，因为进行输出重定向
3. 管道:第一个命令的输出当作第二个命令的输入，第二个命令的输出当作第三个命令的输入，并依次类推
?? ?3.1 基本语法 ： COMMAND1 | COMMAND2 | COMMAND3 |...
?? ??? ?Note：最后一个命令会在当前shell进程的子shell进程中执行；
?? ?3.2 管道的使用：
?? ??? ?[[email?protected] ~]# echo $PATH | tr ‘a-z‘ ‘A-Z‘
?? ??? ??? ?结果 ： /USR/LOCAL/BIN:/USR/LOCAL/SBIN:/USR/BIN:/USR/SBIN:/BIN:/SBIN:/ROOT/BIN
?? ??? ?[[email?protected] ~]# echo $PATH | tr ‘a-z‘ ‘A-Z‘ | tr -d ‘U‘?? //删除了 U
?? ??? ??? ?结果 ： /SR/LOCAL/BIN:/SR/LOCAL/SBIN:/SR/BIN:/SR/SBIN:/BIN:/SBIN:/ROOT/BIN
?? ??? ?[[email?protected] ~]# cat /etc/rc.d/rc.sysinit | tr ‘a-z‘ ‘A-Z‘ | more
?? ??? ??? ?more filename,? 可如果用管道则是? COMMAND | more?? 结合管道的定义理解
?? ?3.3 tee : read from standard input and write to standard output and files
?? ??? ?[[email?protected] ~]# echo $PATH | tr ‘a-z‘ ‘A-Z‘ | tee /tmp/io.txt? //既标准输出又保存在文件
?? ??? ?[[email?protected] ~]# echo $PATH | tee /tmp/io.txt | tr ‘a-z‘ ‘A-Z‘? //保存在文件里面的是小写，而标准输出的是大写
?? ??? ?[[email?protected] ~]# echo $PATH | tee /tmp/io.txt | tr ‘a-z‘ ‘A-Z‘ > /tmp/txt.io?? //这里之保存在文件没有标准输出?? ?3.4 理解管道的两点：?? ??? ?第一 ： 前一个命令的结果不会在标准输出中输出?? ??? ?第二 ： 前一个命令的标准输出是后一个命令的参数
