学习日记

1、

http://www.linuxsir.org/main/doc/abs/HTML/untyped.html

4.3. Bash Variables Are Untyped

Example 4-4. Integer or string?

2、

http://www.linuxsir.org/main/doc/abs/HTML/othertypesv.html

The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems.

3、

命令:basename(# Strips out path name info (see 'basename'))

[mdx@localhost abs-exercises]$ basename /home/mdx/abs-guide-3.7/ex15.sh

ex15.sh

4、比较：

5、得到最后一个参数：

6、待查

If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter.

7、在Example 4-6. wh, whois domain name lookup中，域名查询的服务器是无效的，可以换成："wh-inn" ) whois $1@whois.internic.net;;

8、生词：

positional

notch

1、echo -n的作用简单的说就是不换行，像java中的System.out.print("something");

结果：Values of "a" in the loop are: 7 8 9 11

2、

下面这段的“!”不懂：

  

3、``相当于$()

4、英语生词：

disguised

naked

trailing

1、双引号内的变量可以保留变量值当中的超过一个空格的空格：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

Example 4-1. Variable assignment and substitution

2、可以在一行中对多个变量赋值，但是在有可能引起版本兼容性问题：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

Example 4-1. Variable assignment and substitution

3、在没有初始化的变量上进行算术运算不是非法的，当作是"0"，但是在有可能引起版本兼容性问题：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

read echo >$2 不懂

(http://www.linuxsir.org/main/doc/abs/HTML/special-chars.html)

Ctl-M

Carriage return.

   1 #!/bin/bash

   2 # Thank you, Lee Maschmeyer, for this example.

   3

   4 read -n 1 -s -p $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'

   5                                   # Of course, '0d' is the hex equivalent of Control-M.

   6 echo >&2   #  The '-s' makes anything typed silent,

   7            #+ so it is necessary to go to new line explicitly.

   8

   9 read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'

  10            #  '0a' is the hex equivalent of Control-J, linefeed.

  11 echo >&2

  12

  13 ###

  14

  15 read -n 1 -s -p $'And Control-K\x0bgoes straight down.'

  16 echo >&2   #  Control-K is vertical tab.

  17

  18 # A better example of the effect of a vertical tab is:

  19

  20 var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'

  21 echo "$var"

  22 #  This works the same way as the above example. However:

  23 echo "$var" | col

  24 #  This causes the right end of the line to be higher than the left end.

  25 #  It also explains why we started and ended with a line feed --

  26 #+ to avoid a garbled screen.

  27

  28 # As Lee Maschmeyer explains:

  29 # --------------------------

  30 #  In the [first vertical tab example] . . . the vertical tab

  31 #+ makes the printing go straight down without a carriage return.

  32 #  This is true only on devices, such as the Linux console,

  33 #+ that can't go "backward."

  34 #  The real purpose of VT is to go straight UP, not down.

  35 #  It can be used to print superscripts on a printer.

  36 #  The col utility can be used to emulate the proper behavior of VT.

  37

  38 exit 0