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?

20 c=BB34   21 echo "c = $c"            # c = BB34   22 d=${c/BB/23}             # Substitute "23" for "BB".   23                          # This makes $d an integer.   24 echo "d = $d"            # d = 2334

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.

bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"    bash$ du  bash: /usr/bin/du: Argument list too long

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：

16 if [ -n "$1" ]              # Tested variable is quoted.   17 then   18  echo "Parameter #1 is $1"  # Need quotes to escape #   19 fi

5、得到最后一个参数：

1 args=$#           # Number of args passed.    2 lastarg=${!args}    3 # Or:       lastarg=${!#}    4 #           (Thanks, Chris Monson.)    5 # Note that lastarg=${!$#} doesn't work.

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.

1 variable1_=$1_  # Rather than variable1=$1    2 # This will prevent an error, even if positional parameter is absent.    3    4 critical_argument01=$variable1_    5    6 # The extra character can be stripped off later, like so.    7 variable1=${variable1_/_/}    8 # Side effects only if $variable1_ begins with an underscore.    9 # This uses one of the parameter substitution templates discussed later.   10 # (Leaving out the replacement pattern results in a deletion.)   11   12 #  A more straightforward way of dealing with this is   13 #+ to simply test whether expected positional parameters have been passed.   14 if [ -z $1 ]   15 then   16   exit $E_MISSING_POS_PARAM   17 fi   18   19   20 #  However, as Fabian Kreutz points out,   21 #+ the above method may have unexpected side-effects.   22 #  A better method is parameter substitution:   23 #         ${1:-$DefaultVal}   24 #  See the "Parameter Substition" section   25 #+ in the "Variables Revisited" chapter.

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");

19 # In a 'for' loop (really, a type of disguised assignment):     20 echo -n "Values of \"a\" in the loop are: "     21 for a in 7 8 9 11     22 do     23   echo -n "$a "     24 done

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

2、

下面这段的“!”不懂：

  

10 a=`echo Hello!`   # Assigns result of 'echo' command to 'a'     11 echo $a     12 #  Note that including an exclamation mark (!) within a     13 #+ command substitution construct #+ will not work from the command line,    14 #+ since this triggers the Bash "history mechanism."     15 #  Inside a script, however, the history functions are disabled.

3、``相当于$()

2 R=$(cat /etc/redhat-release)

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

33 hello="A B  C   D"   34 echo $hello   # A B C D   35 echo "$hello" # A B  C   D   36 # As you see, echo $hello   and   echo "$hello"   give different results.   37 #                                      ^      ^   38 # Quoting a variable preserves whitespace.

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

57 #  It is permissible to set multiple variables on the same line,   58 #+ if separated by white space.   59 #  Caution, this may reduce legibility, and may not be portable.   60   61 var1=21  var2=22  var3=$V3   62 echo   63 echo "var1=$var1   var2=$var2   var3=$var3"   64   65 # May cause problems with older versions of "sh".

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)

An uninitialized variable has a "null" value - no assigned value at all (not zero!). Using a variable before assigning a value to it will usually cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable.    1 echo "$uninitialized"                                # (blank line)    2 let "uninitialized += 5"                             # Add 5 to it.    3 echo "$uninitialized"                                # 5    4    5 #  Conclusion:    6 #  An uninitialized variable has no value, however    7 #+ it acts as if it were 0 in an arithmetic operation.    8 #  This is undocumented (and probably non-portable) behavior.

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