Shell学习笔记-基本点

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1.echo

“ 双引号引用的字符串，可以引用出了 $ \ ` 之外的任意字符。其中对于 \ 转义字符必须加 -e 选项才会进行解释转义，否则默认不转...
‘ 单引号则屏蔽一切转义字符
-n 去掉要打印字符串的换行。

2.重定向

command > filename 重定向输出
command >> filename 追加输出

command 1 > filename 命令的标准输出重定向到文件
command 2 > filename 命令的标准错误输出重定向到文件

command > filename 2>&1 命令的标准输出和错误输出都重定向到文件，意思就是 1和2 都输出到文件
command >> filename 2>&1 命令的标准输出和错误输出都重定向到文件，意思就是 1和2 都输出到文件,与上边的区别就是追加

command < filename 文件内容作为标准输入
command << delimiter 从标准输入中读入，直到遇见 delimiter分界符为止，意思就是输入串很长，但是真正的输入就是这个分界符之前的内容

command < filename1 > filename2 一个是输入一个是输出

command <&m 文件描述符m对应的文件作为输入
command >&m 文件描述符m对应的文件作为输出
command <&- 关闭该命令的标准输入

>&m 或者 <&m m为文件描述符，也是重定向，把m作为输入或者输出....这个运算符前后的参数都可以理解为文件描述符，其实 command时也可以理解为标准输出1...
其实就是文件描述符对应的文件内容之间的重定向...

3.test
     test EXPRESSION
   [ EXPRESSION ]

   ( EXPRESSION )
   EXPRESSION is true
   ! EXPRESSION
   EXPRESSION is false
   EXPRESSION1 -a EXPRESSION2
   both EXPRESSION1 and EXPRESSION2 are true
   EXPRESSION1 -o EXPRESSION2
   either EXPRESSION1 or EXPRESSION2 is true
   -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

4.各种括号总结

[ $var1 - gt $var2 ] 判断条件和 test 的意义相同
${Var} 取变量的值，和$var意义相同
$[ var1 + var2 ] 中括号内位表达式，这个是计算表达式的值
$((var1+var2))和上边的意思是一样的...都是计算表达式的值
待续

5.脚本参数

shift n 表示跳过n个参数，是$n表示参数内容发生变化

$n 表示第n个参数
$# 表示参数个数

getopts
getopts 参数可能值 var
例子：getopts afdg var 意思是 var可以去 afdg中的某个值，调用脚本时书写形式为 -a -f -d -g ，如果输入错了，还会有提示....